NEW Project - 6146B Plate modulated by Hammond 1628SEA and 85W SS Amp

[KC2ZFA]

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

[N4LTA]

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

[K1JJ]

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

[N4LTA]

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

[K1JJ]

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

[KC2ZFA]

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.

[K8DI]

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

[K1JJ]

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)?

[K1JJ]

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.

[KC2ZFA]

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

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

[W3SLK]

Who doesn't like a Nash Metro??

[K1JJ]

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

[Chuck...K1KW]

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.

[w8khk]

snip....
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.
...snip

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.

[K1JJ]

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.

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!

[w8khk]

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.

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.

[W1RKW]

 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. 

[K8DI]

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

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

[WD5JKO]

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

[K1JJ]

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:

[K1JJ]

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.

[K1JJ]

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

[w8khk]

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!

[KB2WIG]

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

[K8DI]

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