The AM Forum

The journey starts with the first step....    Destination:  A self-contained, homebrew Ranger III in the Twilight Zone.

For your entertainment, picture below a virgin  Bud 17" wide  X 15" deep  X 6" high aluminum chassis.  It's a rather big chassis for a single self-contained 6146B transmitter, but compare it to a Johnson Ranger and we see it's not THAT big.  Lots of room to mount parts underneath. EZ to work on. The front panel will be about 15" high. FULL metering with four meters; grid current, screen current, plate current, HV.

Large 2400 pF loading cap, large 500 pF tuning cap, tank coil suited for a 300 watt transmitter.  Lots of room and great efficiency.  No need to have loads of fixed caps switched in like the Ranger uses.  Cover 160M to 10M, of course.

Featured will be a 300 mA 800V, Variac variable HV step-start supply to eek out at least 50-60W of AM carrier -  with stud-warrior audio peaks from the Hammond/ SS amp.  It uses three big beer can filter caps of drunkenness.  

The Hammond 1628SEA "mod xfmr" is superb; (over-designed) a single-ended  4/8/16 ohm to 5000 ohm 48 Henry  eleven pounder (rated 20-20,000Hz) that will accept 8 ohm audio from an 85W SS module about the size of a pack of cigarettes.  No Heising reactor needed.  My power transformer for the audio board is big:  +- 35V @ 5A = 175W rated.   I already built a 4D32 version of this rig using a 1642SE Hammond and a Dayton 150W analog audio amp. It is possibly the best sounding [transformer] rig for AM that I own.  So the 6146B rig should be a shoe-in.

I plan to add a DDS VFO for exquisite stability and knowing where the hell am I?

This rig will double as a stand-alone Pissweaker rig at 15-60 watts -  or as an RF driver for my pair of 4-1000As in grounded grid (with regulated grid and screen voltages) linear service.  

The paint scheme will be interesting. I plan to duplicate the two-tone colors  (dark maroon/ gray) and artwork of a Johnson Ranger.  The area where the maroon/brown comes up and circles the Ranger VFO will instead be a plate glass area that displays the 6146B and possible DDS readout.   I may name it  my " Ranger III,"  dunno yet.

Lot's of work yet to do. All parts have been picked or awaiting delivery.

More later....

Are you not entertained?

Tom, K1JJ

The primary winding is 5000 ohm; secondary is the 4/8/16. So will you run the transformer backwards - solid state module into the 4/8/16 side and the 6146 B+ to the 5000 ohm side?
Is this audio module something you're building yourself, or a commercial make?
Sounds very cool.

So, the destructive testing zealot within me is asking…

A 6146B is rated for 65w ICAS in plate mod class C. The plate dissipation is 23 watts.  If you run this at 60w carrier, and have oodles of audio for peak positive modulation, what will give first? Arcing from the instantaneously high plate voltage? Melting from the increased heat? Or will the cathode just give up on giving more electrons and no matter how high the plate peak voltage is, some emission limited point is reached where it just won’t give any more positive modulation?  There’s gonna be some limit some place…will you be seeking to find it? I kinda dare you to put continuous sine-like (asymmetrical tone) waves into it…..

Sounds like fun. Although that tube looks puny on that big chassis!

Ed

Hi Bill -

Good questions.


Use this ITEM # on eBay to see the audio board. Just needs +- 20 to 35VAC  CT  to work. All the rectifiers and filter caps on board.  Only $14 for 85W board.
Paste it into the ebay search  window:    143464647535  (the URL is too long)


This is the data sheet for the actual audio chip, TDA7294:  Check out page 20... Notice it has great THD% below 75W but falls apart above 75-85W, depending on 4 or 8 ohms.   That's OK cuz it's just a single 6146B to be plate modulated.

https://www.st.com/resource/en/datasheet/tda7294.pdf


Here are the line of 1628SEA and 1642SE hi-fi Hammond transformers. The 1628SEA is very under-rated at 30 watts and 11 pounds. Probably more like 60W + I would guess.  Or use the 75W version, 1642SE for more $.

https://www.hammfg.com/electronics/transformers/audio/1627-1642


Yes, the "Output/modulation" xfmr is hooked up as you described, 5K to the 6146B and 4/8/16 ohms to the SS audio amplifier.

BTW, I received some time worthy info concerning screen voltage techniques (using a screen to plate resistor vs: a choke and supply) on a class C amplifier.  Let me ask permission to post his comments first...

T

Hi Ed,

Yes, 40 watts output is more like it, as demonstrated by the average Johnson Ranger, but 60 watts @ ~80% class C efficiency  is my a goal.  I always seem to get more power out of my tube projects than the datasheets say.  I've had four 6146s in a class C PDM rig once and it did way more than rated.  That's why I use a Variac and power transformer that will let me experimentally bring up the HV and other parameters until things start breaking. Once that point is reached I can back the rig way down and it will never crap out.

A single 6146B is more for the fun -  and it's cool running a pissweaker. The dB game can be easily played when I switch on the dual 4X1 linear with the 6146B as a hi-fi driver.... :-)

BTW, credit to Jeff/W2NBC who first gave me the idea of using the Hammond xfmr  back a year ago. He also has a working rig using the technique.
And credit to Chuck/K1KW for his suggestion to try the inexpensive PDA7294  audio chip.

You can buy a Hammond 1628SEA from Hawk Electronics drop shipped for $156 plus shipping.

Here is the building of "Summer Breeze"  a 4D32 plate modulated by the big Hammond 1642SE and a 150W Dayton amplifier. My cleanest rig using a "transformer."  It's like Valiant power - 125 watts out all day.  But my series modulated 813 rig is the cleanest of them all.  (transformerless)

http://amfone.net/Amforum/index.php?topic=45973.0

T

Some very intriguing stuff. The modules are available on Amazon, too; just ordered myself a couple. There are a few Hammond items I've been meaning to order, and I might just toss a 1628SEA into the order while I'm at it.
Thanks for the inspiration -

Yes! Hawk has become my "go to" for Hammond parts: they're cheaper than just about any supplier out there, and they ship like greased lightning.

Bill,

There's actually a used 1628SE on eBay for $110 plus $12 shipping.   Older 1998 date code.

https://www.ebay.com/itm/275138284407?hash=item400f850b77:g:bN0AAOSwoNRh8GCm


Funny but the very first rig I ever built back in 1971 was a 6146B plate modulated.   So we've come full circle exactly 50 years later.

Let me know how your progress goes in this project too. This should keep me busy posting here for at least a month or two, including final testing and optimization.

BTW, your screen modulated DX-60 using the QIX circuit sounded very good the other night when we talked on 75M.

T

Speaking about naming this rig.... the Johnson "Ranger" name is about a tough, military man.  So keeping in the spirit, how about "Commando?"  It's not used anywhere on a rig and is corny enough to be funny.   Imagine a 50 watt rig being called a Commando.  Like a miniature Poodle named Rex.


T

Tom,
  Thanks for the compliment on the DX-60. Like I tell people on the air, I can't take credit for the engineering - it's Steve's genius - I just did the soldering. It is fun to hear people's reactions when I tell them it's a DX-60; the guys who are familiar with the DX-60's stock controlled carrier sound are often amazed.
  My current project is the pine-board transmitter that I posted about a few weeks back. The intent was to perfect that RF deck and then build a modulator for it based on the Hammond H301533 modulation transformer - the one with the "ultra-linear" screen taps - with the modulator circuit based on a "high-fidelity" tube amp right out of the RCA Receiving Tube Manual.
  But now I'm thinking of trying the concept you describe instead. It'll be a slightly easier build than all that point-to-point tube wiring.
  I've never given a name to any of my radio gear, but years ago when I was into building high-powered flashlights, I built a flashlight using an array of high-powered LED's, all mounted on a round aluminum heat-sink that I machined, complete with a cooling fan to dissipate the heat generated by the LED's. My wife named it "Dinosaur Breath" because of the roaring sound of the fan when I turned it on.

Hi Bill,

Cool on the potential plans.  Yes, the concept works very well. I think it may be the most efficient method of running an AM rig except for a PDM rig.  The final runs class C and the SS modulator is running very efficient depending on the amplifier. At 100 watts carrier, the 4D32 barely gets warm (with a quiet muffin fan) and the audio amp is cool. Scale that down to a 6146B final at 40 watts with a class D audio amp and we are freezing in the winter time... ;D


BTW, back in 2007, Stu / AB2EZ had experimented with these Hammond xfmrs and SS audio amps. Check this out:

http://amfone.net/Amforum/index.php?topic=12117.0


You mentioned the RCA tube manual hi-fi amp. That was the first audio amp I ever built. It never worked real well for me on the air, but that was most likely from my inexperience back in 1972.

I'm kinda excited about this 6146B project and plan to do a little each day. The front panel will be here soon, so I can bolt it to the chassis using strong side gussets and that will mark the beginning of serious drilling and blasting.

Keep me updated how your project progresses and I'll do the same.



Definition of "Commando"

1 South Africa

a:  a military unit or command of the Boers

b:  a raiding expedition

2a: a military unit trained and organized as shock troops especially for hit-and-run raids into enemy territory

b:   a member of such a unit

going commando
 *****slang : to wear no underwear***



T

These little 85W modules are linear class AB, rather than class D, unless I'm missing something? Looking at the specs, I'm not seeing any switching going on. Some of the adds on eBay do call them "digital", though.

You may be right, Bill!

Somehow I just assumed it was a typical class D Chinese switched amplifier and I would take my chances cuz the THD% looked so good.  

*A friend says that it has a "bootstrap" circuit so there is some switching going on. I'll just have to test it to see.


The Dayton 150W amp that I now use with the 4D32 is a class AB amp too - and as you know, I just love the performance of that rig with the Hammond 1642SE.  

This module will require a heatsink of sorts. No problem. I plan on a small muffin fan to sweep a little air over the 6146B and heat sink anyway.  

We were talking with the guys tonight on 75M. Just as an experiment I want to see how efficient I can make the rig -  and how much power I can get out maintaining big, clean audio.   We'll require very deep into class C, bigger tank components and max HV, etc.  Oughta be fun and may pop a few 6146B tubes doing it...  ;D

Thanks for pointing that out.

T


Features:
• Very high operating voltage range (± 40 V)
• DMOS power stage
• High output power (up to 100 W music power)
• Muting/stand-by functions
• No switch on/off noise
• No boucherot cells
• Very low distortion
• Very low noise
• Short circuit protection
• Thermal shutdown
Description
The TDA7294 is a monolithic integrated circuit in Multiwatt15 package, intended for
use as audio class AB amplifier in Hi-Fi field applications (Home Stereo, self powered
loudspeakers, Topclass TV). Thanks to the wide voltage range and to the high out
current capability it is able to supply the highest power into both 4 Ω and 8 Ω loads
even in presence of poor supply regulation, with high supply voltage rejection.
The built in muting function with turn on delay simplifies the remote operation
avoiding switching on-off noises.

  Yeah, I saw that "bootstrap" thingie, too, but there's nothing on the diagram to indicate any clocking or switching devices. But, maybe the manufacturer is keeping some of their proprietary goodies under their hat. In any event, it's a neat little device with some impressive specs, regardless of how they're accomplishing it. And you can't beat the price.
  The neat thing about this little module is that you can just hook it up to an output transformer - no "real" modulation transformer needed - and run with it, without dealing with a maze of 12AU7's; 12AX7's; and 6L6's or whatever.
  There are also some 500 watt class D boards out there, but I think output transformers capable of handling that sort of audio power would be hard to come by. The 1642SE primary is good for 3500 volts DC at 300 mills, so there's a KW DC input to the final PA, but the specs say only 75 watts of audio. Too bad - I could see modulating a pair of 4-250's with that otherwise.

I think the Hammond transformers are way under rated. If we use the old TimTron rule that each pound of weight is good for 10 watts, then a 100 watt audio (modulation) transformer is good for 1000 watts, which makes sense to me.    In this case of the 1628SEA, 11 pounds is good for 110 watts.   30 watts seems really under-rated.  Probably these are audiophile pristine music program numbers.  

My 1642SE is really massive for a 75W  rated transformer and 28 pounds = 280 watts.  With the 4D32 rig I saw signs of saturation well past 75 watts of audio from the amplifier.  After all, at 20-20,000 Hz xfmr specs, there is a lot of mass needed to pass 20Hz cleanly.  The average human voice has little energy below 60 Hz.  Though, I like my rigs reasonably clean down to at least 20 Hz if possible.

The tone tests will show what we need to know once the 6146B is built with the 1628SEA lashup.

BTW, speaking of modulating something bigger... I'll bet a pair of big 1642SEs in series parallel could modulate a KW rig.


T

When considering the weight based rule of thumb, keep in mind that in a regular push-pull audio output transformer (not a modulation transformer) there is no DC magnetization effect, the primary DC is balanced/equal and opposite, having no net DC magnetization to contribute to saturation.  Modulation transformers for push-pull to finals will have DC magnetization in the secondary from the carrier standing current. If you use a mod reactor and a capacitor to couple the audio, this magnetization is gone; a good reason to do it that way at high power levels.  With your single ended audio transformer, it's sorta like a mod transformer...in audio use, the primary has standing DC current like a mod transformer would.

You should probably add the standing power from the carrier DC input to the audio power to see if it really is that much above the weight rule of thumb, it is likely closer than you think.

And then, there's the audiophile part...Nobody builds SE audio amps with tubes except esoteric audiophiles, so response below 20Hz is expected, and with that, the degreee of magnetic saturation is greater, as well..


Ed

Ed,

Good points about the single-ended magnetization vs: conventional push pull.   And the DC carrier current effect.   Then maybe the specs Hammond suggests ARE reasonable in the single-ended service we are planning.

I have a friend who built an 807 using the 1628SE and says he can modulate the pants off the 807 with very clean audio, so we should be fine with the 6146B.  At maximum power we will look for the limitations of saturation beginning to appear when sweeping the very low audio frequencies.

So my question is:  If we wanted to modulate a big 500W+ watt rig with the 1642SE or a pair,  would a large inductance mod reactor and coupling cap give us the full use of all the 1642SE xfmr core audio capability?  (using a single-ended 8 ohm amp)

* ahhh...  my aluminum front panel and some parts just showed up.  Time to do a little drilling.     Need to cut four meter holes and viewing window for the mighty 6146B.


T

I received the 85 W audio module today.  1/2 the size of a pack of cigarettes. The chip uses the same sil-pad and footprint as a 11N90.

Assuming the chip puts out 60 watts and dissipates 60 watts,  (120W total)  I wonder if I bolt it directly to the large  chassis .... (17" X 15"  X 6" )  will that be enuff heatsinking for 60W or will I need a finned heatsink up on top too?  I also have a quiet 4.5" muffin fan that will be blowing air past the chip heatsink and onto the 6146B glass.

I found an inexpensive metal shop in town to CNC  plasma cut my front panel holes for the meters, viewing window and DDS readout.  Big PIA to do it myself.  The rest of the metalwork is a breeze in my cellar.

T


**  Below, as motivation, is the dedicated amplifier that the 6146B rig will be driving. [when needed]    "Commando" driving "Rico Suave."  The amplifier will be very efficient running in class C for AM linear use.  Notice the "Class C" bias selection on the front panel.  Classic old school rigs!

A thread about running a more efficient linear amp on AM:
http://amfone.net/Amforum/index.php?topic=32635.0

  So how would we connect for this? Primary would be easy: two 16 ohm windings in parallel to make 8 ohms, or two 4 ohm windings in series to make 8 ohms, etc, but what about the secondary? Two 5000 ohm windings in series would give an unrealistic 10K to the final PA, while two in parallel would give a somewhat more acceptable 2500 ohms.
  I'm looking at this primarily as an exciter-class rig, but it's tempting to ponder how it would scale up. A pair of 1642SE's wouldn't be cheap, but good "real" mod-iron isn't dropping out of trees these days.

Hi Bill,

It would be best to experiment as I have done to find the best taps on the 1642SE xfmr.  But what you described at 2500 ohms would probably work well. There is a lot of versatility with all those primary and secondary combinations.   When I was testing mine, it was clearly obvious by the audio tone peaks tests which set of taps were correct.

T

I think that when all is said and done, the ratios count more than the actual values, anyway. The proof would be in the tone testing, like you say.

I think the rule is to try to use as much of ALL the windings as possible.  IE, the best power transfer for the given iron should be all of the primary and ALL of the secondary working at one time to be ideal.

In my case, I found that all of the secondary  (5K)  and the (8 ohms) of the primary was best. The 4 ohms popped the breaker on the  SS amplifier in the bridge mode.    16 ohms worked but with lower peaks and earlier peak saturation.

T

[/quote]


slang for electronics gear: to use no fuses

I'm taking my time with the Commander.   A little each day. No rush.  Actually the hardest part for me is cutting out and filing out all the big panel holes.    Please don't judge this project for looks until I clean all the parts, shine the chassis and sand and paint the front panel. The various panel cuts need a little more straightening with the file too.

The CNC plasma guy couldn't convince me that the holes would be nice and straight (or round) so I suffered and did them all myself using a drill, saber saw and file for the rectangles or hole saw for the meters. What a job. They will clean up FB.

But we're off to a good start. Can't turn back now.  The side gussets (side rails) are fitted and the panel is stiff. Shown is the Hammond 1428SEA audio transformer and the 85 watt module.  I have a big hank of Teflon wire for the wiring job. No plasdick wire in my shack! Notice the big tank coil. It figures out to be perfect to cover 160M with a Q of about 12.  Lots of room in there! I am expecting big things from the efficiency of the 6146B final, maybe 85%, so the tank is big and the bias will be very deep into class C.

Most of the missing parts have arrived so I have no excuse to linger on.

Next I'll lay out the parts and see where everything will be mounted....

More later...

T

The Commander awaiting birth like a Frankenstein movie:

I love the idea of a tube window, but it's way more fun with tubes whose plates glow.

I don't think I can recall when a 6146 has been quite 'show-cased' like that! 8)

For "showcasing" tubes that don't glow, I prefer the 807, 'cause it's got that classic shape and buzzardly look, like my wife (the classic shape part, not the buzzardly part).

Ya know, I was thinking the same thing.  It's still not too late to switch to another tube type. No chassis drilling yet.  The 6146B will certainly do the job, but it is so common and plain. No color in the plate unless something is wrong.

In contrast, I once built a single 3C24 /  24G  old buzzard triode rig from the 1930's.  It's a tiny tube only 4" tall and 1 3/8" diameter. About the size of a 2E26.  My rig was one modulated by a pair. It had 2KV on it or 4KV+ peak under modulation. Insane, but within extreme pissbeat ratings. The tube plate is rated at only 25W of dissipation but a single one will put out 47W, 60W or 90W depending on the voltage.

I could easily do 1250V with the supply I have.  It would need to be neutralized, of course.

The best part is how that plate glows under normal power... just like a 304TL.  The plate is so tiny but the efficiency is marvelous.

I could even put in a pair -  for a conservative 60 watts out, easily exceeding a single 6146B but more fun to watch.  Plus a better match for the 5K Hammond xfmr.   I'm not kidding... the filaments  light up like a floodlight.  The modulators and final plates of those 3C24s / 24Gs glowed like a hot electric stove burner, and that was normal color. It was doing near 88% final plate efficiency.

They are very fragile tubes and out of 15 I have, 10 have bad gas (purple glow) and 5 are still FB as far as I know from using them 10 years ago.

It IS a possibility and gets me more excited than a 6146B..  [yawn]



Check out these specs and typical operating parameters:  (check out page 3, plate modulated service)

https://frank.pocnet.net/sheets/201/3/3C28.pdf


(The pics below are taken off the web -  not my rig)

Now that is a nice glow. Looks like a couple of Coleman mantles in there.

4-65?

After doing the calculations on the 3C24/24G,  6LF6 sweep tube and  4-65...   other than the lack of plate glow, I'm disappointed to find the 6146B is still the best fit for the job.  I won't get into details, but that old 6146B is hard to beat.   

Also, the 6LF6 sweep tube (I have four) has huge emission and peak current capability for big audio peaks.

Still thinking about it.

T

I have a couple of RS 1003. Specs for class c telephony are as follows ( as well as I can interpret German)

800 volts at 130 mA plate
300 volts at 25 mA screen
Grid bias  at -90 volts 14 mA
Plate dissipation 34 Watts

power out at 100 % modulation 70 watts

Tube max anode dissipation 60 Watts.

Is the 1628SEA up to modulating this tube or should I use the larger 1642SEA. I have a 1642 and a 1628 coming.


Pat
N4LTA

a small disgression on your sigline:

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Every time I see this I want to dive into LTspice..

Hi Pat,

I would say the 1628SEA should have no problem fully modulating the RS 1003, being a 30 W dissipation tube.  Your simple RF final plate impedance is about 6K. That is pretty close to the xfmr 5K.

From my own experience with Summer Breeze, the 4D32 rig; the 1642SE modulated a 150W carrier quite nicely. So interpolating down to the smaller xfmr with your smaller tube makes sense.


I've pretty well decided on sticking with my original plans of using a 6146B with a variable supply that will go up to 850V easily.  Frank / GFZ runs his four 6146Bs at 800VDC under load and is getting out 250W carrier. This is about 62W each. He has big positive peaks to boot.  He uses four sweep tubes in P-P parallel as modulators, so has plenty of mod power.

T

Patrick,

I solved the audio filter problem by picking up a DSP  DBX  DriveRack unit.  It does brick wall audio filtering in any bandwidth desired as well as EQ, limiting, etc.  About $100 used. Such a sweet box and what a deal.

T

This is the fun part - visualizing where everything goes and what parts are missing.

The big beer can caps will be mounted underneath, as well as the input L/C  matching network lying in front of the front panel.  The switches, fuses and lights will be on the front lower panel also mounted underneath.  Except for the meter wires, there will be very little wiring on top, except for the RF tank.  Notice the RF tank is extreme overkill. Who knows... someday I may replace the 6146B with a pair of 4D32s using two Hammond 1648SE transformers.  

I like the 6146B choice more and more. It will be like a Ranger 3 but built to last and a little on steroids. I heard someone say an old, unrestored Ranger went for $500 on ebay today, so I feel better about the whole thing... :-)

The 85W SS audio module will be mounted underneath with a heatsink on top with some air from the muffin fan. I'm hoping to get a slight breeze on the 6146B somehow. The muffin fan will use one of the small Variacs, so will be inaudible. The bigger Variac will control the HV supply, up to at least 850VDC under a heavy load. The three beer can caps will work out to be 833 uF at 1050 volts in series.  The HV transformer will be in full-wave CT config and is rated at 300mA. The 6146B will be pulling about 140 mA, so all is conservatively rated.

Notice the transformers in a line. There is a LOT of weight there. I will add a reinforcing angle rail underneath to firm up the chassis under the transformers. The transformers from right to left: (back view)  Hammond 1628SEA, 6.3VAC 6146B filament, SS Audio module transformer and the +- 800V CT HV xfmr for the 6146B.  There is also a fixed bias transformer to be mounted underneath.

T

DDS rf source, and 6146 grid…. What is the plan for in between?  The grid, per specs, needs a half watt, at a decent voltage. Of course the DDS is well below on both counts, so how will you get the rf power and drive voltage?

Ed

Good question  Ed....  shows you understand the lashup.

Background:  I have a DDS chain that presently drives the 4X1 plate modulated rig and the 813s X 813s.  The DDS drives a simple 11N90 MOSFET using an input 50 ohm unun toroid and an output toroid at about 24V VCC. (I had the 11N90s lying around)  That puts out a few watts, maybe 5W, IIRC.  I then go into  a quad MRF-150 kit amplifier that gives me an easy few hundred watts drive available for any big rig in the shack.

So for this 6146B project, I will simply duplicate the 11N90 circuit (and DDS) and include a drive level pot to drive the 6146B gird thru its 50 ohm L/C  input circuit.  I will also have a switch to select and drive the 6146B with any external driver in the shack directly into its L/C network without the DDS or IPA involved.  (to use the FT-1000D driver, SDR driver, old DDS, etc.)

T

Tom

I think I will build a rig with the RS 1003 similar to this one. I spent a little time gutting the old amp from the fire and ended up with some good salvage. Will build a separate
power supply with the salvaged parts. I have a Hammond 800V center tapped power transformer - 278CX and a 300 mA 10 H choke for the plate supply, a 15H 125 mA choke for the regulated screen circuit and I'll use a small 125 volt transformer for the bias supply. I'll regulate the screen with a couple of VR150/0D3s. I salvaged three 270uF 600 volt snap in caps that I'll use as filters for the plate supply(2) and the other for the screen supply. Also salvaged a plate choke that should work. I'll use an "economy" power circuit for the 800 volt plate supply and 400 volts for the supply to the VR tubes for the screen.

Hope to get started on the power supply tomorrow.

I have attached a few photos of the old amp chassis and the salvaged parts


Pat


.

Pat,  

Cool on your plans to build one of these rigs.  Keep the updates coming!

**  I was talking with Rick / W8KHK.  He always has good advice regarding my rigs. He suggested that I use TWO 6146Bs to have more emission and lower impedance to get better linearity... and not have to beat on one tube.

In addition I could make a rectangular hole for the tubes and install an aluminum plate that could hold the sockets  -  and the plate could be switched out for some other tubes in the future, if desired.

I then thought, hey, I could always unplug one tube and run a single 6146B as planned if I wanted to experiment and try this.  So here we are. I have TWO 6146Bs standing side by side. All of the metering and supplies will be basically the same and the rig can now do from 20W (lightly loaded one tube) to as much as 100W carrier out with two. Or 40 watts (two tubes)  just loafing as a clean driver for the 4X1 linear amp. I like it! This is a good compromise.

T

I still need to clean up the rough edges with a file and get a window bezel mounted:

 

BTW, when she is done,  I want the new Commander (Ranger 3)  to basically look like this.  I still plan on a two-tone panel paint job and lettering similar to a Ranger. (shown is 813s series modulated rig)

T

This idea is my plan for the next RF deck I build. I had been amassing parts, but put it aside when I happened on the broadcast rig I’ve been working on.  The one additional thing I planned is to make the socket mounting plate mount from underneath, so it can be below the chassis on spacers. Some tube types need to poke thru a hole for shielding, and with a fan mounted underneath, I could also ensure tube base cooling. To control airflow and improve shielding a second plate with holes fitting the tube envelopes could be top mounted to close off most of the big rectangular hole.

Ed

Hi Ed,

Yep, the tube mounting plate and adding another one on top to control the air flow and RF shielding is interesting. I will give that some thought.

This may be common knowledge to some, but Rick brought up another interesting point that I've never thought of -  the height of the chassis (in my case 6" high) has a big effect on how strong the chassis is to flexing/ collapse.  I was concerned that the four heavy transformers in the rear needed rail reinforcement underneath.  But as it turns out, a 6" high chassis is VERY strong!  Compare this with a chassis that is only 1/4" high. It is almost a sheet of flat metal and would bend easily. But a chassis that is 10" high would be like a rock and could handle big weight due to more metal cross-sectional area.  Like bending a 4" pipe compared to a 1/8" tube.  The chassis is still strongest at the ends and weakest in the middle of the span, of course, so put the biggest transformers closest to the ends and outer edges.

I noticed this effect by picking up my 6146B rig with the transformers sitting on the back. With the  existing gusset side rails, it could be supported by the front panel if needed, though most of my rigs sit on a table or rack supporting plate.  This is something I never thought about before -  regarding overall strength based on chassis height.


BTW, my choosing this 6" chassis was not intentional. It was just the best deal I could find on eBay for a Bud chassis. I was looking for a 3-4" but lucked out here.

T


Clear example:  Imagine pushing down on the rear of this 6" high  chassis trying to collapse it.  (even with these shorty side gussets)   I tried standing on the chassis and it was solid.

Tom

Are you planning to use a similar input circuit as you used in the 4D32.  I am looking to do something similar with the R 1003.

Anything that I should keep in mind?

Also - is that 41H of  total reactance in the screen lead?

My screen impedance is about 12K ohms  (300/.025).  Hammond makes a
157G rated 30 H at 40 mA. and I have a 9H 50mA rated choke. In series I should get 39 H which will take me down to 50 Hz. This should be enough, I think?

Thanks

Pat
N4LTA

Hi Pat,

I have been using that circuit, an L - C input to the high impedance grid for class C AM rigs.  For linear amps, I use a C-L-C reverse pi-network to go into the 50 ohm cathode of the usual GG amp.

The L-C is very simple and works well once the input coil is optimized for each band via a small rotary switch.

BTW, I did up a new schematic for the (2) 6146Bs that includes the entire rig and supplies, etc.  I want to check it a few more times and then post it so I can get some more critical eyes on it before I build.  You can use it as a guide for your  R 1003 rig.  

I have about 50% of the metal work finished. It's all coming together FB and will now be in the class of a DX-100 or Apache with two tubes.  I still plan to run it "Ranger 3" class -  around 50-60 watts for cleanliness.  

I'd like to make a viewing window brass bezel, similar to what Chuck did on his 4-400A rig.  That is a cool looking add-on.

Oh, I see you added another question....  good you asked.  I do NOT plan to use a screen choke on this build. A screen dropping resistor from modulated B+ to screen is more linear and not prone to audio series resonances like a iron core choke.  On BIG rigs the power loss is too much, but even an 813 rig can do well with a screen dropping resistor.   You will eliminate the screen supply and choke.  You can also use some fixed grid bias to limit final current when drive is gone. Plus the screen will never be in danger because it will be drawing current only when the plate voltage is on. The resistor will be flat from 1 Hz to  15 KHz, no problem.

I have gotten MANY rigs to perform flawlessly using a screen supply and choke, but recently a friend pointed out that the dropping resistor may be a better way to go with these smaller rigs.

T

Tom,


I look forward to building this rig along with your build.  Getting rid of the screen supply and the regulator tubes will save a lot of space. I am trying to put all of this in a 12 x 12 chassis.

I have a power supply PC board that I have used in the past. It uses snap in caps as will save quite a bit of room. I plan to use two 270 uF 600 volt caps and a 10H choke to keep the voltage under control. I'll use a 800 Volt  transformer (Hammond 278X) for the plate supply.

Microchip makes a simple low cost high voltage regulator in a T092 package. With a high voltage transistor, it make a nice variable regulator for up to 400 volts or so. I have some PC boards for it that I used for the  power supplies for my transmitter in the AWA Bruce Kelly. Should work fine for a grid bias supply. I may try it. The tube data shows -90 volts for AM operation of the R1003  with 800 volts on the anode and 300 volts on the screen.

Looking forward to see your preliminary schematic.


Pat

Hi Pat -

Yep, if you build the rig it should optimize out well. Frank/GFZ has been sharing his parameters. He is getting 85% plate efficiency and 250 watts out with 2KV peak voltage on the plates with four tubes.  He is using a very large relative value for the screen dropping resistor, which I want to experiment with.  Very FB running rig.

12" X 12" for your rig may be a little small, depending on if that includes the power supplies and all iron.  

Maybe I'm reading your plans wrong, but you should not need any regulated supplies... there is no fixed screen voltage, the plate is stiff and if you bleed the fixed grid bias heavily with a bleeder resistor, it will not move around on you.     The only possible problem is the grid fixed supply and I wud tackle it only if it becomes unstable by charging up from RF rectification or whatever.

You may not need the 10H choke either, considering that's a good amount of capacitance you have in the HV supply.

I'll get that schematic posted probably tmw once I  stop making changes... :-)


** I ordered some brass bar stock and brass thumb-wheel screws from McMaster-Carr today. Gonna make up some brass bezel viewing windows  ala K1KW's 813 rig.  That should spruce up the new girl... :-)

T

Chuck's 813 /572B rig showing brass bezel:

Tom

Thanks for the info. I am slimming things down.

I was thinking originally using two VR150s to regulate the screen. I can take that out now.

I was thinking about and using a regulated grid bias supply. It is probably not worth the problems.

The choke was to keep the plate supply from rising at light load. I do not plan to make it variable with a variac. I had a bunch of small ones good for 3  amps or so, but lost them in the fire.

It is overkill, but I like to make the plate supply stiff using a choke and loading it to the critical value. I can drop the choke I think.

With the choke, I'll need to keep 80 mA load with a bleeder resistor to keep the voltage at 800 volts DC with a 10H choke. That is a lot of wasted power and space. Probably not a good idea to dump 64 watts of heat into the chassis.

I have two 600 volt filter  caps in series for the plate filter, so I have some room to find a suitable bleeder with a capacitor input filter that keeps the maximum voltage below 1200 volts

With no bleeder, the 800 volt AC will charge the cap to about 1131 volts which is safely below the rating of the two filter caps.  The voltage will drop to near 800 volts when keyed, When in idle or unkeyed, the higher voltage should not be a problem I think?

So it looks like I can reduce two VR tubes, a large heavy 10H choke, the screen power supply filters and the screen power supply choke also the screen audio choke.. I may get it in a 12 x 12 x 4 chassis yet?

Pat

OK Pat -

Normally when using plate modulation and a screen supply with a choke, the DC screen current during transmit is constant, so no need for a regulator. But since you will be using a dropping resistor from the modulated B+, the regulator is still not needed.   Screen regulation is mostly used in linear operation because the screen current varies widely and may cause distortion if not regulated.

A solution for your HV soaring when the rig is unkeyed can be fixed with a step start in the 120VAC line. Refer to the schematic when I post it. Use a ~20 ohm resistor in the 120VAC line that gets shorted out after a fraction of a second. Also, a Variac for the HV is very important for testing and later having control of the overall rig power, etc..  I would highly recommend a 3A or larger Variac.  You can always turn it all the way up to "bypass" its slight loss later on.

A 1200V capacitor voltage rating is also too narrow when soaring to  1131 V DC in the HV supply.  The step start and Variac both can help here.  850V max is OK if the caps are healthy with equalizing resistors that balance the load well.

Maybe you can stuff a lot underneath the 4" X 12" X 12" chassis,  but it will be a battle servicing and living with later.  Don't forget the LV transformer for the 85 watt audio module, filament xfmr, heatsink, etc.  More room is always appreciated.  As is, my own chassis of 17" X 15" X 6" is starting to fill up.  But we always end up getting it all in no matter what the size, huh?   :-)

Keep the HV bleeder current as low as possible. Wasted heat. Used mainly for safety. The AM carrier helps a lot to pull steady current anyway. Acts like a big bleeder when in transmit. But the step start is the ultimate in safety, power economy and preventing the HV from soaring.  IE, the 20 ohm step start resistor limits the secondary voltage long enuff until the carrier draw takes over.  

These pre-build discussions are very valuable to make less mistakes later on. I've already corrected a few things in my schematic that I spaced out...

T

regarding that little audio amp to drive
the 8ohm winding of the mid xfmr:

is it supposed to totally float ? is there a
chassis ground point anywhere on the
board ?

Peter

Did some power supply board surgery and converted my "economy" supply board to work for this application. Now has three 270uF 600 volt filter caps. 90uF should be fine for the plate supply filter. The board uses a 1600 volt bridge rectifier and has 100K 3 watt equalizing resistors. Should loaf along at 800 volts. I will use a variac  on the plate transformer.

I also ordered a larger chassis. I bought a 15 x17 x 5 inch one, should be here next week. I have started AutoCad drawings of the front and rear panels and have plans to order laser cut panels from Cut - Send - Cut as soon as I get all of the details of the panel components.

I have used a toroid and ceramic switch for the grid input circuit tuning in the past. I'll put one together this weekend. The grid bias in the RS 1003 specss for plate and screen modulation is -90 volt at 14 mA. How much negative voltage is usually generated with grid leak and how much with a negative bias power supply? Is there  rule of thumb?

Here is the completed power supply board.

Thanks

Pat
N4LTA

Excellent Pat!!

You made some good progress and everything you mentioned should work FB.  Cool on the auto-cad.  OK on the 15 X 17 X 5" chassis. You will always be thankful of the room to work in there in the future.

The fixed grid to grid leak voltage ratio:  Some run ALL GL, while others run JUST enuff fixed to let the tube idle in class B (near cutoff).   In my case, because I have a 30V transformer available, it will be -40V fixed and about -80V GL.  (-120V total bias)   A 50-50% mix is also done.     I would shoot to just cut the tube off with fixed and then get the rest by playing with the GL resistor.  Expect to play around with the clip leads later during testing to get the screen dropping resistor and GL dialed in for optimization.  Frank spent a lot of time on his rig, as well as I have too to get them right -  doing big peaks and good efficiency.  Or, I may use a screen clamp tube and eliminate the fixed bias supply.

Oh, one thing.. the toroidal input: An unun has no real selectivity. So if you are planning to run a cheap Chinese DDS with spurs down only -40dB, then it would be better to use a tuned circuit. My DDS worked FB after going thru the 4X1's tuned grid and plate tank circuit.  I realize that the Q of these circuits is not that high, but it made all the difference in my case pushing the DDS spurs down to -80dB or so. I am using on-air reports and my SDR spec analyzer as references.


Peter: To answer your question about grounding the audio module to station ground... yes, ground it. So one side of the 8 ohm xfmr will be at ground.  The 5K winding will float in series with the B+ as usual.

I see on the board that there is a ground that runs all around the board to the input and output common, the filter caps and the CT to the xfmr. It is marked "gnd".  

Response from a friend: "You may have to play with the location of the ground to prevent ground loops depending upon your layout.  I grounded mine at the CT of the power transformer.  No issues."

 
T

Tom

I plan to use a tuned circuit . The toroid will be powered iron type 2 high Q inductor just like your circuit. I'll tune it with a 360pF variable.

Same circuit as yours, just the inductor is a tapped toroid.

Thanks for the info on the grid bias. I may go for 50/50 as I have some small 25.2 and 24 volt transformers  Any beed for the supply to variable?

Pat

FB, Pat -

Yes, the toroid as an inductor in an LC input circuit will work fine.

The fixed grid supply, because it is not critical in voltage level, does not need a Variac.  But I've seen several rigs where the input RF drive charged up the fixed filter capacitor of this bias supply. Then it needs attention because the grid bias will increase without limit and need more and more RF drive to maintain the grid current.  A heavy bleeder on the fixed supply usually works. Make the bleeder pull at least twice the current as the tube grid current normally is. A simple zener regulator also does the job.  Just another thing to check when you get the rig up and running.  

50-50 bias is OK, though I like to get as much GL as possible and still protect the tube in case of RF drive loss. Look up what the class AB1 grid bias idle voltage spec is and try to get the fixed bias near that voltage.

I should have the hand drawn rig schematic posted tomorrow.  Hopefully we can find some mistakes and refine it some more.

After you get the schematic, work on your grid, screen and plate calculations to find ballpark resistor values as well as tank Q for proper tuning cap values for the various bands, fuses, etc. Write down and know what all voltages and currents should be for your rig.

T

thanks for the grounding info JJ.

to feed the module audio from XLR output
would you just short pin 1 to 3 at the input
plug if the module ? thinking about ground
loops.

Avoiding ground loops and shorting pin 3 to ground on an xlr are different things.  

If the preceding device (preamp, eq, processor, etc.) has an active balanced output (most do), shorting one leg to ground is not the best choice.  Balanced lines work best when balanced...not with one leg shorted. The currents in the legs will be unbalanced, because the loads (amp input on pin 2, short on pin 3) are different.  To both eliminate ground loops and have the best possible balance, a transformer is the best solution.  Barring that, the best choice would be to connect pin 3 to audio ground through a resistance equal to the amp input's input impedance at mid-band frequencies, and connect pin 1 to the audio ground.

Since this entire modulator circuit has no actual need to tie into any of the RF or control grounds, having a separate audio ground makes sense. Tie the xlr connector shell to chassis (either because the jack is metal and it just happens, or if plastic, there's a fourth terminal). Tie the audio ground to chassis through a 10 ohm resistor paralleled with a .01 or .1 uF capacitor. The point of the resistor is to allow for them to be at about  the same AC voltage, but not pass any real current.  

Ground loop currents are large.  As an extreme example, consider two "grounds" that are pretty close, say 0.01v different. Connect them with 2" strap. How much current will flow with 0.01v across a foot of 2" strap? The resistance of 2" x 0.05" strap (DC/low frequency, at room temperature) is about 0.00007753 ohms.  0.01v/0.00007753 ohm = 129 amps!!!  This high current creates a strong magnetic field, which is what then induces the ground noise into the audio circuits.  Thus, 10 ohms is large to a loop current, but insignificant for shielding.  This will keep the loop-induced noise and buzz to a minimum.

Ed

OK, here we go.  I welcome all comments concerning the schematic of this pair of 6146Bs plate modulated by an 85W audio module and Hammond 1628SE transformer.

This does not show the DDS but pretty much everything else.  The part values and current/voltages are roughed in for now.  Hope the schematic is clear enuff for now...

Also, check out the new brass bezel window. I need to clean it up and fit it perfectly, but what a nice addition.  [Chuck's idea]

I am also looking for ideas to paint the front panel.  My last metalwork project is to populate the chassis with terminal soldering strips in key areas. That's about it. Then it's time to sand it all down and paint the front panel.

I was thinking of painting it all my classic blue color but still thinking of a two-tone panel.  That is difficult cuz it's so easy to screw it up or decide later the design looks like crap.  I imagine a loop of color that starts at the bottom and curves up around the 6146B window.  A gray bottom and a blue top loop might look FB.  But I need to decide cuz it's almost the next step.

*Someday I'll break down and use software for my schematics.

T


Corrected schematic changes:

1)  Thanks to Rick, W8KHK who sent me an email with some suggestions. I was able to eliminate the 50K cathode keying resistor and relay. The cathode goes directly to ground for best stability. The screen bypass cap directly to cathode.  The HV step start, fixed bias  and drive/ant relay system will key adequately and keep a more constant load on the HV supply when unkeying.  The updated schematic is posted.  Any more?

2) Thanks to Frank and Rick -  removed two relays for audio module and transformer protection - not needed.  Low level audio input should be switched off for receive instead. The only relays left are the antenna relay and the HV step start.  Simpler design.  Remember to float the screen current meter with a Plexiglas mount.

3) Rick suggested some more  ideas related to the HV glitch resistor, current limiting on step start up and wire fuse -  also other assorted ideas for improvements like back to back diodes on all current meters.  Much appreciated!
4)?

pic1 and pic 2 -  How about these two colors, white and blue?  White at the bottom and  blue looping  over up and over the top of the tube window?

Or would a medium gray at the bottom and the blue loop swings up around the window?

Or three tone with the meters at the top a white?

This is like picking a car at the dealership.

T


No that's not MY car... just a photoshop.  But interesting paintjob.

Ed thanks for your input. I’m an audio ignoramus and a little more schooling me may be in order.

I attach a pic of the modulator audio input female xlr plug. The white wire is on 1 and the braid on 3, this is a mistake, but now have 1+3 shorted at the amp’s audio input.

Are you suggesting the following:

1) connect the braid (now mistakenly at pin 3) at the plug to the white on
pin 1 and ground pin 1 to chassis at the female’s mounting screws (effectively
turning the xlr cable piece going to the amp’s input into a simple shielded
cable.

2) ground pin 3 of the female to the chassis via the parallel RC values you
suggested.

Did I get that right ? The driving device is a cheap Behringer Eurocom spl3220
processor.

Sorry for the dumb questions but this audio stuff and xlr whatzits are very new to me.

Peter

Who doesn't like a Nash Metro?? ;D

What are you building with the audio module, Ed?  Please post some pics and description.  You may be the first one to test the viability of this module and Hammond lashup.

I'd be especially interested in what you plan [if anything] to protect the module and Hammond xfmr.  I have nothing in the schematic now except to kill the line level audio during receive.


Otherwise I am almost done with metal work.  I'm kinda stuck at the panel paint design point, so behind you somewhat.

I ordered some terminal strips and they broke apart with little effort. Junk.  Had to reorder some strapping ones from the 1940's era. So I am awaiting parts too. Still lots to do.  I have a friend who may be able to do a panel paint photoshop idea for me. Something cool, but not too outrageous; so I don't come into the shack each day with regret... :-)

T

What the heck...might as well jump in here.

JJ - Black wrinkle all the way!!!  Now that you are using that nice brass surround, the only thing that goes with it is BLACK WRINKLE!  Go to your local Harley dealer and get a can of it.

According to the TDA7294 datasheet, it appears to include functions to either mute or put the device in standby mode:
https://www.st.com/resource/en/datasheet/tda7294.pdf

Pin 9 for standby, pin 10 for muting

To enter mute or standby, apply a voltage less than 1.5 volts to the respective pin.  To enable or unmute, apply a voltage greater than 3.5 volts.
 
Can you inspect the module to see if you can access either of these pins?  If they have a pull-up resistor on either of these pins, and no other connection, you should be able to override the pull-up voltage with a resistor connected to a relay, or a bipolar transistor to shunt the pull-up voltage.   Small pins, likely rather delicate surgery is required.

Chuck,

Yep, brass and black wrinkle certainly look great together.  A friend is working on an artwork design of the front panel in Photoshop for me.  We'll see what he comes up with before deciding.  I'll post it here.

Rick, I spent some time on the pin 10 disable feature as you described. As far as I can tell, the board I have does not have the resistors or jumper J1 interface to pin 10 as described. There seems to be different boards out there. I was also advised that these disable features can be noisy and produce transients, so I decided not to use it. Instead, just kill the audio line input during receive.

However, no matter what method is used to disable the audio or operation, if that chip smokes for whatever reason, it could generate a transient and arc the xfmr over anyway. Is a fat electrolytic capacitor in the 8 ohm winding a good idea for better isolation?


T


The latest "improved" schematic:

Frank suggested to add a 2.5A fast-blow fuse to the 8 ohm Hammond winding. That will hopefully save the xfmr in case of chip failure or overdriven audio. Great idea, tnx!

Tom, I believe a fat electrolytic would simply pass the transient on to the transformer, and a very large electrolytic would be needed to pass the low audio frequencies.  The only reason to use an electrolytic would be the case where there is a DC offset at the output of the amplifier driving the transformer.

I would suggest a snubber, a resistor in series with a capacitor, in parallel with either the output of the audio amplifier, or at the secondary of the transformer.  The spec sheet for the audio amplifier device suggests 2.7 ohms in series with .1 uF.  You could use a larger resistor, and a somewhat smaller cap to absorb high-frequency transient energy in the resistor, while passing lower frequencies without attenuation.  If on the secondary, much smaller capacitor and larger resistor would be required.  Spark gap is another option on the secondary, keeping the induced high voltage impulses at bay. 

Another option is simply adding additional resistive load to either the primary or the secondary of the transformer.  With all the power to spare, adding a resistor to keep impulses low enough to protect the transformer should not detract from the modulator performance.

 put a DC sensing circuit and relay on the output of the TDA7294.  when the TDA7294 settles to zero V, the relay closes and connects to the 1628SEA. if the TDA7294 decides to go TU the relay will open to prevent DC from getting to the 1628SEA. 

I'm not exactly sure from your picture which wire is on which pin.  Here are a couple of XLR pictures.  Note that all the connectors are oriented the same way in the group shot.

A little about the XLR or "QG" for quick ground connector.  There are three pins and an overall metallic/shielding connector body. It is considered bad practice today to connect the connector body to any pin, except at signal generating devices (microphones). The connector body is an extension of the chassis covering the connection, essentially moving the wiring inside the chassis for overall shielding.  If you look closely at the single picture, you will see one of the three pins is shorter than the rest. It is the same length, actually, but sunk in deeper...and thus out towards the face further.  This is the quick-ground. This pin always is connected first when you plug in a cable...the shield first, so you don't hang a hundred feet of hum-pickup into an amplifier input until it is shielded and not a hum-pickup any more. That pin is the shield, and the shield is ALWAYS pin 1, no matter how many pins on the connector (yes, xlr goes from 2-7 pins; only 3, 4, and 5 pin versions are common).  The hot pin on  a three-pin audio xlr, is ALWAYS pin 2, the hot, or in-phase, or in-polarity (people argue for days over which word to use) signal carrying pin. The last pin, pin 3, is ALWAYS the cold, out of phase, reversed polarity signal carrying pin.  Due to the layout, pin 1 to pin 2 miswiring is the most common assembly error -- pin 2 is across from pin 1, pin 3 is the middle one. When looking at the back of the connector (the solder side), with the center pin away from you, pin one is to the right on female connectors, and on males, the left.  There is always a fourth connection point, which usually looks different. this fourth point is the connector shell. It is never connected to pin 1 internally in the jack itself. 

The connector body/shell terminal is the chassis extension. If the connector is metal, like most older connectors, the proper mounting of the connector takes care of this connection (lockwasher/scraped off paint/etc.) If it is plastic or you don't want to chance it, connect this fourth terminal to chassis.  I am specifically saying chassis, not ground, because they do not have to be the same thing.  We ALWAYS bond chassis to the electrical safety wire, the green "ground" of the power system. We do not always bond the circuit common to the electrical safety ground. An example..consider your alarm clock radio. It certainly has a circuit common, or "ground" in the radio and clock. It almost certainly has a two wire power cord. This circuit common is not bonded to the electrical safety ground. So, to reiterate, the connector body/shell, the extra terminal is connected to the chassis.

In a well-balanced circuit interconnect. the shield does very little.  The induced hum pickup is rejected by a combination of the differential input of the following stage rejecting common mode voltages, and that the pickup is reduced by the intrinsic geometry of a twisted pair of wires carrying the same signal in opposing polarities. To this end, to eliminate ground loops, we often do not actually connect pin 1, the shield, at both ends of an interconnect. The shield is essentially a "chassis extension".  As the chassis are already electrically bonded, this extension is complete without actually being connected at both ends.  Connecting it at both ends can create a ground loop, with the attendant buzz.  In permanently installed facilities (radio station studios, recording studios, etc.), after careful planning, usually a decision is made as to how the shields will and won't be connected. Often a "shield connected at source end only" approach is taken, sometimes everything is based on the shield originating from a patchbay, or the mixing console.

So far, I've mostly talked about the shield.  Now, about interconnecting balanced and unbalanced devices.  First, non-transformer, electronically balanced gear, which covers anything modern. Electronically balanced circuits, especially outputs, almost always are referenced to circuit common.  They are essentially two amplifiers driven with reversed polarity versions of the same signal, two outputs, equal and opposite.  The whole twisted pair/common mode rejection thing is based on them being the same. In addition, as we know, it is generally bad to short out an amplifier.  This is why you don't want to connect one of the pins 2 and 3 to ground when connecting to an unbalanced input. Although most gear is designed with the idea that this WILL happen and will therefore behave nicely, that is, not distort or break, it is still wrong. 

The basics:  if you are using unbalanced cable to an unbalanced input from a balanced source, connect the cable hot to pin two, the cable shield to pin 1, and leave pin 3 floating at the source. If you are using balanced cable from a balanced source to an unbalanced input that you aren't in the middle of building/have design control of (i.e. not the transmitters this thread is about), connect pin 1 to the cable shield, pin 2 to the cable hot, pin 3 to the cable cold (this is entirely arbitrary, the cable has two wires, they are the same, just be consistent), and then at the unbalanced input connection, leave the cable cold floating. Ideally, we want balanced currents in the two wires of the pair, but not knowing what is inside the box, any decision you make to load the unused pin 3 to balance the currents will be wrong, so it will not be any more balanced than leaving it floating.  On the other hand, in this thread, we are building the box, and we can know what the pin 2 load looks like. Thus we can load pin 3 to be similar.  That right there is what I was getting at when I said connect a resistor from pin 3 to circuit common equal to the pin 2/amp input midband impedance.  There is a second advantage to doing it this way, vs. floating pin 3:  In the event you connect some vintage piece with a transformer output to this transmitter, it will still work. Transformers generally float, with NO reference to circuit common.  With pin 3 floating there would be an open circuit, and no signal would flow. With pin 3 connected to common through a resistor, current would flow and the vintage piece could be used. Note that some vintage transformer inputs and/or outputs tie the center tap to ground (or allow that option) which would provide a reference, but many do not.

I am happy to go off on audio interconnect further, but I think this is enough for one post.  I will state, emphatically, that there are different schools of thought on how best to do grounding/shielding in large audio installations. You can find differing opinions easily. Mine are based on forty years of wiring up sound systems of all sizes, and with that, the training one sees along the way....

Ed

Ed, Good audio stuff on balanced feed, deserving a 3 page thread all by itself. Those modern guitar shop boxes with balanced in and out (and single ended), and a wall wort need special attention when there is an environment high in RF. Op-amp CMRR falls away at Ham Radio HF frequencies.


Tom, Following your thread with interest, as always. You do some pretty neat stuff.

Looking at the circuit, I wonder about the condition of keying up with little or NO RF excitation. The fixed grid leak bias may not be enough because the 6146 screen voltage will soar due to the lack of screen current. With a 6146, designers usually used a Clamp tube to lower the screen voltage to a safe value when this condition occurs. Traditional clamp tube circuits are troublesome, need extra circuitry, a pot, a negative bias supply, and other stuff to get them to function well.

The Gonset G76 (later versions) did a novel trick. I attach a blown up image of that area of the schematic here. There is only grid leak bias used, yet on CW, key up, the PA cathode current is zero. This works really well, and no need for a negative bias supply. The trick is to use a VR tube (OB2) in series with the screen dropping resistor (low side), and add a screen to ground dropping resistor. The Clamp tube (12AQ5) hooks up to the high side of the VR tube. With a lack of excitation, the
Clamp tube conducts heavily, and the VR tube goes OUT. The PA screen voltage goes to ZERO since there is a screen resistor to ground.

The circuit also has another advantage because many Plate Modulated beam power PA tubes excessively modulate the screen. Here we have a voltage divider action between the screen dropping resistor, and the screen to ground resistor. That ratio can be played with. With the G76 using a 6DQ5 PA tube, the screen is modulated  50% without factoring in the fixed 105v drop of the VR tube.

I have modified my G76 such that I can hit 150% audio peaks. The stock screen circuit as described is not an issue with pushing the modulation capability.

Jim
Wd5JKO

Rick:  Good suggestion about the "snubber."     I will probably try the snubber once the rig is working... maybe even a spark gap, as you suggested.

Bob: FB on the sensing relay for the module  8 ohm output. Another possibility.

Jim:  I've been studying your Gonset G76 clamper circuit.  I like the idea that it wud eliminate the fixed grid bias supply and is very simple.   I cud easily add something like that and solve the screen voltage soaring problem when keyed and the RF drive is gone. Still thinking about it.  Frank and his four tube Valiant [with clamper circuit] says it has saved his [6146] ass many times during the testing phase.

Ed: A very nice post about the  XLR stuff. Hope the moderators put that in the AMFone Handbook.

I've gotten a few questions as to why I want to use a step start. It's just an old rule I have in my shack -  I leave NO HV on anywhere unless it is keyed on with the PTT switch. Just a safety precaution to prevent a senior moment. In fact I have a big red bulb that goes on whenever a HV supply is keyed on.  I have learned to really like step-start HV activation. But the 3.5 KV/ 4.5 KV big supply with the contactors really makes a racket. I have to use a 500 mS audio delay when I key it up to mask the sound. This same delay masks all the other rigs' step starts when used.  Hardly any bleeder power waste too.  Anyone building this rig can easily eliminate the step start and just use a switch and fuse in its place.

I might have some crazy looking front panel "Commando"  photoshop concept artwork pictures soon.  From my "Secret Santa."  Stay tuned.

T

***  Here's what I decided to add to the schematic:

1) 2.5A fast- blow fuse in series with the 8 ohm winding
2) 2.7 ohm in series with 0.1 uF capacitor across the 8 ohm TDA7294 module output. (snubber)     *** UPDATE - Looks like it is already on the stock board - Rick
3) Set of brass spark gaps on the 5K winding of the Hammond 1628SEA
4) Screen clamp tube circuit (6AQ6, OB2) as per Jim's recommendation -   G-76 schematic
5) Eliminate fixed grid bias supply.

Miscellaneous needed:  2 Variacs, 5 Fuse holders, 4 toggle switches, 3 relays, 1 small rotary switch for RF input taps, 1 larger rotary switch for plate tank taps.

OB2, 6AQ5 Clamp Tube circuit now part of the rig, but cannot fit on schematic yet:

Updated Schematic:

Here's  a simple homemade spark gap for the 5K secondary of the Hammond 1628SEA audio transformer.   This spark gap, the TDA7294 module power supply fuse, the 2.5A fuse in the Hammond primary, the built in snubber and the audio mute during receive should keep the module and transformer safe.

I used the same brass thumb wheel screws as the window bezel. They have a nice, flat, polished surface to keep sparks even.  The wire lugs will go under each nut.

T

Below:  The first artwork for the Commando decal.  I like the look of the Johnson 500 front panel and may use it as a rough guide - maybe different colors.

Here's a first rough PhotoShop.  I will be using big 2" black knobs and the round black meters.

Should the light blue be a little darker? Should the dark blue curve down to the bottom edges like a Johnson 500?  Should the nameplate go under the window, centered?

Do you like it as is?

Wat ya think?

T

Color scheme and layout looks great as pictured. 

Perhaps you might add some "striking" lightning bolts to the left and right of the tube window?

I like the logo right where it is.  It should be done in milled/engraved brass, just like the tube escutcheon. No short-cuts now.  No turning back, OM!

Hopefully you will also have all meter and control legends silkscreened.  Or engraved, if you wish, but no Dymo or PTouch labels!

It looks FB, but the window clashes with the clock. If you can, maybe a cheep, round black plastik framed clock, echoing yer metering,  will greately increase feng shui.

klc

what about this..

the horizontal brass strips going all the way across the panel, instead of the lines you've got there now. They'd divide it off yet pull it together. The full width accent would make it look sleeker and smaller.

Ed

Thanks for the suggestions, guys!

Yes, I like the idea of continuing the brass to the sides.  The Collins 32V series does it too.  I am out of brass material, but there are excellent gold pin stripes available in the same 3/8" width I could use. I might also add a border around the whole panel using  1/4" width pin striping with curved corners....  All this can be done later after I paint the panel slate blue and the middle dark blue as shown.  

I am stuck about what to use for lettering. I don't want to go with engraving the panel or a silkscreen.  But I'm game to find a transparent stick-on lettering that comes in black or white to cover the two shades of color.

I saw some cool gold anodized aluminum knobs, but decided to go with some all black, 2" knobs.  The nameplate will probably be gold brass with the engraved "Commando" as shown.  So the main focus of the panel will be the middle window 6146B tubes with the gold bezel frame and thumb knobs. The panel will start to look busier once I start mounting the controls, big knobs, switches, meters, etc.

Any suggestions appreciated before I start the sanding and painting process. [You probably didn't know that I was once an auto body apprentice for a summer back in 1970...]   ;)

T

I use the P-Touch labeling system for my transmitter panels.  Clear with either white or black lettering, like you mentioned above.  Various font sized available, very easy to produce and apply to the panel, inexpensive media, and the P-Touch machine is around a Benjamin, give or take.

Hi Rick,

Good idea on the label machine. One of the local hams offered to print me out some black or white transparent labels.  That should work FB.

We're just about decided on the panel colors and style.  I found that different top and bottom colors make the rig look like a junk yard car with different colored doors.  I also found that curved lines and colors do not go well with square panels.  The Ranger has curved panel edges which makes its curved colors work.  This square panel needs squared color borders.

 I like the dark slate blue (or sky blue) along with the dark blue center and added gold horizontal 3/8" thick pinstripe following the brass to the panel edges.

I plan to add 3/8" gold pinstriping around the rectangular DDS window to kinda match the 6146B window..  That thin gold pinstripe will be the same thickness as the brass.

There will be a lot of black knobs and black meters added to better balance the overall bright paint.

Here's some samples. I am favoring the light blue top and bottom or maybe the dark slate blue.  (2nd picture)

They have etching primer for aluminum at NAPA stores.  Almost ready to sand and prime.

Any ideas?

T

All the color schemes look good to me, but of course it is a matter of personal preference. 

The black-lettered  P-Touch clear labels will view nicely on the lighter sections of the panel, while the white labels will contrast well on the darker areas.  In-between panel lightness or darkness may be problematic, with either the white or black lettering providing marginal contrast. 

With good lighting, either may work ok.  But with older eyes, sufficient contrast is your friend.

My experience is limited to black lettering on natural aluminum, and white lettering on black wrinkle panels.  Some trial and error with the labels may help to achieve the desired results.

Real men don't label!!!

Checkout the Four Horsemen!  ;D

FB!

It all depends on how many knobs, switches and meters you have -  and how good is thy memory...  ;D

T

Ya gots a point there Fabio! We ain't gettin' no younger. Memory is the first to go.....I think.

Yup!  That's why the RME-69 receivers never had any labels on the controls.  They felt the ham should just know how to operate the receiver.  But labels on transmitters are helpful when you have guest operators in the shack.    I am sure Tom will be hosting a few Pharties!  

And if you have labels, they should look good!  I never had the patience to do fancy paint jobs like Tom does.

Turning 75, I realize I really miss three things that I have lost.  First, my mind.  Second, my memory.  What was the third thing?  I just can't recall???

Still waiting for all the parts to arrive. Have received the SEA1628 transformer , the 85 watt amp, the band switch, sockets and variable caps. Waiting on the roller inductor, the main chassis and panel, the 85 watt power supply transformer and power supply parts. Hopefully, all will be here this week.

The front and rear panels are almost ready to send for laser cutting. That wont take long but it needs to be right.

Look forward to really getting started.

Pat
N4LTA

Hi Rick,

The painting ain't so bad, in fact kinda fun if you make it the first step before mounting parts. Once done, it's a joy to see it all come to life with parts mounted.  The lettering comes last, once everything is working.  Also, a painted panel with no obstructions looks cleaner since the panel screws are shiny and not painted.  I sanded, cleaned, sanded again and cleaned the panel well.  Prep is the key.  Keeping the dust out is also important.   I am going to use a satin finish for the slate and a gloss for the navy blue.

This panel will now get baked so that the etched primer can handle the taping without peeling -  required to paint the next two colors, the slate blue and the glossy navy blue bands. I figger the window area should grab all the attention with the gold and gloss.

BTW, that black monster on the wall is the acoustic enclosure for the big step start. The contactors inside are extremely loud on start up.

I'm having these obscene thoughts of adding a 3rd 6146B in case I decide to upgrade to a big Hammond 1642SE xmfr later on. I have the tube sitting there on the chassis just calling me to cut the hole. I could always just wire it for three and unplug one, leaving two with no bad effects if desired.  The window lets me view three tubes.  This would be a very compact Valiant class TX.  The big Hammond wud fit underneath that 6" high chassis.  Just thinking...

T

Primer Etched Aluminum and Drying:

Tom...
It's great fun vicariously observing the development and progress of your latest rig.  I was going to keep my trap shut, but when you mentioned the third 6146 it made me throw my vote in for it.  Early on in this series, when you had the mockup with a single 6146 in front of all that iron on the chassis, all I could think of was that fellow standing in front of the tank in Tiananmen Square.  Just one 6146 looks brave, but lonely.
73 de Norm W1ITT

Sounds like a well organized plan, Pat!  The fact you are ordering so many parts, new chassis, new panel, etc., tells me it will be a nice looking rig.  The laser cut panel both front and back oughta look FB.  Did you decide on colors and panel "decorations?"  I really like the gold brass. It's put the rig into a different class. The two-tone Earl Shieb paint job is new for me too.

Norm:   FB on the lonely 6146B... ;D    Two looks a lot better and three makes it a bigger rig. I was thinking of running the voltage down low, like 400VDC, low impedance and three tubes would have a lot of modulation headroom potential running at  80 watts or so.  I just don't know how much the Hammonds will take without saturating. Also, I want to keep the HV down for transformer longevity. The 1628SEA is hi-pot tested at 2KV and the 1642SE is hi-pot tested at 3.5 KV.    So there is some room to play.

But as I said, I can always just unplug a tube or two until it is a good lashup.  I'd give it a 50-50 chance of becoming three tubes at this point.  

T

Update:

The good news:  I added another 6146B for a total of three in a triangle.  I plan to boost the TDA 7294 audio module input voltage from +- 35V VAC up to +- 40VAC, data sheet maximum.    I plan to initially run the three 6146Bs  at a lower voltage (maybe 400-500V)  to have ample independent modulation voltage available from the module and transformer output.   I can always unplug tubes to experiment and see what the rig likes best.

I checked the volume in cubic inches of a Johnson Valiant - and "Commando"  is about the same displacement, but has more room to spare, especially underneath the big chassis..  The Valiant weighs 78 pounds and I'm sure Commando's fighting weight will come in much lighter.

Not-so-good news:   I painted the next panel coat. (slate blue)   I let the spraying get away from me and created a paint run. Too aggressive.  I should have used lighter coats.  Now I gots to sand it down and start over... [sigh]    It does look FB, despite the run, however. :(   It's best to take your time and use very light coats, especially if the humidity is high or the temp is not 72 degrees ideal.

T

** Second Update:  Panel sanded down and repainted. Whole panel now slate blue, satin finish. Looks beautius, no runs.  Next, let it bake on the coal stove for a night and then add the glossy Navy Blue center section tmw.



Pic Below: The Commando is cooking...    (Those heatsinks really work!)

Tom

Looks great. What is the size of the digital VFO cutout'? Is it a readily available digital VFO?

Thanks

Pat

Commando will float like a butterfly and sting like a bee.

Hi Pat,

My DDS is an older one that is not made anymore.  There are plenty of them on eBay that you should be able to find and cut a custom hole.  I may choose to use my main station DDS as a driver and fill the panel with a simple frequency counter that will sample the final output and display a bright set of blue digits to match the panel artwork.  

I decided to go with a "wildflower blue,"  my favorite color for panels here. I found the slate blue to be too bland. The new blue is striking in sunlight and I'm happy.  I'll be mounting parts as soon as this dries.

I ordered some 3/8" gold chrome pin striping. This will extend the brass window to the panel edges and also make a window frame for the DDS/counter.  Once finished, I'm hoping Commando to be the best looking rig in the shack, caw mawn.


Bob:  Yep, with three 6146Bs, Commando is well equipped. Maybe a decal of Schwarzenegger's macho face just above the nameplate is in order?

T


Commando panel baking on the coal stove:

Available Commando decals:

Can you jot down the amp circuit you use for the dds to drive the input LC circuit?

Hi John,

It's been over 10 years since I built it, so was foggy... I looked inside my RF DDS driver and realized that the DDS is actually driving a 200 mW to 5 watt RF amplifier costing only $20 made by a hobbyist QRP company. (16 dB gain)    I used a 11N90 after that, but it is not really suited for the job at that low level. For even more output, then there are bigger and better linear boards like this available.

5W amplifier:
http://shop.qrp-labs.com/pa


Ton of kits:
http://shop.qrp-labs.com/kits


This little SS linear amp should give you ample drive for a 6146 style rig.

They also carry a digital vfo/sig gen.  Something like this may be what you need.  

* Anyone who comes up with an "amplified power DDS" design based on current parts or boards, let us know!*


T

Now it's the fun part - just like assembling a Heathkit!  Most of the drilling and blasting is done.  

I ordered some big 2" rubber feet. Commando will be sitting pretty.     Most of the parts will be bolted on by tomorrow.  The wiring will be the next big push, followed by testing.

Do ya think the tank components are big enuff to handle three 6146Bs?   ;D

There are a few cosmetic errors, but I'll straighten everything out later on.

The dark blue section will be enclosed with chrome gold pin-striping to match the 3/8" brass. Same with the DDS/freq counter window.

The bandswitch will sit directly over the tank coil for short leads.   There will be long standoffs bolted to the panel to support it.

Lots of iron to load on the back chassis.  The TDA  audio module will have a heatsink on top with a small muffin fan.  The 6146Bs will get some of the air.

The ~1"-2"  holes in the back will hold the sockets for the relays - and the smaller 3/4" holes will hold the 6AQ5 and OB2 screen clamper tubes.

Little by little.

More later.

T

Here we go!  Gots most of the parts mounted. We can get a pretty good idea what this puppy is gonna look like.  I haven't weighed it yet, but it might be closer to a Valiant weight now, 78 pounds.  The transformers in the back actually made the chassis stronger due to reinforcement.  Got some nice spinner knobs. Still need to add the gold pin striping around the Navy blue.

I got rid of the big beer can filter caps and went with newer, more compact ones underneath. Notice there is ample space there if I decide to go with the biggest Hammond audio xfmr.  The heatsink is mounted on top as well as the TDA module mounted below. The muffin fan still needs to be added on rubber mounts.

Commando still needs a nameplate. It may be the coolest looking rig in the shack.  Really pleased so far.

T

Pic #3 - glorious!   Commando in the company Titans.  4X1 plate modulated rig on right. 4X1s linear on left.

Pics came out good today for some reason.

We talked about having enough room.  Kinda tight on top, but still some room.   Some room to spare underneath.   Panel space pretty filled up except for the center for a nameplate.  Looks like I forgot to mount the toggle on the left. That's a loading padder for 160M.

Transformers left to right:  850V CT HV xfmr,  TDA Audio module  xfmr +- 40V,  6.3V fil,  Hammond 1628SEA audio transformer.

The blue freq counter really look trippy when lit. Still room to add a DDS later on.


Have six fuses - well protected.

Still need to mount the spark gaps for the Hammond.

Pic 1:  Notice how close the bandswitch is mounted to the tank coil for short leads.

I use 1/4" panel shaft sleeves that keeps the tuning movements rock solid.


#2 pic shows a quite soulful three little 6146Bs backed up by all that heavy iron in the back...  ;D

That's a real silver plated coil. Strapping 5/16" edge wound.

Notice the G-76  (Jim / JKO) clamper tubes, OB5, 6AQ5.

**  3/7/2022 UPDATE **

About 95% of parts mounted.  I added the muffin fan on rubber mounts, vibration free, Variac controlled.  Added the panel gold pin striping.   Made a computer printed temporary nameplate. (Commando)  I want to get a brass one engraved later on.  I added HUGE rubber feet for better ventilation below. They're from an air compressor.  They are mounted on 4" plates which also reinforces the chassis.  

Frank/GFZ suggested I drop the big plate RF choke and move a smaller one to the plate side of the tank for less stray pickup.  RF connections will be very short now. Cleaned up the iron transformers with black paint to make them more presentable.  Fixed a few panel flaws.   Still need to design up panel lettering.

Awaiting some heavy duty terminal strips to start the wiring - new old stock from a 1940s command set.   All the drilled holes are now filled with parts.

I still want to mount the screen current meter on standoffs or Plexiglas in case there is a plate to screen dropping resistor crap out of any kind.

Anyone have more suggestions before I start the wiring?

Are you not entertained?    ;D

T

Pic 1: A proud Commando standing at attention:

Pic 2: I may add a small coil  between  the C1 tuning cap  and L1 tank coil to tap 10-20M.

More:

Notice the muffin fan in pic 3.  I tested it and found there is a good flow through the audio chip's heatsink as well as residual air finding its way past the 6146Bs.  I may keep the 6146B viewing window open for the best airflow. There will be a Plexiglas cabinet covering the sides and top, just like my other rigs.

It's difficult to get all of the gold pin striping to shine by the lighting.  In fact the camera is very brutal on every little imperfection. It's a great acid test to take a shot and then look for flaws. This rig looks so good in person until it gets on the silver screen.  I don't have the patience for detail any more than this.  [sigh]   Yeah, yeah, I know the meters don't match. I used to buy them at flea markets for $1 and loaded up. But I'm out now and they want $30+ on eBay...

Pic 1: That RF choke is very similar to a Valiant's choke.  I've had poor luck with them for QRO rigs, but it may work OK for 550VDC plus modulation. Any opinions?

Pic 1: Notice the "cool" heatsink. It uses heat sinking paste to sink to the chassis. The TDA audio 85 watt chip is mounted directly beneath it.  Muffin air flow through it ca be adjusted. It will be dissipating maybe 40-50 watts under operation according to the datasheet, so we should be FB.

Pic 2:  There is a nice space, 6"X 8"X 6" next to the blue filter caps. This would fit the big Hammond 1642SE transformer if the three 6146Bs stress the smaller 1628SEA.

Pic 2: Check out the rubber to the road feet!  The green caps at the top are part of the compact TDA chip module.  Heatsink on top.

More:

Pic 1: The freq counter/ DDS gold pin-striping is lit up.

Pic 2: Notice the heatsink fins point upwards for best heat flow and in direct line of the air flow. And the muffin fan splits its air between the fins and the 6146Bs towards the front. 6146s usually don't need air; and especially considering how easy they will be run in this rig.

Pic 3: Notice the bandswitch has been flipped 180 so that the connections will be be shorter and more direct.  Its panel shaft is missing the 1/4" sleeve.

Pic 3: Notice the wide berth given to the tank coil for best Q and least interactions.  The muffin fan is plastic near the tank coil.

Pic 3:  The switch under the meter is the 160M loading cap padder to add a 1000 pF fixed cap to the C2 loading variable cap.

3-9-2022, Wednesday update:

This is the fun part... assembling the plate tank circuit!  

The straps are soldered to the bandswitch, but not to the coil until I have it running and optimized. They are in approximate positions for now.  I plan to cold silver plate all the copper once it's  finalized.

This rig will be operated mainly on 160-40M. But I added a 10M tank coil based on the pi-network calculator. The bandswitch is configured for 160M to 10M. We will have to see if this rig will tune up on the higher bands based on minimum C1 and overall stray capacitance to ground.

The 160M C2  .001 uF padder cap is installed with an on-off switch.

I mounted the screen current meter on standoffs and placed a fake meter bezel on the panel.  When using a plate-to-screen dropping resistor, there are failure scenarios where there could be HV on the meter.  All about safety.

Added a safety RF choke from C2 loading cap to ground.  This will short HV and pop fuse in case plate coupling cap shorts and puts HV on the antenna circuit.

I added the three parasitic suppressors to the 6146Bs.

Next comes the power, control and 6146B socket wiring. That can be drudgery, so I'll just do a little bit each day.  

T

Pic 1: Notice the second meter from the right, rear view, is on standoffs. (screen current)  And notice the front panel shot, second meter from left - there is a bezel that I cut out from a bad meter. It looks almost "normal."

I tried my best to use low inductance straps and reasonable lead lengths thruout the tank circuit for both efficiency and stability.  The 6146Bs are hopefully happy.  Next, the grid and screen circuits need good treatment underneath with good bypassing and the L-C input matching, etc..

The copper straps will attach to the FRONT of the tank coil for shorter length. I left them long just in case.

Pic 1: A look at the fan mounting feet.  These are standard rubber chair leg pads found at any Home Depot, etc.  No fan vibration.

Pic 2:  A good look at the floating screen meter and fake bezel.

A big goal crossed off - the wiring for Commando is completed with Teflon wiring.  The wire routing and harnesses still need to be dressed up and cleaned up with tie wraps and clamp-on guides, but I always keep it loose and messy for the testing phase in case there are some changes. I will keep wiring far away from the 6146B finals and input tuning circuitry.

I added in a 1500 ohm 45 watt non-inductive resistor chip from grid to ground for better stability.  The 85W audio module is installed and all hooked up. Theoretically, if everything is wired correctly, it would work FB on the air right now.  But this is never the case.  Human error and bad parts are part of the game.

So far I have tested the filaments, PTT circuits, HV step start sequencing and coax in/out  relay switching. I added in an adjustable time delay on the rear panel for the HV step start.  I found an antenna relay connection wrong.   I usually check all the wiring against the schematic before turning on anything else.  The Hamond spark gaps are installed as well as the HV fine wire fuse posts.

I decided to remove the 10-20M coil and will use Commando from 40-160M only.  These are my favorite bands and if I want to get on the higher bands I will use the modified hi-fi FT-1000D and several linear amps in service.  I could always add 10-20M easily later if desired.

I used ball park grid leak and screen dropping resistors until I can test it for optimization later on.  The screen clamper 6AQ5 and OA2 circuit is wired and ready.

Next comes the fun part; testing and optimizing the rig for power, stability and the highest audio peaks I can get.  

Hmmm...  was wondering if adding a Heising reactor to this specialized  Hammond single ended xfmr would help to increase its saturation capability?    The point being it is rated at 130 mA of DC current and a single 6146B is around 120mA.  IE, to take the DC off its core for handling 2-3 tubes.  Just thinking out loud.

One thang I keep reminding myself of; it takes just as long to build and test a little rig as it does for a big rig. Except for more panels and metalwork, there are just as many parts to mount, to wire up and then test.  eBay takes just as long for parts and the UPS truck takes just as long to get here...  ;)

T

These old buzzard rigs take a lot of wires to work...

Pic 1:  at the top is the 85W audio module -  heatsink on top of chassis.

Pic 2 : Two Variacs - one for muffin fan and other one at bottom for HV, 0-800VDC.

I am reminded that at one point I thought I had too much room in that 15" X 17" X 6"  chassis. But like everything, space gets filled up based on how much is available.

All solder connections are double and tripled checked for strength and durability.  All crimp-on lugs have molten solder poured down their holes.  Solder connection failure is a rare event with these rigs.  Internal solder connection failure can be some of the hardest problems to find so it pays to make sure in the early stages of construction.
T

Audio amp module snuggled up next to the Air-Dux - no fear of RF getting into the audio? I was thinking of putting mine in its own little cage; maybe I'm being overly cautious.

Hi Bill,

The same thought occurred to me too.  In fact, I got the input tank circuit (Air Dux) optimized for 50 Ohms 160-40M last night and the coil is twice as big as it needs to be.  So I could either cut it in half and move it away from the TDA7294 -  or put a shield between them.  It's not easy to locate things when there are big parts on the top and parts on the bottom.

All of the wiring needs to be tied off and bundled - neatly away from the underside components anyway.

I added the parts labels (in the pics below) which makes it easier for troubleshooting and optimizing in the future.  I will send the panel labels lettering design to my local friend Bob/ W1RKW  so he can print them out for me.  We are getting closer to switching on the HV and seeing if Commando works at all.

T

Teflon wire, baby!

More Details:

More:

Tom,  I really like the "Tom & Yaz 3/20/22" signature label. Like an artist that signs their paintings.  Nice touch OM!  BTW, the rig looks excellent.

Yes indeed, a great big virgin chassis seems plenty big until you start putting stuff into it, just like an empty storage shelf seems big until you start loading piling stuff on.
I don't think I could ever cram stuff efficiently into a small space nearly as well as a commercial manufacturer. Looking at something like an SP600 or R390, or even a Ranger, it's really quite impressive when you imagine trying to accomplish that yourself.
Love the idea of labeling everything: when I'm building my stuff, and for the first few months afterward, it all looks so comfortable and familiar, but revisit it a couple of years later and it's all a big bowl of colored spaghetti.

Hi Bill,

Yep, those Rangers and Valiants really pack in the parts.  The drawback is when you have to replace a Chernobyl resistor or equivalent. I watched the procedure on YouTube and decided I could do a better job building a low power rig; especially the reliability later.

The inside labels really do help to refresh the memory later on.  I have eight homebrew rigs now - and  five (including Commando) use Plexiglass clear cabinet covers. I'm just a building fool!  I get a real thrill looking in at the rig's inner workings as I walk by. The labels help to get back in sync.  The biggest problem is not being into a rig for a few years and it craps out. The labels help a lot.

Well, I'm glad you mentioned the input coil being kinda close to the TDA7294 chip.  I cut the coil in half and mounted it away from the chip and closer to the bandswitch and tuning cap. Much cleaner looking arrangement.  I will try it like that - and if there are any problems I'll add some shielding.   BTW, that L/C input circuit is so simple and gives close to a 1:1  50 ohm SWR every time.

Bob/RKW:  Thanks much for the comments!  I think once the wires and harnesses are all laced up and neat, it will be one of my better looking rigs of the harem here...    ;)  I'll be sending out the panel label info to you soon.  That will be the final touch.    Yep, I wonder what someone might think if they come across an "antique" K1JJ & Yaz signature rig 100 years from now at a hamfest.

The current phase for Commando is the refinement phase. Redoing the JS'es and testing, optimizing.  I cleaned up the shack of building debris and back to a clean environment for the refinement. It's all a mental game now. A matter of attitude to file off the rough edges.


T

Tom,
Just something to ponder before you put the big power to everything. Regarding the audio module and RF shielding.  I noticed when I play with my 2m handheld that it's RF will affect some of the audio devices here and from several feet away. Things like computer speakers and some other things. Granted these consumer devices aren't the best when it comes to RFI but maybe it could be used to test your audio module before applying the power to Commando. I'm wondering if you can power up just the audio module and put a scope on the output and do a 2m test to see how it would behave at 2m RF. Would it be an indicator at HF??  Not sure. Thinking out loud.

OK on the labels.  Whenever you're ready, I can support.

bw

** Commando 3-23-2022 UPDATE **

Made some good troubleshooting and testing progress.... so far no fuses popped or loud bangs.

The HV filter cap circuit had a 50K  to ground measurement.   That shows something is wrong. I found a wiring error on the screen clamper circuit. Fixed it and turned on the high voltage.   The high voltage worked FB from 0-800VDC using the Variac.   The power transformer is actually 600-0-600V  CT.    So, using a fullwave CT, I get about 850VDC max, no load.  I plan to run no more than 550V under load since there are 3 tubes and plenty of carrier available.  The Variac was set for boost; I reduced it to normal.

I didn't have a reading on the 0-1000V  panel meter. Turned out that a ground connection for the meter was floating on a plastic cable guide. Connected it to metal and it worked... however, it was off by 20% from my calculations.  I changed the divider resistor and HV now reads exactly as my VOM.

Using the MFJ-259 I  tuned the taps on the input coil for 50 ohms for ~1:1 swr on 160, 75 and 40M and soldered them. The 6146Bs use a 1500 ohm, 45 watt chip resistor to ground, so the load is stable. I don't expect the extra 10 mA grid current to effect it more than 10K of load, which is minor.

I turned on the +- 35VAC TDA7294 power transformer with no problems. The chip was hooked up and stayed cool. I didn't go any further until I test out the 6146B finals. I might even use a power resistor as an audio load at first.  I found there is 50VDC on the chip. That is about the maximum it can be run, so my plans for a boosting transformer are not needed.  50V should be plenty.


The HV stepstart delay was acting flaky and would not hold a delay setting. Turns out that the rheostat was intermittent. I replaced it with a 5K pot and now have a stable step start delay from instant to about 1 second.   I test cycled the PTT by keying the power supply at 800V with no problems.

I turned on the fan, 6146B fils and OB2/6AQ5 clamper circuit fils.  No problems, though I still need to put some real RF drive into the grid circuit to test the screen clamper.  It should light up the OB2 when enuff current is detected at the gridleak DC point.   No grid current and the OB2 should go off as it is now, killing any screen current.

I notice that the 50K off the screen to ground (part of the screen clamper circuit) is showing 10 mA of screen meter current. This is something I need to think about, but may just live with it.

I may increase the step start resistor from 20 ohms to 50 ohms since it presently does not give enough "jump" when it comes on.  I want say, 400VDC on key up and then 600V when the step start shorts out the limiting resistor.

My PTT pilot lamp was hot as a pistol. Turned out it was a 14V bulb on a 24V circuit. I found a 13V tap and all is well wit that.

So next I plan to put some 1 watt RF into the rig from my station DDS and tune it up on 160M and see how much carrier power it will do. I still need to determine the tank bandswitch connections and solder them permanently.  Then I'll back it way down to a few hundred volts and carefully add some audio to the TDA7294 to see how it likes it. I'm still not sure if a 1 volt line audio signal is enough. I want to add an input audio gain pot to the front panel once I determine if I will need another gain stage or not. I am having trouble determining just how much audio signal is needed to drive that module. I read that it is good for 36 dB? gain, but it is not clear.   My guess is 1 volt will be enuff drive.

So it looks like all is going very well with no big surprises yet.   The RF power and audio tests will really tell the story next.

Bob/RKW:  FB on the suggestions for the TDA7294.  I suspect if anywhere, it will act up on 40M and higher, but it is easy enuff to add a shield around it if it has a bad spell...


T

    Tom,

   That 50K should not pull any current when the VR tube is not lit. You may be seeing the 6aQ5 plate current. Kind of depends on where you are measuring that screen current.

The clamp circuit might be on the edge of capability with three 6146's. Data sheet screen current for a plate modulated 6146B is around 9-10ma / tube..That OB2 is rated for 30ma..

Was thinking that an OC3 (VR105) Octal would give you more margin. Then there is that 6AQ5....might be fine if it clamps hard enough, but if not something like a 6W6...

Looks like fun!!

Jim
Wd5JKO

Thanks for the input Jim.

Yes, that current is likely the 6AQ5 plate current doing its job to shunt the screen current.  It is conducting heavily when there is no grid drive. It appears to be working FB so far.  The screen current meter is connected between the modulated B+ and the pin 5/ pin 5 connection of the 6AQ5 and OB2, so it makes sense to see the 6AQ5 plate current there with no RF drive.

A friend suggested I try a 6AH5 in place of the 6AQ5.  Three 6146Bs are probably at the limit for this clamper circuit.  

Question about the 50K from screen to ground...  Lets say that normally without a screen clamper, the screen dropping resistor is 47K.  By adding the additional 50K to ground from the screen, how will that affect the screen modulation swing and will I need to reoptimize the screen dropping resistor to compensate?  It appears to be also acting like a voltage divider, thus reducing the screen modulation?

I'll run some tests and see how the OB2 and 6AQ5 act today under full drive and let ya know.  I don't want to cut new octal socket holes if I can help it... ;)

T

Update 3-23-2020:

Today I fired up Commando into a dummyload for the first time.  For a good hour there was no output at all.  I slowly eked out 10 watts, barely.  Something was obviously wrong.  I disconnected and tested every circuit I could think of and all worked FB.


So then it dawned on me... maybe the tubes are bad!  The obvious is possible but unlikely. I went to my questionable used 6146 stash and started testing the tubes one by one.  I was also considering ripping the 6146 tubes out and going with a single 4D32. But I already have a single 4D32 rig so that is the only reason I stuck wid the 6146s.

As luck would have it the set of three I had in the rig were all barely working. Some arced over. But little by little I was able to get each good tube to put out 50 watts at 600 volts and 90 watts at the maximum  voltage around 750V.

Now I have a box full of bad 6146s.  The three good ones were very strong, however. I put the three into the rig together. After playing around I was able to get a very stable output on 75M of about 75 watts at 550 volts. At 600 volts I was up to about 90 watts and at  700 volts the Bird kissed 150 watts out.  This is comparable to a Valiant that also runs three 6146s.

I think that 550V at 75W out will be a good goal for this rig considering the Hammond xfmr. (Unless I spring for a Hammond 1642SE big mofo then its 125W)

Earlier on the screen clamper 0A2 tube was working but sometimes flashing like crazy at 600V and above. I disconnected it for now. It seems that with the HV step start doesn't really need the clamper cuz the HV bleeds off quickly. But I will try to get it working as another backup.

The input circuit tunes FB at 1.3:1 or better swr on the bands.  

So next comes some audio. I like the HV Variac feature. I can easily change the carrier power out from 25W up to 150W. The Hammond and TDA7294 will be a good independent match-up for the adjustable carrier, for whatever audio I can put out.  The Variac will also give me versatility to drive the 4-1000A linear at the proper RF level.

I notice my 250 mA plate current meter gets pinned at full 150W power output.  It was planned for one tube. Gotta find a 500 mA meter I guess.

So after some more repair changes, I'll try some audio.

T

I am testing the TDA7294.   With a 50 watt carrier, the module starts to modulate the carrier well up to about 10%, then falls apart and the audio folds back and collapses.  The rig is staying solid on all parameters, so it must be the module.  The heatsink gets luke warm, so it is working.  I tried shielding and cap bypassing, but no luck.

To make the story short, I turned the transmitter completely off and ran the TDA7294 by itself. I put an 8 ohm 20 watt resistor across its output as a load.  Looking at the scope when the audio input is applied, the 45V VCC is solid and the input 1 volt audio waveform is clean. The resistor is warming up to about 10 watts and is clean. But as I cross the 10 watt level output it falls apart.  When the scope is across the 8 ohm load resistor, the collapse takes place in the form of flat topping and a general fold back distortion look.  It is definitely in the module since everything is disconnected except for it. The VCC stays rock solid at about 45V.

Any ideas how to proceed?

Here's the module datasheet:

https://www.st.com/resource/en/datasheet/tda7294.pdf

T


Here is the eBay module:

https://www.ebay.com/itm/143464647535?_trkparms=ispr%3D1&hash=item216728cb6f:g:LxkAAOSwu5thoqKs&amdata=enc%3AAQAGAAAA4H3LEDT9X5xyUJzlSVX0ybAeNUa5aSKJKyUrv0oR3UO8%2FxHdOjRzwMN3Rf5Q43u%2Fp9%2BA8nmo%2BHaVHJb%2BwzhILu3RSZYfkK0DyCvf9OvTDyQgnES2zkGNbdmeKXawl3rKFx7XRxJCSNqVSUT7EcjLsJJwuBmoJR1veDFUACEykIe90XHOUbB4CIiDGwpKWVQsGYz97Rw2XK8pxLhz4ylI059M687dMazzmVicSiUeYgP3%2FEpWVRIR36Sn6uswX1oLGnbVEd6EMHcjxcRfP8QCtAjbHducyYsSzQ3sltgwM3wI%7Ctkp%3ABFBMjNGiqvdf

*** UPDATE:  I notice that the required input AC voltage on this eBay module shows +- 23V maximum.  This is a new change. It used to say +- 35.     +- 35VAC is what I am feeding it.  I wonder if the VCC is on the hairy edge and the module is shutting down a protection circuit?   They talk about 50V DC max in the datasheet  so it is confusing when we have a X1.4 factor after being rectified.  I'll try a lower voltage and see what happens tmw.  The audio looks quite nice when running under 10 W.

The data sheet test conditions are +/- 35v.  The power supply range (where they say it works) is +/- 10 to 40v.  The it’s going to explode absolute max is +/- 50v.  The AC input is not nearly as important as the actual DC at the chip…so measure that, and make AC changes based on the DC voltages…    Remember that solid state data sheet maximums don’t have anything like the wiggle room of a transmitting tube data sheet maximum…

That said, although I don’t like being the bearer of bad news, a $15 module from China that should cost $50 most likely has a counterfeit chip…they work ok at low levels then either shut down or blow up at high levels.  In repairing amps with this sort of chip, unless I can get it from a real supplier/it’s still in production, I’ve resorted to buying used pulls with old date codes, because all the new ones on places like eBay are fake and don’t work right.

If it were me, I’d hook up an external, known good, audio amp to the transformer and get on with the testing, and circle back to the module. Then I’d get the DC voltage right, and buy a new chip from a reputable supplier (Digikey has them in stock for $6.87).

Ed

Tom,
The rail supply voltages +Vs (pin13) and -Vs (pin15) should not exceed 50Vdc which is also what the 2 large filter caps are rated at.  So you'll  need to determine what the AC side would be for bridge rectified for whatever rail supply you want to drive +Vs and -Vs at.  

EDIT: Ed brings up a good point about counterfeit components.  I have the exact same module and looking at the chip, I am suspect of it because the ST trademark is absent. In fact, there's no trademark of any kind on it at all.

The ST Micro datasheet states +/-50VDC absolute maximum. So about 35VAC would be the absolute max AC input for the module (using a real TDA7294).

Bob and Ed,

Thanks for the valuable info. You are likely correct about bad fake chips.   A search for  "counterfeit TDA7294" brings up a rat's nest of comments.   I would order a new chip from DigiKey, but I don't think I can unsolder and solder that tiny package without destroying the circuit board. Or maybe I will order a proper chip from Digi-Key and give it a go anyway.  

Or,  I might think about a different module.  Anyone have a suggestion for something similar and reliable?  

Yes, I could use an external 150W Dayton amp, but I'd still like to do it self contained here as planned.

I'll take your suggestion, Ed, and continue testing with an external amp for now.

In the meantime, I read where one guy tried decoupling the board using bypass caps. I notice it broke into parasitics when folding back, so maybe I'll try some cap bypass experiments.


What a PIA.


Fake chips:

https://www.google.com/search?q=tda7294+counterfit&rlz=1C1GCEA_enUS908US908&oq=tda7294+counterfit&aqs=chrome..69i57j33i10i160l2.11259j0j15&sourceid=chrome&ie=UTF-8


T

Tom....
Last year I bought a "digital soldering station" thing on Ebay for less than $100.  As with most of that ilk it's not there now, but others of the same type are.  It has a small hot air gun with controlled air temperature that you can dial up to a specific temperature.  This is sometimes referred to as "reflow" and it how the chip was soldered on in the first place.  You can use it to unsolder the fake chip and replace it with a genuine article chip, if your hands are still steady and your eyes are still fairly sharp.  And you will most likely do it without damage to board or components as all this stuff is designed for this treatment.
My unit also has a conventional small soldering iron on the other side that I can dial up to temperature and be soldering SMD chip capacitors and resistors within a minute,  It may not be an American Beauty, but it's still a beaut.
73 de Norm W1ITT

Hi all, I once ordered some big three phase rectifier modules from a China based Ebay seller. I get an email from they seller, in translated english,
basically asking me the exact part number to brand on the side of the module! I knew this was bad, but they came in, looked perfect, and worked fine, over a dozen of them!!

As to the situation in hand here, a 4 or 8 ohm speaker has several ohms DC resistance in the voice coil. A slight DC offset from the amplifier will then only be a minor issue. Conversely, driving the 4 or 8 ohm winding on a transformer will be a huge problem if there is a DC offset, or if there is an asymmetry in the AC waveform. The winding resistance could be 0.1 ohm, and with a 1V DC offset would cause 10 amperes to flow! Might try two big electrolytics in series - + + - to form a poor mans AC capacitor. Something like 4700uf or thereabouts x2 at 25v would be a good test. Put the two series caps going between the AMP and the Transformer primary.

Jim
Wd5JKO

Tom,
to desolder the TDA, cut the long leads first close to the body of the IC then fold back the IC to get to the short leads then cut those as close to the body of the TDA as well.  Clamp the board down and extract the leads one by one by heating them up and pull out with needle nose pliers or tweezers. Tinning them a bit might help with heat transfer.  Clean up the holes with solder wick.  that's about the fastest and cleanest way to do it with basic tools with minimal board damage.

EDIT:  I looked at the link on counterfeit  parts.  Holy cow is all I can say.  I watched this youtube video:  https://www.youtube.com/watch?v=1IlTckCE8CM

Very interesting on all!

I gave it another try and this time I was able to get it working on the bench.  The rig was off.  I'm not sure what is different out of the rig. I just extended the leads with clip leads. I could smoke an 8 ohm 20 watt resistor with a clean sine wave.  But after about 5 seconds it folds back and looks crappy again.

I tried it with the rig running and Hammond transformer.  At about 15 watts carrier, I can modulate it up to about 250 watts cleanly.  But after 5 seconds it goes away and folds back again. Maybe I am seeing the effects of thermal protection shutdown, I dunno.  I am using a smaller heatsink on the bench and it gets warm quickly and works best and longest after a cooling rest.

I tried using shielded cables, big ferrite beads on all leads but it still folds back. There are also some parasitics mixed in when it happens.  BUT for the first 5 seconds the chip seems to be performing well.

Now thinking about ordering a soldering station as Norm described and ordering a REAL chip from DigiKey as suggested by Ed.    I've been thru this stuff before with the SS MOSFET audio driver and finally got everything working. Mostly layout and RFI getting in.

I might hook up the 150W Dayton analog amp that I use with the 4D32 and Hammond big 1643SE to prove out this rig.  It it works well with that outboard amp, then I can have confidence it can work eventually with the TDA7294 down the road.

Jim I already tried the back to back caps and no difference.  The module appears to like the 16 ohm best. It is most stable there.  The whole lash up appears to have big potential if I can tame it down.

Bob: FB on the chip removal procedure. Tnx.

* Some progress. I think it is thermal shutdown after 5 seconds.  I let it cool off and the waveform was clean for 15 seconds before the heatsink got hot and folded.  I'll return it to the inside rig on the bigger heatsink and try again.  It will handle a 65 watt carrier quite well with huge peaks when it is in a good mood..  Lots of things going on in there.   I may need a big shielding job  via an alum box around it.   It IS sensitive to higher RF power for sure.   At 36 dB gain, it drives easily with my station one volt audio source.

** Put the TDA back on its heatsink into rig and fired it up.  The triangle wave looks big and beautiful for the first 10 seconds and then starts to fold back.  The chip is staying cool on the heatsink with the fan blowing.  So something else is changing and folding it back. The signal going in is FB.  I will try a 16 ohm resistive load on the output without the rig running and go from there.  It may end up I buy a new REAL chip and replace it.


T

Update 3-24-2020:

There is definitely something wrong with the TDA module.  I replaced it with my Dayton 150W analog audio amp.  The rig ran perfectly with clean triangle waves and peak audio power. There was no limit for time.

So the TDA module wants to fold back after 5-10 seconds. The more audio power I draw, the quicker it drops out.  Maybe a bad critical part inside that is heating up?  I've seen this before.... runs good then gets flaky and jumps around like James Brown and dies. Over and over.

Anyway, I'm on the fence as to whether to get a new solder station and new chip from DigiKey -- or return this TDA to the eBay vendor and buy two more, one as a backup.  What keeps me hanging on is that it works perfectly for 5-10 seconds...   BUT, the circuit board itself may have a bad part which would be another waste of time if I replace the chip.

T

Tom... 
I just looked at the TDA7294 in a few examples on Ebay.  I think the hot air soldering station would not be the tool for this job.  They are meant for little tiny SMD devices and the ICs with pins so small that you can hardly see between them.  It's a fun tool, but I think it's for smaller stuff than the TDA, if what I'm looking at is the same thing.  I think I'd just suck it up and buy a couple whole replacement amplifiers and hope you can get one that was made on Tuesday.  If you are just honor bound to replace the chip itself, I'd do what someone else suggested and snip the leads then unsolder the debris and clean with a spring loaded solder sucker tool.  I think my old one was made by Ungar and still works.  But the hot air station just doesn't look like the man for the job.
73 de Norm W1ITT

OK, Norm, TNX.

Some progress: I probably won't need the precise soldering station anyway.  After a lot of testing and trying various stuff, I've found that the TDA works OK, but it does NOT like the Hammond transformer.

When run alone into a 8 ohm resistor it is clean up to 90 watts, no problems at all. But as soon as I connect the output to the 8 ohm winding it gets squirrely as I have described.  I tried the back-to-back capacitors to no avail.  (2  10,000 uF @ 50V)  The best tap is 16 ohms and the worst is 4 ohms FWIW..

Even if the TDA is connected to the 8 ohm resistor load and the transmitter is off, when the clip lead touches the Hammond primary winding it folds back.  This tells me it is not RF getting into anything, just that the TDA does not like an inductive load.  Again, to back this up, the Dayton 150 watt amplifier works perfectly at full strap. Any further ideas?

I want to use the Hammond and the general rig as wired. But unless I find a completely different audio module, I may pick up another Dayton 150 as an outboard SS amp.  

There is a possible advantage... it will leave room when the 35-0-35V TDA power transformer is taken out and the muffin fan moved closer to the tubes. I could add a strapping mod reactor and coupling cap to take the DC off the Hammond. Right now I am drawing about 200 mA+ and the Hammond is rated at only 130 mA DC current. I think this would give me more audio capability to modulate those three tubes better. Also, the DC would be off the xfmr for less breakdown arcing risk.

I already tested the rig with the Dayton amp and know it works FB. (even has 150W bridge mode into 8 ohms) The added room for the choke and additional headroom might work out.

So that's it for now.  This is what happens when we try untested ideas.  I might build the TDA and transformer into a nice station amplifier. I presently use a small Chinese 25W box - and the wall speakers wud like something like this. But I'm still open to ideas before I give up on the TDA....

T

Here is the Dayton 150... a very nice amplifier for about $140 used on eBay: (2 pages of data)

https://www.manualsdir.com/manuals/294570/dayton-audio-apa150-150w-power-amplifier.html

Hi Tom,

   Take a look at the App Notes for the old LM12 power op amp, specifically on pages 8-10. See PDF attachment.

The clamp diodes from the output to each power buss could be important with your TDA, and easy to add.

Also, any audio transformer has a self resonant frequency, often up in the tens of Khz range. If the TDA has enough gain up there, it may oscillate at that frequency.

A possible fix is to insert a series R-C shunted across the transformer primary such that R is somewhere between 4-16 ohms and the reactance of C is low at the transformer resonant frequency, might try 0.5uf and 10 ohms.

BTW, I have two NOS LM12CLK devices dated 8849. Yours if you want them.

https://www.electroschematics.com/lm12-150w-audio-amplifier/

Jim
wd5JKO

I work in an R&D lab where I do lots of precision soldering. Pretty much the only thing I use the hot air pencil for is removing QFN and BGA and similar type packages where there are absolutely no exposed terminals - all solder connections are hidden under the body of the component. Otherwise, it's solder-wick for most SMD stuff with leads, hot-tweezers for SMD resistors and caps and such, and the vacuum solder-sucker for through-hole parts.
A good rule-of-thumb for soldering iron tips, whether soldering or de-soldering, though it might seem counter-intuitive: use the largest iron tip that will contact the leads of the part without incidental contact with any other parts. The larger tip has more mass and will transfer heat to the leads more quickly than a smaller tip will, thus requiring less contact time. The smaller tip will require longer contact and could result in thermal damage to the board material and the part. Faster is better.
And, while I agree with the previous advice to cut the leads, be very careful - a wire clipper cuts not by removing material, like a file or a saw, but rather by displacing material (it's basically a wedge), which means that it has the effect of momentarily expanding the lead's length, and can result in mechanical stress tearing foil traces from the board.

Jim, as usual you appear to have a good solution.  I also got the schematic below from Jeff / W2NBC and hooked it up.  I tried 8 uF and 5 ohms and the TDA 7294 is now stable!  I'll try to trim the values back. But the resistor gets just luke warm and appears not to be hogging too much power for now.

So far so good. I'm hoping to optimize things and see how many tubes are best, best HV level, screen dropping and grid leak values, etc.

I'll keep ya informed.

Tnx for all the suggestions, guys!

T

3-25-2020 UPDATE:  Random thoughts...

After some refinements and tests, all it took was a 2 ohm resistor in series with the 16 ohm Hammond winding to make everything stable.  Though, I think I will still need a cap across the resistor to keep voltage drop at bay.  I'll tinker with it later.

I also found that ONE 6146B final was the optimum choice for the final.  The audio peak power limitation is as expected - the transformer and the TDA 7294.  This combination is ample enough to modulate a single tube at 40 watts output @ a very robust 150% positive, but any more carrier and there is saturation.  

That's OK cuz it was my original plan to build a Ranger III for QRP and to drive the 4-1000A linear. .  The frequency response is quite good and very clean. However, it starts rolling off at 5 KHz which is odd. Alone into an 8 ohm resistor, it sweeps flat over 40KHz.  I must look at the screen bypass caps.  I only use 500 pF in the plate circuit.  Why is it that every new rig has frequency response limitations until it gets worked over?

I see no signs of audio foldback or parasitics with the 2 ohm 10 Watt in place, though if removed from the circuit the problem reappears.  I can live with the small heat loss that is barely luke warm after a sine wave tone test of 1 minute.

BTW, the TDA-7294 shows barely any heat after an extended sinewave tone test of 100% modulation. This is a very efficient method of modulation. It really didn't need a muffin fan.  The single 6146B itself barely shows any color after a 50W carrier and 100% tone test. The 100 Hz triangle waveform looks FB and sharp.  I settled on about 650VDC plate voltage for now.

I feel good about the "Ranger level" 40 watts out with lots of headroom audio. Parts are way conservative.  There are plenty of rigs in the shack that will do much higher power levels. Commander is now the baby.  A FB low power rig of high efficiency.  The rig is as quiet as a church mouse with the muffin fan turned down.  Fantastic.

Still lots more testing and refining left to do.  I'm happy I didn't have to start over with the audio section...   I may try out the Hammond secondary spark gaps later.

Thanks to the guys in this thread -   and Jeff/ W2NBC and Frank/GFZ for their email suggestions.

T

UPDATE NOTES:

A few things...  I found the Bird wattmeter does NOT read audio tone peak power well.  I was watching the peak reading bird and thought it was rolling off at 5 KHz when in reality, I looked more closely at the scope and SDR analyzer and see we are flat up to about 20 KHz. And it even goes higher to 40 KHz with some roll off but still a clean sinewave.  It is near impossible to get that kind of performance with a standard tube plate modulator.  In the past I can remember saying some sss's and the Bird hardly moved. Now I see why.

Also, I ran the rig hard for a couple of hours with the muffin fan off. The TDA-7294 DOES get warm to the touch thru the heatsink but only a warm, not hot at all. I turned the muffin fan on and in a minute it was back to room temp.

I'm trying to find the right values for the resistor and cap for the 16 ohm input.  I want to get as much power out as possible but still keeping it stable.

I did a harmonic distortion test  and see that side products are down at least -35 dB.  This is good and shows no problems. I was able to sweep the audio sine wave up into the 20KHz and higher ranges and see the full signal dancing up the band. The bandwidth is quite impressive. I will use the DSP audio filter to be a good neighbor, of course.

So far nothing has crapped out or smoked since the rig was built. Just one 16 ohm fuse popped when abusing it. This is highly unusual and needs a full house investigation to figger out why..  :o

T

'JJ

you still have the series 2.7 ohm .1 uF across the output of the TDA module ?

Peter

Hi Peter,

Yes, the 0.1 uF and 2.7 ohm are there on the board.  I tried padding it with different values, but no difference with the problem discussed below.

I've made some big changes since the last post.  After doing a closer inspection of the rig's performance, I found that Commando works very FB, BUT there are some small audio parasitics riding on the desired audio whenever I put a 60 Hz or lower tone thru.  The tone triggers the parasitics.  I had been fighting this problem in different forms for a few days and thought it was licked.   I tried everything I could think of and also talked with a couple of friends who know the score. I could have rolled the audio off below 60 Hz and been done with it, but decided I didn't want a band aid rig with a hidden problem.  This particular TDA-7294 module just can't hack the Hammond below 60 Hz.   It may be just this particular TDA with a fake chip, but I had to cut my time losses short and move on.

A more powerful and dependable solution: My good friend, Jeff, W2NBC offered to send me his spare Hammond 1642SE, the big MOFO 28 pounds, 55 Henry 300 mA beast.  I will be using it with an old Dayton 150 Watt audio amp I've used for years with Summer Breeze, the 4D32 rig.  I put the three 6146s back in and tuned up Commando.   I cut out and installed a slick AC power socket outlet on the back of Commando that will turn the Dayton on from Commando's front panel. I also added a -41 volt fixed grid bias supply and got rid of the screen clamper. The clamper worked OK, but the three 6146s were a little too much for it load-wise.

So I'll be experimenting to see what is the best tube complement.  (1,2 or 3 tubes)   The Hammond is good for 300 mA of DC current so would be a good match for two tubes anyway.  

The space that was freed up by taking out the +/- 35V power transformer, the fan and the heatsink will easily accept the Hammond 1642SE onboard. The Dayton amp will sit by itself out of sight behind Commando remote controlled.   I know this lashup will work FB since I tried a similar version already - and is an improvement to making it a super Ranger if not a good match for a Valiant power.

I relocated the muffin fan on the inside front panel closer to the tubes, directing air downwards. At 30% spin rate it keeps the finals in a good state.

I'll take some new pics when the new Hammond is installed and working.

So bottom line is anyone building the original TDA version should run a bench test with the transformer hooked up to the TDA  and into a 5K resistive load.   Look for tiny buzzing parasitics below 60 Hz when drawing audio power via a tone below 60 Hz.   Be sure to try a TDA with a real chip.  I estimate the parasitics to be around 300 Hz or so with a pulse character to them.  I did not measure the frequency.


T

Yo Tom,

Why the TDA-7294?

You should be using the TDA-7293.

Check out the differences in the specs.

The '94 can not handle what the '93 will do into low impedance loads which is what you get when going lower in freq into a transformer.  Probably why the '94 craps out below 60 cps.  Also, the '93 has a higher voltage capability in addition to the much better load tolerance. 

I've got my '93 based board driving the 1628 into a dummy load at 60W cleanly down to 7 Hz.  That is with no DC current however so this isn't a good test. 

Getting ready to modify my Ranger with this set up.


-Chuck

Tom, thanks for the info. Good thread.

Peter

Interesting info, Chuck, thanks!

If you were able to get your TDA7293 to work with the transformer and load connected as described, you should be OK.    7 Hz low end is amazing.   My TDA7294 took off even without the rig connected, no DC - it just required the transformer connection with or without a load to misbehave.

So what in the TDA7293 datasheet (below)  stands out that it would be a better choice for low frequency, low inductive loads?   I do like the higher 100V and 100W specs.  I may still modify a real Ranger later on if you get yours working well.   These little rigs are so much fun to play with.

https://www.st.com/resource/en/datasheet/tda7293.pdf

T

If you overlay the graphs for the '93 vs the '94, it's obvious on the 4 vs 8 ohm loads vs voltage.

Plus, I've got both the '93 and '94 boards when I bought these from epay.  I bought a bunch of them from the various chink vendors.  Tried them both, the '94 board had issues like you found and the '93 board had none and drove my 1628 just fine into the dummy load.  Interesting that the '93 and '94 board were the same except for the chip!  Different chink vendors...

Also, on one of the HiFi forums, there was a discussion about these two chips.  A Google search will help but I didn't save it plus you will find lots of interesting info vs the two.  Bottom line is that the '93 is much better and there are a lot of '94's that were chink labeled as '93's.  So if you have a '94 it's really a '94.  Try a real '93.   There is a you tube video of a guy comparing the two on a subwoffer that doesn't reveal anything...so ignore that.

Chuck,

OK on the 4 / 8 ohm loads vs: voltages datasheet comparison for the '93 vs: '94.      

Later on after I get the Dayton amp going, the overall '93 concept  still seems like a good way to go for me. More compact and all onboard. Also, I like the idea of bridging a pair to get 200W.  There's examples of that on YouTube too.  I just received the big 1642SE Hammond from Jeff today, so will be testing that with the 150W Dayton amp soon.  But down the road I will likely look into a pair of '93s since I will have more audio power capability to play with using the 1642SE and three 6146Bs.  We'll see how your Ranger mods work out first.

Thanks for the info and shedding some light on this audio parasitic problem.    

T

We're now in the testing mode.  The Hammond xfmr 1642SE is strapped on and working.  The Hammond got slightly warm as well as the 800V power transformer.  The Hammond popped my arbitrary 5A fuse in the 8 ohm line so I replaced it with 8A.  The spark gap was arcing at first so it's been widened. I can hear some talk back from the core with tones, but we'll see how it works on voice.  The rig must weigh 80 pounds now.  So far I'm very pleased with most performance parameters.

160, 75 and 40M are optimized for tuning and power out.  160M took  an additional coil and it needed some loading padding. But all bands put out 125 to 150W carrier at about 700V.  I can modulate the 125W carrier to at least 150% positive, just what I wanted. It will do a 10 Hz sinewave looking good. Check out the pic below of 20 Hz at 100% modulated.  It sweeps to 20 KHz with a FB waveform.  The Hammond is amazing.

I optimized the screen dropping resistor  to 20K and the grid leak to 5K plus -41V fixed protective bias.  Total of 140V screen voltage and -105 grid voltage, class C, carrier conditions.

The freq counter is working and uses a loop coil pickup near the tank coil. I tuned up on 20M by accident when I intended 40M and it let me know. That's a good check.

The band switching is fast, maybe 30 seconds.  The big rigs are very slow in comparison; too much rowing around town with the vac variables.  With the preset labels on the panel, it will be even faster. I want to get more active on 160 and 40M, so will be putting up a pair of stacked 40M dipoles at 130' and 65' fed in phase for USA coverage.  The ant on 160M is a dipole at 190' so we are looking forward to seeing how well 100W can do on AM.

The 6146Bs run gray plates, though I am working on plate efficiency right now.

There is rear chassis space on top if I want to put in another TDA7293 audio module and power transformer.

The HV step start and -41V fixed bias  (and dropping resistor) all work well together with no screen problems.

For the first time on the back panel I put in a proper AC cord receptacle and even a 120VAC socket to feed the Dayton amp. The Dayton is turned on by the Commando front panel switch.

Commando is turning into a handsome soldier if I may say so. I didn't take any pics of the bottom until the testing is over and I can tie wrap up the wires neatly.  All Teflon wire, of course.

The input L/C network gives about 1:1 for each band when tuned.

I still need to do some triangle testing and voice tests as well as key down torture tests. My philosophy is to beat the heck out of a rig during testing, looking for failure parts. Then back it way down to operate and usually nothing will fail for long periods. The only failures are human error mistakes.

I might even have Commando  on the air this weekend or early next week.  Ya never know.

More later -

T


First Pic: Commando actually transmitting a 125W carrier on 3885.00 into a dummyload.

2nd pic:  It's hard to appreciate how heavy and massive that Hammond is for a little rig. It's a 28 pound brute with 55 Henries of low end strap. Notice the wide washers used to support the chassis better top and bottom.  The fan has been relocated and does a great job cooing the 6146Bs.

Pic2:  That plate coupling cap is the size of a lemon. Only thing I had...     Fan on rubber feet - no noise.

Pic 1:  First time doing the AC cord right!

Pic 2:  20 Hz 100% modulated.

Pic 3:  Both coils used for 160M, so no tap.  Full big coil tap for 75M.     1/2 big coil tap for 40M.

Commando is nearing graduation...

Pic 1:  Close and personal with the trio running the show.

Pics 2 and 3: Probably the best photos of the bunch.  

Pic2: Notice the Bird wattmeter deflection on the right.

Pic 3:  Dayton 150 amp remotely powered  by AC socket.

Update  4-4-2020:

Some changes... back to the start again.  The rear chassis empty space has been filled in with the +/- 36V audio power transformer and soon to be TDA7293s.  The chassis and panel are now filled! No more room.

Here's why:  After successfully running Commando hard using the single 75W audio channel, I doubled the audio by bridging the two channels to make 150W. I did not like the performance when bridged.  

So, I decided to go back to the TDA7293, but this time I ordered a DIY kit board for $9 that has TWO chips and claims  140W.  This shud be enuff to hit the audio hard but still not blow out the Hammond if there is a surge.

Also, because of the fake chip issue, IE, boards with pirated TDA7294 chips, I ordered a few new REAL TDA7293 chips from Mouser for only $5.25 each. I'll populate the kit boards with these REAL ones right from the start;  fake chip problem solved.

I always did want a self-contained rig anyway.  The downside is I gotta wait two weeks for the China shipment.... a slow boat from China.

To see the TDA7293 eBay $8 DIY 140W kit using two chips in parallel, search on eBay:  (The link is way too long... why do they do that?)

"TDA7293 Power Amplifier Board Mono Audio Module Kit DIY 140W"

More to come.

T

I was just going to say that cap looks really cool there.

I see the fan mounting. Those mounts remind me of the rubber caps that go on the floor-end of a cane. I also like the back-set screen meter. Is that due to too much high voltage?

UPDATE  4/6/2020:

Rick, W8KHK has given me some great information via emails based on his vast experience with solid state amplifiers.   My particular problem is audio talkback from mic to transformer.  The internal thermal resistance is a factor that can put the device on a point of no return. Rick has taken a close look at the TDA7293  and has made some suggestions. I am still working on my rig as planned and hope to have the new TDA boards here within 2 weeks.

Pat: Thanks for the comments. The screen meter is offset from the panel for safety. When using a screen dropping resistor, there are scenarios where 300VDC+ can appear on the screen, thus the meter was floated. The other meters are at low potential.


I received Rick's permission to post the info below. If you are planning to do something with these types of devices, this is a worthwhile read:

---------------------------------------------------------------

Rick said:

I have been following the Commando progress, but have just stayed back on the sidelines, quietly cheering you on.   With lots of experience on both tube and solid state power amplifiers, I can say with confidence that heavy strife testing of tube equipment rarely results in a crap-out, but with the solid state devices it is very easy to reach a point of no return, especially with extended sine or triangle wave testing.  These amplifiers are intended for voice and music, but not constant banging with the 100 percent duty cycle testing.

The reason I bring this up is to help you prevent the same type of failure on the new dual TDA-7293 module.  If you look carefully at the spec sheet, under the section 4.2 High Efficiency, you will see some cautions.  I will post some of it here and highlight the critical phrases.......

You may download the specific datasheet here:  https://www.st.com/resource/en/datasheet/tda7293.pdf

Their high-efficiency amplifier is similar to the class H modulator typified by the build Steve/QIX did a few years ago.  Note that in the ST TDA schematic for high efficiency, they implement dual plus-minus 50 volt supplies for the peaks, and dual plus and minus 25 volt supplies for the average power, and employ external darlington pairs on the pos and neg rails to take on the excessive dissipation external to the chip modules.  

My concern is that you might reach a failure point very quickly if testing at max power using sine waves, if you are just using the TDA device without the external darlington pairs in your build.  You should be fine with voice duty cycle, but I would certainly tread lightly if doing sine or triangle testing at max power.

Hope this helps..

73, Rick


Here is the excerpt with highlights:

  High efficiency Constraints of implementing high power solutions are the power dissipation and the size of the power supply. These are both due to the low efficiency of conventional AB class amplifier approaches. The circuit below in Figure 8 is a high efficiency amplifier which can be adopted for both hi-fi and car-radio applications. The TDA7293 is a monolithic MOS power amplifier which can be operated with a 100-V supply (120 V with no signal applied) while delivering output currents up to ±6.5 A. This allows the use of this device as a very high-power amplifier (up to 180 W peak power with THD = 10% and RL = 4 Ω); the only drawback is the power dissipation, hardly manageable in the above power range. The typical junction-to-case thermal resistance of the TDA7293 is 1 °C/W (max = 1.5 °C/W). In worst case conditions, to avoid the chip temperature exceeding 150 °C the thermal resistance of the heatsink must be 0.038 °C/W (at a maximum ambient temperature of 50 °C). As the above value is practically unreachable, a high efficiency system is needed in those cases where the continuous average output power is higher than 50 to 60 W. The TDA7293 was designed to work also in a higher efficiency way. For this reason there are four power supply pins: two intended for the signal part and two for the power part. T1 and T2 are two power transistors that only operate when the output power reaches a certain threshold (for example, 20 W). If the output power increases, these transistors are switched on during the portion of the signal where more output voltage swing is needed, thus "bootstrapping" the power supply pins (13 and 15). The current generators formed by T4, T7, zener diodes Z1, Z2 and resistors R7, R8 define the minimum drop across the power MOS transistors of the TDA7293. L1, L2, L3 and the snubbers C9, R1 and C10, R2 stabilize the loops formed by the "bootstrap" circuits and the output stage of the TDA7293. By considering again a maximum average output power (music signal) of 20 W, in case of the high efficiency application, the thermal resistance value needed from the heatsink is 2.2 °C/W (with VS = ±50 V and RL = 8 Ω). All components (TDA7293 and power transistors T1 and T2) can be placed on a 1.5 °C/W heatsink, with the power darlingtons electrically insulated from the heatsink. Since the total power dissipation is less than that of a usual class AB amplifier, additional cost savings can be obtained while optimizing the power supply, even with a large heatsink.  



Just out of curiosity, I ordered a couple of the dual TDA-7293 modules like yours, and a few of the authentic chips from Digi-key.  Shipping was less than what Mouser wanted for a small order.

You will probably be up and running before I receive my package from across the pond, but the price was right, I was tempted, so HERE WE GO!

I am most interested in the THD spec, where they list it as around 10% at full output.  That seems kind of high, and I am hoping that the numbers are much better at lower sustained power levels.

I assume you are going to use some large storage filter caps external from the board, as the tiny two on the board are just local bypasses for high frequency impulses, and they will not provide any storage for low to mid freqs.  I think the earlier module required only an external power transformer, where this one needs a fully-filtered supply.  I do not think regulation is in order, so long as the caps are BIG and the transformer resistance is low.

I have a Sony power amplifier with a couple dead modules, but it has a husky power supply and massive heat sink.  So I was thinking this is an opportunity to replace the guts and have a decent amp for the surround sound in my home theatre, UNTIL I finish the tube amplifier with a quad of 6336 carbon anode bottles.  These are like 6AS7/6080s on steroids, double the dissipation, so a pair per channel should easily do 100 watts of clean triode power.  But in the meantime I can probably tolerate a solid-state amplifier, and listening tests will probably tell more than typical distortion measurements.  I am NOT one of those audioPhools, but I do understand that steady-state measurements do not tell the whole story when it comes to impulses and attack time, where ringing due to inverse feedback is very apparent with music, but undisclosed with a sine or triangle repetitive waveform.

I am of course interested in any thoughts you might have......



Tom, there is one other option you might consider to protect the TDA module during turn-on and turn-off surges....

The module has a standby input and a mute input.  I do not know if your printed circuit board exposes this circuitry with an interface terminal.  If it does, you may protect the device, and it is ok if both standby and mute are tied to the same input through RC circuits.

First, figure the time constant of the RC circuit interfacing the mute and standby input signals to the module.  The spec sheet recommends a separate 5 volt supply to control these inputs.  It may or may not be necessary to use a separate supply, as you may have another DC supply which could be regulated and isolated for this control function.

The idea is to make sure the standby (and optionally, the mute) control lines are not enabled until the power supply is fully up.  If there is not enough delay on the built-in RC circuit to accomplish this delay, it is easy to add a 7805 sourced from some other supply, followed by an R and C that will delay the enabling of the TDA until the main power supply is up.  Just milliseconds, but you really want the two pos and neg rails up and stable before enabling, such that an offset between the two is not passed on to the output transformer.

For elimination of surges during shutdown, you could take a main power switch with an extra contact, and use that to remove the standby signal from the module, connecting it to ground instead of the 5v supply.  This is best done by a diode to discharge the capacitor (AT THE MODULE) if there is an RC circuit for this function on the PC board. (Discharging via the exposed standby terminal on the PCB may discharge very slowly, due to a resistor in series with the capacitor.)  
This will ensure the module is disabled to standby status before the main filter caps start to discharge.  If you have an external R C circuit in addition to the one on-board, then BOTH capacitors should be discharged by separate diodes connected to the power switch, or, optionally, a relay that is triggered, (without delay at turnoff), by the power switch.

This technique will provide considerable protection to the module at turn-on and turn-off.  At other times, the caveat is to make sure your audio chain has adequate limiting and clipping, such that there are no impulses overdriving the amplifier.  Of course, audio connections to the module should be secure and not connected or disconnected while the module is enabled.  That little thing packs quite a punch for its size, and you need to help him make sure he does not punch himself!

  Excellent thread - been following it intently. Thanks, Tom, for blazing the trail. I've got some parts here: Hammond 1628 and 1642 (what a beast!), a couple of TDA-7293 modules and also a couple of class D 100 watt jobs - a Sure and a Dayton. For now, I'm holding off doing any drilling and blasting until I see where this project takes you.
  Curious, though: since we're not passing DC through the "primary" - the 4/8/16 winding - does it really matter that these Hammond transformers are designed for "single ended" applications? Hammond makes an output transformer - p/n 1650W - good for 280 watts and probably underrated, 4/8/16 and 1900 ohm windings. Could probably modulate a 500 watt input amp with one of those and easily a KW with a pair. Any reason why we need to use a single-ended transformer?

Hi Bill,

Looks like you are about ready to roll with all those FB parts!

Well the main reason I went with the 1642SE transformer last year was building the 4D32 "Summer Breeze" rig.  A friend of mine was using the smaller 1628SEA and reported  tremendous audio results, so I tried it in a bigger form.

The gapped transformer eliminates the need for the modulation choke and coupling cap which can add distortion itself. (so called "modified Heising", better low end, less saturation)   So single-ended was the choice for this little rig. The second big 1642SE (recently given to me by Jeff/W2NBC) has about 55H of inductance which would be hard to replicate physically as a mod reactor using regular chokes. Lots of weight and space needed.   It was intended to be used for class A audiophile enthusiasts, so they did a really great job designing and manufacturing it for hi-fi.

Another thing is there was no requirement for big power on this rig. The original plan was for a Ranger 40-50 watt output power level. To put everything in one package (the mod transformer, reactor and cap) in the form of one single-ended gapped xfmr was appealing. I already have a 4-1000A plate modulated rig with BC iron and two smaller 813 rigs, so the motivation to try the higher power push-pull 280 watt xfmr was not there. This was supposed to be a semi-PW rig. But now it will be about 125W, depending on how the new twin TDA7293 works out.

Let me know how the 280W xfmr works out if you go that route.... or any rig you end up building.

Everything but the 2 watts of drive from the DDS is now self-contained. I just finished putting in the +/- 50VDC supply with  new Variac - and TDA heatsink and other connections for the TDA 140W version. I'm hoping it will work without the audio parasitics this time.  I also replaced the screen bypass caps with 1KV 300 pF knobs and also increased the plate coupling cap's voltage rating  just in case it was arcing that was causing the weird trash below 60 Hz. Maybe the TDA was on the way out, I dunno.  I bought three kit boards from China and six TDA7293 chips from Mouser, so there shud be no excuses if it does not work right this time. The only other unsolved problem remaining is microphonic feedback with the 1642SE.  I may have to surround that pretty xfmr with acoustic foam.


** Once I tie wrap all the wiring under the chassis, I'll take some new pics. There have been a few additions on top too.  There is absolutely no more room for parts on that rig.


Out of curiosity I put Commando on the digital scale;  87.6 pounds!    (out of the cabinet)

T

I suppose I'm showing my lack of knowledge - I'm not a hi-fi audio guy - but I always figured that the "single ended" output transformer design was to alleviate core saturation due to passing all the DC plate current for the amplifier tubes through the primary. The DC plate current is going in one direction up through the primary, whereas in a push-pull configuration we're still feeding the plate current through the secondary, but since the B+ is applied to the center tap the currents are opposing and cancel out. In the case of output transformers, the designers weren't concerned about DC currents in the secondary because the secondary is only connected to a speaker coil - no DC current. Now, in our case, there's no DC current flowing in our primary because the amplifier modules supply their own current, which appears across our transformer only as an audio AC voltage. We do however have DC current flowing in our secondary - supplying the plates of our final tubes, so that's where the saturation comes in. It's just the opposite of the design intent of an output transformer; we've got DC current in the secondary but not the primary, where in a single-ended tube amp configuration you'd have DC current in the primary but not in the secondary. Either way I guess you'd still benefit from an air-gap transformer core.
No, I've got no plans of trying to scale this approach up to something really big - like you say, it's probably a concept best applied to exciter-class rigs. Also, the windings on these transformers are tested at only 2KV - if you wanted to modulate KW class rigs you'd probably be looking at higher B+ voltages than that. Anyway, these transformer's aren't cheap; you'd most certainly be able to come up with a "real" mod transformer for a KW class rig for less than a couple of Hammonds. You might not be able to duplicate that frequency response, though.
87.6 pounds - yikes! I toyed with the idea of applying this concept to a B&W 5100B that I have, which has a bad driver transformer, but the 5100B weighs in at 85 pounds already!

See my post further up on this, too.

You are right that in a push pull audio amp, DC core saturation isn’t an issue. But the transformer is designed for a particularly boutique style of high end hifi amp, the SET single ended triode. They run a single triode in class A, like a WE300B, or even a transmitter tube (there’s at least one SET amp maker out there using a single 833A) running all the B+ one way through the primary (which we use as the secondary and run the RF tube B+ through the same way).  This, the gapped core is necessary, just like in a conventional modulation transformer.  None of this has to do with the 8 ohm winding, neither in an audio amp nor transmitter..

Ed

Ed, you bring up a very good point.  These high-end single-ended audio amplifiers do make very good sense to the audiophiles.  A triode single-ended amplifier avoids many of the issues of push-pull tetrode and pentode amplifiers, and the difference is truly audible and amazing.  

Not only are they implemented with 211s and 833s, there are now companies producing the SET amplifiers with the beautiful Eimac 304-TL bottle!  
https://wizard-highend.blogspot.com/2016/03/alum-rock-technology-304-tl-stereo.html
Ya gotta admit that is a pretty sight with the golden plates and cool blue mercury vapor rectifier bottles!  Too bad it doesn't do RF!

As we complain about the audiophiles running up the cost and depleting the availability of our beloved classic transmitting tubes, I was thinking that perhaps we might better be thankful to them as they create a demand (and supply) for these gapped single-ended output transformers that fit so well in the home-brew tube final transmitters with solid-state modulators.

Is that your HB audio amp, Rick?  Very FB.

As a friend told me: "The current and voltage rating of the high impedance winding in a single ended amp is exactly what you get in your application flipped around. Operating at higher voltages or currents will push the core to saturation and degrade your low end. The only way to extend the power is to heising connect and get the DC current out of the winding."

When you think about it, these TDA and other "easy" solid state amplifiers ARE single ended amplifiers and require a single ended transformer to perform in our service.... so that's another reason to do this single-ended modulator trip.

Though, I wonder if a pair of these TDA amps could be run push-pull using a solid state driver to drive a p-p output transformer without too much trouble?   It gets to the point of then comparing it to a class E solid state rig for an easier build.  

The bottom line is we are mixing tubes and solid state -  hybrids here.

T

I had that thought, too. Or, would it be possible to solid-state phase shift the output and create push-pull "artificially"?

Or, use a two channel module and phase shift one of the channels?

UPDATE:  4-8-2022


These pics show the wired bottom tie wrapped "more prettier."  Also added the TDA7293 box and heatsink as well as the Variac  to control the +/- 50V audio VCC.   Still awaiting the twin TDA kit from China.

The rig ran FB (carrier) after being tie wrapped. Sometimes we get surprises, but not this time.  I immediately noticed how stiff and solid the wiring harnesses made the wires.  It adds and overall strength to the individual wires as we would expect using bundled wires.

The chassis is stuffed. As you can see, I had to stack vertically for the TDA box and the Variac to fit.   87.6 pounds is a lot to heft.

Pic 2:  This is the only time we can see all of the front panel gold pin stripping due to the light angle.

Pic 2:  Notice the red freq counter pick-up loop to the right of the tank coil.

** Blow up the  bottom view pics with your enlarge button to get a better look at the wiring.**

T

Yaz 4 is now five years old. English Springer Spaniel.  What a great guy!

** Blow up the  bottom view pics with your enlarge button to get a better look at the wiring.**


Are you not entertained?  :-)

Commando lives.

Pic 3:  The empty slot next to the heatsink -  I plan to add a small fan to blow air thru the fins.  The forced airflow makes a huge difference on the heatsink, assuming the thermal resistance of the chip core is reasonably low enuff.

Pic 1:  That is a 6" high chassis. A lot can fit under there. I needed every square inch. I pulled out the freq counter so you can see that covered area better.


** Blow up the  bottom view pics with your enlarge button to get a better look at the wiring.**

Pic 1: A pretty picture of Commando at attention. All self-contained except for the external 2 watt drive from the universal station RF DDS and the 1 volt line audio processing.

Pic 2:  Another pretty Commando. He's getting ready for his 3 mile daily walk/run in the woods.

Except for any surprises with the awaiting twin TDA7293 audio project, we are nearing the end of construction and this thread.  I'll post the results of the overall testing in about a week or so when the parts arrive.  If it fails, then we will continue on until it works.

** And panel labels need to be made up.

Thanks to the many guys who helped this project along!  I will add a credits list near the end.  I see the thread hits are over 8,000, so there must be a bunch of interested builders out there.


T

No, Tom, the amplifier is not mine.  It is a product of Alum Rock Technology in San Jose, CA, the proprietor is Robert Potter.  According to info on the web (which may or may not be current data) he has sold seven units with the pair of Eimac 304-TLs, and one employing 75-TL bottles.  The price for the stereo 304-TL product borders very near 100K ($100,000.00).  Apparently they can afford to acquire the Eimac bottles!

I once considered building an audio amplifier with the 304-TL, the closest I came was the modulator for another pair of 304-TLs, push-pull in the RF final.  I don't have enough spares for those games!

Tom, you mentioned "these TDA and other "easy" solid state amplifiers ARE single ended amplifiers..." and while they have a single-ended output, they are really push-pull amplifiers with single-ended output.  They are actually called single-ended push-pull.  If they were not push-pull, they would be running extremely inefficient class-A, but in actuality they are running class-B, with just enough forward bias to minimize crossover distortion.  Solid state amplifiers have beem using this topology for over 60 years, and some of the "output-transformerless" amplifiers employing tubes (for example, the Futterman with 6AS7s) have been around since the 1940s.

The general idea is the use of a pair of output transistors, either bipolar or FET, in series.  The top transistor collector or drain connects to the positive supply rail, and the emitter or source to the speaker or mod transformer.  The bottom transistor collector or drain is connected to the speaker, while its emitter or source is tied to the negative supply rail.  By alternately forward biasing the top and bottom transistor, the positive and negative currents are produced in the output load device, relative to the ground, which is the return path for the positive and negative power supplies.  Thus you have push-pull output devices into a single-ended load.  Proper biasing and feedback maintains the output point at zero volts DC.

Now you may take a pair of these amplifiers, such as the TDA-7293, and parallel the output, thus increasing the current available, providing higher power output.  

Though I would NOT attempt it with the 7293 device, a pair of these single-ended push-pull amplifiers may be driven 180 degrees out of phase, and the output may be taken from the output terminals of both amplifiers, instead of one amplifier and the ground terminal.  This series output arrangement (referred to as "Bridge Mode") doubles the available output voltage, thus quadrupling the available power into the same load impedance, assuming the device is capable of providing the current demand without excessive dissipation.  The Safe Operating Area (SOA) of the 7293 precludes the use of this configuration, due primarily to the thermal resistance from the die to the heat sink. The thermal energy cannot be dissipated fast enough to allow this sort of abusive operation.

Many, if not most, of the solid state audio amplifiers of today are no longer analog.  They basically operate very similar to the PWM modulators we use for AM, and include an LC filter in the output to remove the switching artifacts.  This is how they can be made so small and inexpensively, requiring very little heat sink and minute power supplies.

So, if it turns out you are not satisfied with the dual TDA-7293 push-pull analog amplifier performance, I would suggest looking at available devices or modules providing high-efficiency audio output through PWM or PDM.  They will play well with your existing power supply and modulation transformer.

OK Rick,

Yes, you are correct about the TDA7293 being a p-p analog amplifier that requires a single ended configuration.  I was referring more about these application requirements and stated it wrong.  What impresses me about the chip is it is not a class D switching device for such a small size.  But we see that the thermal resistance is a drawback. The class D stuff never gets to that point heat-wise as the linear chip does.  I plan to be very careful with microphonic feedback this time around. My only TDA-7294 is also dead as a result.

FB on the difficulty of interfacing two TDAs in push pull.

T

T,
I haven't seen the underside of Commando in several weeks.  Holy cow! the "miles" of wire.  What an undertaking.  My hats off to you OM.  I really love how you fit stuff in and make it work ie, the little variac with long shaft.  What is in the white tube surrounded by the coil? 

Tom,

It speaks //volumes// that your mod xformer is twice the size of your HV transformer!

Great looking rig.  Hope to hear it one day from the mighty blue waters.

--Shane
WP2ASS / ex KD6VXI

Bob:   Yep, there's a lot of wires underneath.  I always thought a rig looked its best just after the parts are mounted, without wiring.  It's almost like looking at a person's face and then realizing how much intracity there is inside.


Shane:  I sometimes forget that we are using two power supplies. The RF final is powered by that 800VAC CT HV transformer - and the modulator has its own power transformer of +/- 40VAC with lots of amps.  So I suppose we could combine the two cores mentally and say we have a really big [show] -  a big cored transformer equivalent, maybe as big as the Hammond 1642SE.

[BTW]  Bob/W1RKW is working on the panel labels with his label machine.  That shud put on the final touches.  I also want to get a brass engraved plate reading  "Commando."

T

UPDATE: 4-15-2022:

Labels and Brass!

Bob/ W1RKW sent me some FB labels.  I need to redo a number of them, cuz the camera flash shows the imperfections of repositioning after I make an error. Some look good. They really should be applied once and scrapped if they come out crooked - not salvaged. Clean the surface with alcohol and try again. TWEEZERS are the key for everything. Clean hands...dirty fingers are NG! Live and learn.   I gotta buy one of these label machines.

But anyway, the "Commando" brass plate arrived and I drilled and tapped the panel with 4-40s to secure it. It's hard to get the proper camera light to show its black lettering on brass.  But it's beautiful...satin brass, like a trophy.

I also decided to do 160, 75 and 40M "presets."   Pretuned marks so that I can change bands and have a full 125 watt carrier in about 15 seconds.  

Next, the Chinese TDA 7293 dual chip 140 watt audio kit board will arrive and I'll get back into the testing mode again.  If that all goes well, the rig is essentially finished, except for inevitable modifications.

T


** Pic 2 low lighting really brings out the brass trim and plate... **   (I need a tripod badly)

Commando, get ready for service...

My memory is too short not to use labels and preset markings.... ;)    [15 seconds to change band with 125W full power  - no brainer procedure]

The brass plate looks excellent. Nice touch!

Really classy.

Now that's the two nicest comments I've ever received about a homebrew rig... TNX!

The camera's extremely bright flash brings out insignificant problems. Glare and washout.  Rigs always look better in subdued  regular room light with the naked eye. I just ordered a 72" tripod for steadiness and longer exposures without flash.   Watch the improvement in the future.

Still awaiting the dual [analog] TDA7293 audio power chip kit board.   Here's the rough schematic of two TDA7293s in parallel.... 140 watts.    Rick will be doing a software version soon. He has been doing some research and plans to test a dual chip version out soon. We'll let ya know.


* For those who already have TDA7293 single chip boards, there's no reason why combining [in parallel with mods] two SINGLE chip boards will not work with this schematic. Though, just buying a dual kit board is easier. I have not seen any dual TDA7293 assembled boards yet.

T

Commando's artwork is finished....

Bob/W1RKW sent me the remaining labels to finish up Commando and add some presets to the other rigs.

Check out the new larger Plate Loading, Plate Tuning, fan variac and band presets. Easier to see.  Also the big "Commando"  label lying flat in the tube window.

T


** The camera's extremely bright flash brings out insignificant problems. Glare and washout.  Rigs always look better in subdued  regular room light with the naked eye. I just ordered a 72" tripod for steadiness and longer exposures without flash.   Watch the improvement in the future.

I'm no photography wiz but I wonder if reflection off of the shiny parts messes with exposure to some degree. How one would adjust or compensate for that I do not know other than trial and error.  Maybe a polarizing filter on the flash and/or lens to reduce glare would do the trick. 

That rig looks ballsey and very stout.


The meters, control knobs, window and the labels dare one, no Demand one to to sock Atomic Chainsaw Yeah-A-Lows and thereby Command the aether.

Very fine work.

klc

Thanks for the comments Kevin!   Yep, I think you coined it... Commando might be considered a ballsey and stout looking rig. Reminds me of a big frog, ready to leap... or even a face like Ernest Borgnine.   Hopefully he can take rapid, punishing chainsaw yallos...  ;D ;D  I'll use that description later on when on the air with him.

Bob:  Yes, I'll experiment with artificial lighting as well as daylight to see what works. Even a polarizing filter may help. Right now, no matter how well I hold the camera, without the flash I get a tiny wobble that shows up as blurriness. The tripod will help a lot.  Your labels worked very FB. I'm now putting them on the older rigs as band presets.


More TDA7293 details:    (The Commando 140W modulator)

Rick emailed me some more great info about mounting the TDA7293s.  The kit boards arrived yesterday via USPS from China and went together easily as shown in the pic below.  He suggested a bracket to hold the center of the chips onto the heatsink with better even pressure. The normal chip screw is off center, favoring the top of the chip.  The bracket cannot touch any part of the PCB board or parts. It is grounded into the heatsink and sits across both the plastic TDA chip covers to give an even downward pressure. Better heatsinking using the bracket.  The three screws all need to be torqued evenly.

** The chip has -VCC (-45V) on the metal tab, so needs to be insulated from the heatsink.


I also ordered some nylon screws and sil pads to better custom fit the chips.  The TO220 silpads are too small.   I need to cut a clean screw hole into the silpad. Anyone know how to do this? I suppose a paper punch wud work.

So in summary, to mount I will use a nylon screw, washer , then the chip -  and silpad into the heatsink.  Then on top a small bracket with a screw in the middle to add a more even pressure.  No paste needed.

Hopefully everything will work when the nylon screws and silpads arrive from Mouser. I hope I didn't damage the chips during installation.  Rick gave me some refined installation precautions after I did the job.. I will post them here for those who are building a TDA module.

I plan to add a small 2.5" 120VAC muffin fan next to the heatsink blowing air thru the fins. It will be powered off the same Variac that feeds the 6146B fan. At 20-30% spin rate both should be very quiet and effective.


I see there are no more dual TDA kits or assembled available. But there are THREE chips in parallel TDA boards for about $30 assembled on eBay.

T


Pic #1 below:  Two dual TDA7293 KIT boards (one as a spare) almost ready for testing into a 10 ohm dummy load, showing the loose holding bracket.  (140 watts)    (The two outer screws will be nylon - the center holding the bracket will be stainless; all 6-32)

Valuable info from Rick / W8KHK  about mounting and installation of power chips.  (TDA 7293)

T


------------------
Written by Rick and posted with his permission as follows:


Hi Tom,

Good news on receiving the kits.

They normally do not provide mounting hardware if the kit comes without heat sinks.  The mounting is usually done with a machine screw ( i prefer stainless steel) and a mica insulator or sil pad.  To insulate the tab on the IC from the screw head, a nylon or teflon washer with a flange is used between the bolt and the IC mounting tab, thus insulating the screw heads from the  tab, which is at minus 45 ( on the VEE bus).

The sequence I prefer is the screw head, flat washer (to allow sufficient mounting pressure) nylon flange insulating washer, IC tab, si lpad or mica, then the heat sink.

If you can get enough pressure with a nylon screw, give it a try.  Use the largest screw that will fit through the tab hole.  Heat sink compound normally is not used with sil pads, only with mica insulators.  Most of my experience is with mica and heat sink compound.

The device has overload AND thermal shutdown, so as long as you do not do full-power tests very long, you should be ok with a sil pad and nylon screw.  Use a flat washer to allow the screw to be tightened with minimal turning friction on the screw head, with the goal of making maximum pressure on the tab-to-heatsink junction without breaking the screw.

Often that type of tab transistor or IC is held down to the sil pad with a bracket that rests on the top of tie IC body, with a screw to the heatsink on either side of the chip.  This allows much more complete contact and thermal transfer.  

I have seen (and extracted) these types of brackets from used PC power supplies.  They attach the switching transistors to the heat sink metal bracket, which is usually just a plate of aluminum.   You might be able to make the same type of bracket with a scrap of thick aluminum.  After mounting the devices with the nylon screw, place a piece of aluminum across BOTH devices, and with a hole in the center of the aluminum strip, run a hole with a 6-32 or 8-32 machine screw into a tapped hole in the heat sink.  This method provides even better heat transfer than just a screw at the top of the tab (which only applies contact pressure at the top end of the device, not the best by any means.)
Hope you have success in your tests.  

---------

 It is really better to use ESD protection but you'll probably get lucky this time without a problem. When I attach heat-sinked devices to a PC board, I normally attach them to the heat sink first and then solder one connection on each device.   Then i make sure everything is aligned.  I solder the other terminals alternately to give it time to cool. One of the things we need to keep in mind is that even though these devices look very strong, they are susceptible to damage if too much strain or stress is put on the individual leads, especially when mounting to a heat sink.

 Another thing to try might be to use metal instead of plastic screws but insulate them in a different manner. If you have some nylon or plastic or fiber flat washers to insulate the screw head from the device, it's possible insulate the screw threads from the hole in the device with a piece of sleeve, this is often done with nylon or Teflon but you might consider a short piece of shrink sleeving over the threaded portion of the bolt along with just a flat washer to allow you to still use metallic grounding screws for the devices.


----------------
 I saw your post with the photo of the proposed bracket. Looks VY FB, thick aluminum bar.  As long as it is thick enough so it does not bend or distort, it should apply very even pressure to the tops of both chips.  Just do NOT tighten it so much that it bends, snug is fine, heavy duty torquing is not FB.

Yes, you may post my comments, and even the photos of the PC power supply sample if you believe they would be helpful.

That chip is pretty well protected internally, I doubt very much if you damaged it with ESD or soldering temp.  Just remember in the future, to solder one lead, let it cool, then do the next one, etc.  But if it was wave-soldered in a manufacturing plant, it would probably be hotter than what you caused with your iron.  No worries!

I would be VERY CURIOUS to see how warm that nice heatsink gets WITHOUT the fan.  It has lots of surface area.  If it is oriented so air can flow upward through the fins, maybe a fan is not needed.  The amplifier should be very efficient, ans with the voice duty cycle, it should not get very hot.  Steady-state audio will warm it up, but voice should not be a problem, even with moderate compression.

GL on the RF TESTING!

PS, if you have some sleeve and non-metallic washers to insulate the machine screw from the tab, I would use a metallic screw, instead of nylon.  But with the clamp it may not matter.  Notice that on the PC sample, they used one big sil pad, and NO SCREWS in the tab, just the metal clamp.  But then again, the devices and the heat sink were all secured to the same PC board.

As long as your interconnect wiring does not put a strain on the board position, you should be good just mounting it the chip leads holding the board.  But for mechanical rigidity, I would prefer an angle bracket holding the PC board stable in relation to the heat sink.  You do not want vibration or bending forces constantly moving the position of the chips and board, held only by the chip wires.  With a double row of pins, they should be rather strong, but the bending point where the leads enter the chip is the failure location if stressed regularly.

-------------------

you're experiencing a slower shutter speed in low light conditions.  Camera is trying to compensate but can only do so much. I've experienced the same and I've tried messing shutter and aperture priorities and ISO to override it but it doesn't help really.  the tripod will definitely take care of the blur in low light.  Glad the labels are worked out.  Did the white on black work out good too?

Hi Bob,

Finally!  The tripod arrived and what a difference it makes. The following samples were all made with a single lamp for light, no flash.  Steady tripod mount with no shake. Some shots took up to a full second to shutter. It is much more controllable and what you see if what you get.  I believe the F-stop and / or shutter speed is automatically controlled. The camera stays on depending upon the light available.

The first pics are standard size while the last ones are resized larger and show the beginnings of graininess.  Check out the closeup of the Commando brass plate. There was no way I could get a closeup like that before due to flash glare and unfocused light. With the tripod it's a cinch to get good quality. I will continue to experiment and hopefully my pics are much better in the future.

The  light balance across the whole picture allows better detail and also eliminates the harsh imperfections that are not visible in normal room light conditions.

T

Pic #1:  Notice how easy it is now to get all the gold/brass to come out and shine!

Pic #3:  "YAZ'  is a single 813 series modulated by a pair of 813s.  No audio transformers. All have Plexiglas cases. There is little glare on the Plexi now. It was brutal before.   All rigs use the GFZ MOSFET audio driver board.

*** NEW DISCOVERY***   Click on the pic below and get a 1/4 page sized pic.  But click on the blue fie name at the bottom of each pic and get a full page pic that can be enlarged using your "+" icon or mouse roller. You must click on the file icon that appears at the bottom of your screen to activate the full screen feature..  Use the mouse roller to enlarge.  I never knew about that feature. The enlarged pics are really something else.

Pic #1 is an enlarged pic of the brass plate. Impossible to get before... I tried many times.


Pic #3 has reasonably balanced color and light.. about what I see in person.  One of the best front-panel Commando pictures taken to date.


*** NEW DISCOVERY***   Click on the pic below and get a 1/4 page sized pic.  But click on the blue fie name at the bottom of each pic and get a full page pic that can be enlarged using your "+" icon or mouse roller. You must click on the file icon that appears at the bottom of your screen to activate the full screen feature..  Use the mouse roller to enlarge.  I never knew about that feature. The enlarged pics are really something else.

Check out pic #3:    With the flash before, the details were very harsh and beat up looking. This is more what it looks like in natural light with the human eye to me.  I was relying on a flash to make a fast shutter speed to prevent camera shake.  Now the light is controlled before the shot and the longer exposure is steady in the tripod..

T


Pic #1:  813s modulated by a pair of 813s. The best shot I ever made of it.  

Pic #2:  Single 3-500Z linear with 1500V - super clean linear, -45 dB 3rd.  Great driver.

These older rigs are now getting the pre-tune label mark ups for each band.

This should be enuff pics to see the difference.  

This pic has been enlarged a bit. Notice the graininess.  Just experimenting.

Live and learn.

T

Tom,
Those pictures look excellent and the rigs are sweet looking.  yes, tripod in natural room light is the ticket. stability makes all the difference in sharpness.   

I did an online comparison of xenon flash vs nature light and xenon apparently is absent of orange, yellow and some red portions of the light spectrum so blue, green and even UV are the main color components of a white xenon flash where natural light has very little loss of color spectra.  I figured xenon covered all the colors giving it its whiteness and brilliance but apparently not.  And camera sensors are sensitive to UV too. All of which might explain that harsh look.

Hi Bob,

FB on the lighting. It's hard to beat an incandescent lightbulb.


I found out two more things:  Most of the images I posted in the last group are only 640 X 480.  So they cannot enlarge much without graininess.  But the one Commando brass plate pic is 1920 X 1440.  I'm not sure why the others are not too.  

Also, if you click on the photo .jpg picture link and then right click on the pic, there is a selection called "file information."   It shows the pic size and the shutter speed, f-stop and mm size.  For example one pic shows:  640 X 480     1/2 sec   f/2.8   6mm.    That 1/2 sec and f/2.8 are indicative of a close up shot.  I just need to get the camera to do 1920 X 1440 next time to allow exquisite enlargements.


You axed about the black on white labels for white panels use... they worked out very well.  The white background blended in FB. I'm working on them now. I need to fire up every rig in the shack on every band to find the best presets. Lots of work but valuable info for jumping on a rig and getting tuned up at random.

T

Tom,
the only thing I see when I right click is image size and file size.  I can see all those parameters in Photoshop though if I download the file.  must be something unique to your system.  Not sure if you see this in AMFone, but the file size and image size is displayed next to the filename.  What is interesting is that your image size numbers are indeed different.  Back on page 7 image size is 1280x960 for most and on page 8 they vary to something smaller as well as the 640x480 size.  I can't think of a reason why a camera would change that on the fly but maybe some cameras do that to speed up write time. Just a guess but it would explain graininess when enlarging.  I don't think my Nikon does that. Of course it has so many features, functions, menus and buttons it is impossible to know and keep straight.  But at least you nailed getting pix the way you want them to look or should look.  
 

Glad the black on white labels match up nice. I have black on clear now too.  

Different subject.  Your plexiglass cabinets, how do you get nice bends for the corners on such a large piece of plexi and have everything line up?

Yo Bob,

The Plexiglass bending is all done on a table edge using a regular propane plumber's torch.   First I measure the width, length and height of the proposed cover area.  I add about 1/2" for each bend to curve properly and use up some material.   So if the rig panel is 19" and I need the Plexiglas to fit around it, I will add 1/2" plus another  1/2"  for the two bends making the cut 20".    I then use the edge of a table to let the Plexiglass hang over exactly on the + / -   19.5" distance for each bend.    I make the Plexi sheet  a big 'U'  -  with no covering in the front or rear of the cover.

To bend with the propane heat, always keep the torch moving on the bend line. Heat BOTH sides evenly.  Too much heat and it will make internal bubbles and a white opaque.  If you mess up, you can always tape it off and spray aluminum colored paint to simulate 1" aluminum rails.

The Plexi will heat up after 1 minute or so of heat and naturally fall (with a slight push from you) and make a nice 90 degree angle bend. The table edge will dictate where it bends.  Before it cools, eyeball it to be sure it cools at exactly 90 degrees and hold it steady for a couple minutes.  Be sure to apply torch heat evenly and keep moving.. watch out for the opaque look of overheating at all times.  The edges are very prone to this overheating.

Another method is to use 1" aluminum angle  with 6-32 nuts and bolts  to hold it together. They both look good if done right.

I like the Plexi for my smaller rigs just to look at them every day I walk by. When in a rack it's easy to forget your work inside.  For the bigger rigs, I use metal cabinets or racks for RFI, no Plexi anymore.

I usually let the rear of the Plexi cover overhang a few inches to better protect the parts in the back of the rig.  I like clear Plexi - the tinted stuff may look good but is harder to see thru.


T

FB on the plexi.  I'll keep that in mind for the future.  I've wondered about how to do big sheets and get the sides to line up.  

I'm going to do some soldering tonight. Got a bunch of parts in this afternoon for the GFZ PS board.  Some of what I ordered before didn't quite fit correctly.  These should finish the job.  then it's time to plan fitting things into the 813 rig. Got some heat sink tapping to do too.  Baby steps.  

Thanks.
rkw

Update 4-25-2022:

The TDA7293 dual chip kit boards were a success. No more parasitics.  I had my doubts that I would be able to successfully build this tiny board but it works!  I used Mouser  chips and the dual board from China.  On the bench the new board was putting out a measured  200 watts into a power resistor at +/- 45 volts.    I also have the rig making 200%+  modulation peaks using tones.  The heatsink gets warm but not too bad unless I do a sustained tone at heavy modulation for 1-2 minutes.

Now here's the problem...   I have another rig using a 4D32 in class C plate modulated by the same model transformer, 1642SE.   It runs very quiet with no talk back even with high mic gain levels.  But this Hammond transformer in Commando and the twin TDA7293s  sings like a bird at about 2 KHz. The same thing happened with the last two amplifiers. Acoustic feedback between the mic and 1642SE audio transformer.

It does a sustained howl at about 2KHz with as low as 30% mic modulation level.  If I unplug the mic it goes away completely. If I push on the chassis around where the transformer sits, it comes and goes in resonance, as if it's using the chassis as a sounding box.  So looks like I will need to mount it on rubber. But what else can I do?  How about a soaking it in tar or taking it apart for a treatment of some kind?  I already tightened the four bolts with no chage in sound level.

I tried wrapping it in foam sound insulation stuff, but it barely had an effect.  It will be located about 7' from the mic when in operation. At 4' away it is out of control.  The mic is directional, but does not help; the sound seems to be bouncing off the ceilings and walls.

I might have to call Hammond and see what they have to say. The point that I have another identical transformer working well in basically the same job is a strong point. At this point the rig works really well, but impossible to use because of the feedback.

I've had various rigs with talk back in the past that were curable. Most fed back at 90 to 100% mic level, which can be handled. But this feedback is so strong and fixed at low mic levels, it's going to require something clever to fix it.

Any ideas?

T

kool news ! so no RC circuit between the bridged output and the 8ohm winding of the mod xfmr ? just a fuse on the negative lead ?

Peter

Kudos on successful high-level modulation with the new amplifier module!

If the transformer laminations are tight  (as you confirmed) there must be something else loose that is allowed to vibrate and cause the talk-back.  My first guess would be the bobbin may not be securely affixed to the EI laminations.  Perhaps it might help if something non-metallic is wedged between the core and the bobbin.  Wedging it so that it cannot move side-to-side should be easy, but it may be loose between the bobbin and the inside section of the EI core, and this would be a bit more difficult to secure with the laminations in place.  This might be a question for the manufacturer to answer.  (Can I hear a free replacement coming in the mail?)

You are experiencing the talk-back with the plate and screen current of the trio of 6146s through what you are using as a secondary of the modulation transformer.  Perhaps this DC bias on the winding is exacerbating the talk-back problem.  To verify or exclude this theory, would it be possible to place a fixed resistor load on the secondary, and run the modulator into this load with no plate voltage on the final amplifier?  If the talk-back persists, then the DC bias is not the culprit, but if this test reduces or eliminates the talk-back, then a modulation reactor might help.  But unfortunately that device would be BIG and HEAVY, there is no room for it in Commando.

Regarding Peter's comment (KC2ZFA) there should be no need for any DC blocking capacitor, or other RC network between the output of the amplifier and the low-impedance primary of the modulation transformer.  The PC  board includes a high-frequency RC filter at the output terminals.  The outputs of the two amplifiers are not bridged, they are paralleled in what is called "modular" configuration.  With the DC and AC feedback into the amplifiers, the output should not have any significant DC offset.  The amplifier current consumption at idle should be about the same whether or not the transformer is connected.  If the current rises, or the resting power supply voltage sags when the transformer is connected, then suspect a DC offset, and look at the DC feedback components for resolution.  Leakage of the capacitor in the feedback loop could cause a DC offset.  This is highly unlikely though.

Hi Tom,

One power supply company I worked at many years ago could pull the top of the lacquer container into a vacuum. This pulled the air out of the transformer pretty quick.  Yes, talk to the mfr.  Worst case you have to remove the transformer, send it back, and they do the super-duper dunk and vacuum pull and real good bake.

GL OM.

Very good info, Rick - thanks!

I pulled the Hammond  out today and will run the tests you described with clip leads. I also want to sit it on foam rubber and see if the talk back is reduced - showing if there is some sound box acoustic amplification going on or not.

I will try the DC offset test and transformer current test you described.

The good news:  I called Hammond  today for some advice. They are shipping me a replacement, free of charge incl shipping. What a great company.  A friend suggested that to fix it might require an elaborate procedure including stripping out the old varnish and using a vacuum process. Tough to get at stuff once potted. So they took the easier way out.
Hammond has always been a first class company. I remember I used to talk with Mr. Hammond on 75M. He was a generous guy and helped out his ham buddies. One night I called CQ on my 4-1000A plate modulated rig and he came back using a broadcast rig. More tall ships broke in. He was a humble guy and you would not know who he was.

I will compare the new xfmr when it arrives using clip leads.  BTW, I tried Summer Breeze again last night (single 4D32 class C plate modulated with same Hammond model) and it was quiet as a church mouse, even at really high mic levels.  

Commando now has much higher  audio peaks [tones] than SB as a result of the new TDA 7293 dual amplifier. That thing really plays. Again, no parasitics or anomalies as before with the "7294."    I may put the spare dual TDA into Summer Breeze and make it self contained.

One odd thing I noticed... when the TDA sits in idle (and rig unkeyed) it starts to get warmer than it does when in operation. I have to keep it below +/- 35V or so. At 40-45V it runs FB but during idle the heatsink and whole area gets too warm. I'm wondering if the disable function might be needed with a PTT relay.  I ran the voltage up to  +/- 50V and it really played. But I want to keep it at about 35-40V max at all times in the future. There appears to be plenty of headroom for the audio peaks at 125W carrier output.

Tom: We crossed posts... FB of the vacuum process.  It wud be nice to salvage it eventually. We will have to see if the new one solves the problem first. Worst case is the bad one gets used in a sound proof compartment.

T

** BTW, spent part of the day outside building up a pair of stacked wire dipoles for 40M.  Wanna get on that band with the new AM rigs.  Stacked at 130' and 65' high, in phase / out of phase switched for high angle local or low angle USA DX.  Gotta climb up there to put in the ropes and pulleys between towers. Also need to fix my damaged 75M delta loops.  Antenna work is fun but unnerving to climb.

** Peter:  Yes, that's correct... I'm going directly into the 8 ohm  winding. No network needed so far.  Just a fuse, 8A, is the protection for the 8 ohm winding.   5A popped under normal use, so 8A  it is.   If it gets unstable, I will try the 16 ohms instead. But so far FB.     Bridging and 4 ohms has always given me trouble, so I stay clear.   There is plenty of audio power available now.  The parallel TDA technique seems better.  There is a four parallel TDA board available for those of you more adventurous to experiment with bigger iron..

Rick,

I ran the tests you suggested:

The audio  +/- power supply current does not change when the module gets connected to the 8 ohms winding.  So the DC offset is very low.

The transformer is just as loud sitting on the table with foam rubber underneath.  So, the sound box amplification is invalid. A noisy xfmr itself.

 I connected a 5K load across the xfmr 5K winding and keyed it without 6146 HV.  The talk back at 2KHz was almost as loud. I could see the stunted audio on the scope as I talked  due to the audio HV on the tubes.  So the DC current is not the major problem here.  The load resistor got quite warm to the touch in 30 seconds.

The talk back microphonic barely changed in frequency in all the tests, all about 2KHz.

So it appears to be a bad xfmr; something loose inside.  I peeked into one of the xfmr holes and can see a black looking, solidified, shiny urethane material. Looks like it is potted and would be very difficult to pull apart without destroying the windings, etc.  Maybe a heating in the oven for a few hours would help. I might axe Hammond their opinion later on.   No sense wasting a potentially good xfmr.  I also wonder if put in a soundproof box, would the loud squealing / vibration adversely bring on audio distortion in a sonic manner?

An experienced xfmr friend commented:  "The problem with your transformer is the center laminations of the E stack. They are free floating inside the winding bobbin. The bolts are too far away to make a difference. Jamming wedges in the windings can damage the transformer if you are not careful and go too tight. It is likely there is an air pocket in the stack so the laminations can vibrate. Pulling a vacuum on the transformer may not work because the void is likely sealed from the outside world so new varnish can't get into the void even under vacuum. This is the reason it will likely need to come apart."


I'll post again after the replacement is tested.

T

Update 5-2-2022:

From Commando Headquarters:

The replacement 1642SE Hammond transformer arrived.  I installed it and the talk back was much less.  I'm able to run the audio gain up to about 90% of normal before it takes off. Once in the cabinet and relocated, it should be fine.  So the original xfmr WAS faulty.

Rick, W8KHK has been giving me his calculations and ideas about getting the TDA7293 and the general rig working well. He's been a great help. I finally settled on two 6146s in the final and have optimized the screen dropping, grid leak and general parameters. It is doing about 100 watts  carrier output at about 140% positive peak modulation headroom according to the REA mod monitor.  Between 75 to 100 watts is where I will probably run it barefoot.

I ran into problems with heat from the TDA7293 module. I blew out my first module. It just stopped working when I first turned it on after a test session the previous day. Even with voice programming, the heatsink and two chips were getting hot.  I ended up with the best solution: I added a 2.5" X 3" X 3/8" polished copper plate as a heat spreader.  The TDA7293 is white pasted and bolted with tapped 6-32 steel screws directly to the copper plate. The copper is white thermal pasted and bolted with SIX balanced tapped 6-32 steel screws directly to the aluminum heatsink.  The whole structure is floated on Plexiglas because the chip tab and now the whole heatsink sits at -40V.   No sil pads, shoulder washers or other gimmicks. What a big difference in chip heat transfer this made!  I think it will survive this time. The chips now stay lukewarm with the heatsinks and dissipates the heat quickly. I am adding a super quiet 3" 120VAC fan to blow air thru the fins in case of old buzzard transmissions.

At 120 watts carrier output, I am up against the limits of the dual chip module to make 120% positive.  Even the +/- 40V supply sags a little. I may need to do something about that. Maybe a bigger bleeder resistor to pull it down from peak voltage.

The Hammond transformer is handling it well at about 220 mA plate current.  The grid is 5K GL, -95V total GL and fixed bias, screen 15 Ma at 145V using 30K  25 watt dropping resistor,  HV usually run at  600 - 750V on a Variac.

But all in all, when I run Commando at 75-100 watts, there is ample headroom and a very robust sound in the monitor. Very clean with audio response from 20 Hz to 20 KHz no problem.

If I were to do it again I would probably get the three parallel TDA module for a little extra thermal conservative safety and peak audio headroom. It's really about one chip per 6146 required. (at least)   This chip needs special heatsinking care to keep the heat from destroying it. Even short transmissions could kill it when not using a copper spreader and pushing it to its specification limits..

I got the presets on the panel all set for rapid band changing.  There's not a lot more to do but install the fan when it arrives and get on the air with Commando!  I might test it into the 4X1s linear amp soon too.  At 30 watts carrier drive the audio headroom will rock the socks off Rico Suave.

I'll take some pics of the new (and greatly improved)  TDA7293 heatsinking array soon.

T

Update 5-6-2022:

Commando  is finally working as planned. Tested and beat on.  I took some pictures of the TDA 7293 audio module, polished copper spreader, heatsink and fan assembly. The fan is so quiet you wouldn't know it's on - but it made all the difference in cooling!  I can make an old buzzard series of atomic yallos and really slam the audio -  and the heatsink / chips don't even get luke warm now.  The fan blows air thru the aluminum fins and its speed is controlled by the same Variac that controls the audio +/- 46V supply. IE, the higher I raise up the audio HV, the faster the fan spins.

The biggest cooling difference was made by using the 3/8" thick copper spreader and the fan.   Before that the chips didn't last for more than a day or so. One OB xmission and it was all over when using just the aluminum heatsink alone.

The audio is very flat and clean from ~10 Hz to 20 KHz ++.   There is no cross over or peak distortion that I can see.   The SDR spectrum scope looks clean and the harmonic distortion is a little better than most of my all-tube rigs.

I tried some things to get the audio HV regulation improved, but nothing really worked. I tried bigger caps and bleeders, NG.  The only solution may be normal regulation MOSFETs or a capacitor multiplier. Also, there are some +/-  45V /50V  8A switched power supplies on eBay for $25 I might try too.  But I see no signs of distortion or flat topping under normal use or testing.

I plan to seek out some on air contacts soon, so give me a call if ya hear me on.

T

Pic #1:  Notice all surfaces use only white thermal paste for direct contact. No sil pads, mica, shoulder washers, etc.  Everything is easily accessible. I can change out a TDA module in a few minutes.

Pic #2:  The fan pushes air directly into the fins. There is no way out except forced air out thru the top of the fins.  Decent breeze.

Pic #3: Everything in the entire rig will fit under the Plexiglas enclosure, yet to be made.  The front panel sets the height of everything in the rig.

Pic #2 shows the TDA7293 module, copper spreader and heatsink all floating on Plexiglas. The chip tabs and all are sitting at -46V.

I still need to neaten up the TDA installation. It is still in the testing / optimization mode right now.

The rig has been very reliable; other than a TDA module failure last week.   Easy 100 watt carrier; 150% modulation peaks headroom.  DC to light audio. Who could axe for more?

T


Pic #1:  It's getting mighty crowded in here!   Oh, I've been asked... what is that white thing holding the coil?  That's the 160M coil add-on supported by a white ceramic post / pillar.

Pic #2 is a good look at the TDA7293; in this case a DIY kit using two chips for 140W of audio.  It will modulate the heck out of the two 6146s.  Three 6146s are just too much for the Hammond xfmr (150 W carrier) and would require a 3 or 4 chip module to fully modulate up to the audio peaks that I require.  But with two tubes, the Hammond 1642SE xfmr shows no signs of warmth and handles the additional 220 mA of DC plate current.  Possibly the main risk here is the 5K winding of the Hammond. There can be as much as 2KV on it during heavy modulation peaks. It is hi-pot tested at 3500V.  Yes, we ARE on the edge of wetness as usual.  :-)

Update 5-17-2022:

I blew up a second TDA7293 audio module.  Working fine until I dropped it by accident into the Variac hot leads and bam!  Rick sold me his two TDA kit modules and I built up two more. It's all working again with one module for backup just in case.  My bad.

** The TDA module continues to run cool to the touch after normal voice transmissions. I'm actually surprised. I think it is a very efficient way of modulation. In contrast, even running class C, the two 6146s throw off a lot more heat than the modulator. But all said, Commando is a very cool running rig for 100 watts of carrier and 150% audio positive peaks. It is a great daily driver, especially in the hot summertime.

Commando was performing perfectly EXCEPT for a persistent 60Hz AC hum on the carrier. I tried many things to fix it with no luck.

I finally located the problem and I must admit it was thru trial and error. In hindsight it was an obvious problem, as always.

The TDA7293 audio module and heatsink assembly were sitting directly on top of the +/- 46V HV power transformer which powers the audio. I was running out of room!  I relocated the TDA 1 1/2" ABOVE the xfmr on plexiglas and the hum is completely gone. Now below ambient room noise and birds chirping outside.  Evidently 60 Hz magnetic coupling will go thru aluminum and even steel.  The square function of moving it away just 1 1/2" did the trick.  I should have known better but I had to try all the other stuff first, I guess.

So Commando is about ready to be moved into its place of honor within the harem soon.


***  So for the last few weeks I've switched into the NEC antenna modeling and tower climbing mode; fixed the 160M dipole at 190', adding a new pair of stacked dipoles at 130'/65'  for 40M, rebuilding the 75M delta loops at 190' using #10 copper wire openwire tuning stubs and feedline  for full tuning ability from the ground...  No OWL spacers, just tightly stretched between tower and ground.  

And, this is exciting: Inspired by the possibility of a good solar cycle, I'm adding a driven, broadband, open wire fed, 10-20M pair of aluminum tubing stacked lazy H's at  25'/ 50'/ 75'/ 100'.  (effectively eight 1/2 wave elements on 10M)   Lots of new ants to play with soon.  I mean WTF - might as well have some fun.

T

mu-metal? 

mu-metal?  



That sounds like a great idea, Bob!   It would further reduce the tiny residual hum that probably still exists.

I did some reading and see it really does work, needs no grounding but is crazy expensive. I could use a 4" X 6" piece, 0.5mm thick or better.  

Krell metal also works, but is about $1 trillion/ gram and forbidden, so I'll stick with mu metal.

I see small beat up plates going for $40+ on eBay. Amazing.

Supposedly some old scopes have a mu metal plate shielding the rear of the CRT.

Anyone have some scrap, please let me know.

T

Tom, a lesser expensive option might be copper.  I think in some instances it might be an effective magnetic shield but it may be better suited for RF. I've seen transformers with copper in them so it may be possible.  Frank might have a better feel for this than me though.