AM Power Amp Exciter

[W9ZSL]

I'm still dinging around with designing an AM amp.  I started with the 4-125A then considered a 4-65A.  I bought a Stancor 10 VCT 5 Amp filament transformer last week for a song so going with a tried and true single 813 modulated with a pair of 811A tubes wins.

I could build an exciter but would rather use a DX-20.  At this point in time if I don't build this damn thing I never will.  I have a couple amp designs in mind both of which are ARRL standards.  I've outlined all the various power supply options I have in previous posts.  I can do 2KVDC at 300 MA on the final max.  No, I'm not going to use 2 x 813s.

Back to the DX-20.  That transmitter has more than enough drive so what do you suggest to match it with an 813 running at around 300 watts input?  Yes, the amp would have a tuned input.  

[W3GMS]

Since the DX-20 output is 50 ohms, the grid network on the 813 could be a parallel resonated tapped coil for the different bands with a 50 ohm link over the coil.   Sometimes a swamping resistor across the parallel resonant coil helps as well.  Several examples of that are shown in the older ARRL handbooks. 

Have fun with the build!

73,
Joe, GMS     

[fg5fc]

Hello W9ZSL,

Something like that!

[W8VG]

ZSL:

Good luck on the 813 by 811A's.  I built same thing four years ago and am still running it today--no problems.  Is a great combo.

Concur with Joe, 50 ohm link over grid tank coil works great!  I'm driving mine with old recycled IC-730 mobile rig (replaced in car by smaller 706 years ago) and have it cranked down to just a couple watts.  813 doesn't take much drive and IC-730 is great digital VFO. 

With no HV/screen voltage on 813, I applied a tad of drive from the 730 (thru Drake W4 into RF deck) and just tapped up/down a turn or two on the grid side of the coil (re-peaking variable capacitor for max grid current each time) until SWR was minimized at band center.  Also have to be careful not to exceed max grid current for 813.  I put the IC-730 in the AM mode for this (runs less output than CW mode when cranked all the way down). 

The rig is band-switching 160-40 only and had no problems getting reflected power down to almost nothing on each band.  Works like a champ.  813 easily drives to 12-13 ma grid current with just a couple watts from 730.  Get about 190-195 watts of carrier out of 813 (at ~1500 VDC). 

My HV supply is good for 2KV at 400 ma but I run cranked down (via variac) to 1500V (not to exceed ratings of 811A's) and don't think I ever exceed 250-260 ma total current.  So 300 ma so do just fine. 

Modulator consists of solid state audio driver, into Antek toroid LV tranney (wired backwards as audio driver tranney), and the 811A's into Stancor 125w multimatch. 

Good luck and have fun homebrewing.  It's just as addictive as collecting boatanchors <hihi>.

73's Geo
Fullerton CA



 

[W9ZSL]

Hi Geo,

FB!  You wouldn't happen to have a schematic of that amp would you?  I'd like to take a look.  I have 3 different single 813 amps I'm considering and would like to keep things really simple.  As for an exciter, the DX-20 would be fun and super retro but I also have a Kenwood TS440 with auto tuner that runs 25 watts AM and the CW power is variable.

I also have a UTC 125 watt mod iron.  It's a VM-3 (CVM-3in an "S" style case).  I have a wide range of power supply options.  One Thordarson plate iron has dual hi/low primary with 4280 or 3480 CT which would give me about 1500 volts loaded on the "low voltage" taps.  The other Thordarson has 1 primary but 2 secondary voltages: 3120 and 2500 CT. Both are rated at 300 MA.

It would be great if I could get away with using the 1500 volts and use just one supply for the final and modulator but I'm not sure if I'd be pushing the envelope with only a 300 MA rating.

The modulator probably will be driven through a Kenyon 600 ohm balanced line to grids transformer by a limiter/compressor amp I built for my recording studio back in the late 60's.  It still works great!  In fact I used it for compressing a 5-piece brass section in a multi-track mix down I did yesterday.  I needed it so it wasn't buried by a 50-voice choir and pipe organ.  My ears were ringing and my mind was warped, but it sure came out great!  And yeah, when I got done it was Miller time!

Mike
W9ZSL

[W2VW]

Just for the files:

A TS-440 or similar with bipolar power amp will drift in output power as it warms up.

Can be annoying with stages following that need precise drive control.

Some newer ricers use fets which generally do not have the problem.

[W8VG]

Mike,

Great to hear--sounds like a plan.

No don't have schematic......started out as junkbox prototype designed on "back of old envelope" <hihi>.  I had no experience building CL C RF decks and wanted to experiment/prototype a single tube.  Ended up turning it into a rig anyways.  Had a bunch of 813's laying around and was just playing initially.   Used a surplus Bud chassis (oddball narrow size).  Goal was to eventually build something bigger.  Inspiration came from multiple sources:  K1JJ/ZEQ "tesla" design as starting point--modified for single tube.  Used RCA specs to set operating parameters, biasing, screen, etc.  Am using screen resistor on 813 consisting of 50w 25K adjustable (had laying around) in series with couple of 10 watt 10K wirewounds.  Think I'm using around 43K total in screen (from RCA spec sheet).  With one tube, got away with deriving bias from small RS filament transformer wired backwards into rectifier/filter, zener string.  Using combo of fixed and grid leak bias--no clamp. 

Dave, VW, is right on riceboxes.  My 730 tends to drift down a bit as it warms up.  I start out with 13 ma or grid drive when it's cold and after about 15 mins it will creep down to 10 or 11.  So have to plus it up occasionally.  Can be annoying if you have to get up and trek across the shack during a qso.  Bigger problem for me was just getting the drive down to couple watts.  Started out using the 730 in AM position but ended up going inside and cranking a pot that controls the lower power limit.  Only need a few watts to drive the 813 full bore. Not familiar with the 440 so you might start out with DX-20 and then experiment with riceboxes after you get it going.  Or build a nice digital VFO/driver.  Been looking at those too.  My 730 had been sitting around for years doing nothing so decided to use it as driver but gotta admit--is a bit of overkill.   Am not using the receiver in it--RF Gain is cranked all the way back.

300 ma could be on ragged edge.  I run RF deck around 165 ma of plate current and voice peaks don't seem to go higher than 100 ma on modulator (I used REA mod monitor--to get it up to 100 percent plus on positive).  But also have 813 screen, grid, and PS bleeder to account for.  I have MA meter on power supply so will check peaks next time I'm on and let you know.  You might try it with single power supply initially and see what happens.  Might be able to get away with it and save some of the cool iron for next project <hihi>.

I think my PS iron came from old paging xmtr--got it as package deal.  Is about 2KV at 400-450 MA so have headroom there...also came with nice 10H choke and bridge rectifier module--all rated same.  I'm using it with variac and can switch off the HV for tuning the grid. 

As you can probably tell, I like to experiment and have been known to produce plenty of smoke and blue flame <hihi>.  Used the K8KK tank-kit to design the PI net (he has great site) and wound tank coils myself--laying in final tank coil taps using an RF analyst.  Was amazed at my initial success on the RF deck but still ended up blowing some fuzes in process of turning it into a rig.  I'm good at w8vg@arrl.net if you ever want to compare notes.

Have fun and looking forward to hearing you on the air with it!

73's
Geo   

[W2VW]

Funny how the 730 drifted down in power and the TS-440 drifts up

Bet Icalm put some compensation in there and it works well at 35 watts maybe.........

I used a 730 for an exciter a long time back and don't remember it changing anywhere as much as the Kenmore. The 730 has a very fb shape factor ssb filter.

[W9ZSL]

Then again I could get REALLY retro and toss together a 6AG7 with a 6V6 for an exciter.  As for the 813, I have plenty of iron for separate screen and grid supplies.

[W2VW]

But then all the new guys would say you are off frequency  

[Tim WA1HnyLR]

Hi Mike,
Good deal on the 813 transmitter project . A few things to consider: The VM 3 is a bit shaky with the higher voltage. I would use your smaller plate transformer. You should get about 1200-1300 volts with full wave rectification w/choke inpoot filter. The 300 Ma rating is fine. The VM3 should be set up for full primary to full secondary.  Terms 7&12 PRI 9&10tied together is the CT. Sec 1&6 join3&4. This will allow full audio power that the 811s can produce to be able to modulate the 813 to 140% positive.  The RF outpoot should be in the order of 150 watts or so with the 813 loaded to 160-175Ma. This would be a conservative transmitter for sure. The typical screen dropping resistor should be in the order of 25K@100 watts for the lower plate voltage. I have such an 813 rig that I used to use. It is now in storage. It was built by some one else. It had many issues that I corrected. Someday it will return to the air. If you have the components to build a higher power RF deck you could build an 813 pressure cooker. Yep 2500-2700 volts. The tank components have to handle the power. You could start off with a very capable RF section and use your existing lower power modulator and power supply with the future plans to upgrade both the power supply and modulator. A 500 mod transformer like a CVM 5 would work well.Certain broadcast mod iron and mod reactors would be much better than communications grade iron.  You can use 811-As with -12 volts or so of bias to get the idle down to 30 Ma. They will take the voltage. What would be better would be a pair of 572-Bs in the modulator. In therory the 813 is rated @125 watts max dissipation with convection cooling. With forced air cooling, a chimney of sorts and proper air flow I betcha it would be good for 200 watts dissipation or so. You could extract a good 400+ watts of power from the tube. You could use a plate transformer with a 240V primary and operate it @ 120V for low power operation with better audio fidelity when you don't have to run it balls to the walls. The DX-20 would make a great exciter. The only thing is it does not do 160. I build all of my transmitter to work on 160. Typically I drive the 813 to about 15-18 Ma on the grid with a 10K grid leak. When in pressure cooker mode more drive is needed than book value. Hopefully I have planted a few seeds here. Tim WA1HnyLR

[W8VG]

Hey Timmy,
This has got the wheels turning. I have more headroom in power supply and have heard that the 811a (mine are chinese though) can run at higher plate voltage (think Johnson used them in the 500 that way). My fear has been pushing the mod iron. I mounted mod tranney on insulators but have resisted temptation to crank variac anywhere above 1500vdc. Have thought about going with heftier mod iron. Got a 300 watt stancor in garage that I was saving for rainy day but 600 watt version is probably unobtainium for me. Wouldn't mind getting more out of 813 deck. RF tank is very solid.
BTW I hear you out here from time to time coming through on 75 using ur phased array--making the trip.strip
73 Geo W8VG Fullerton Ca

[W9ZSL]

I would have no problem with two supplies.  Got every part I need including 2 Thordarson plate transformers, two matching 300 MA swinging and smoothing chokes and another pair of 200 MA chokes.  I can run up to 2KV @ 300 MA for the final and up to 1500 VDC at either 300 or 200 MA on the modulator. The restriction is the 125 watt UTC mod iron.  I have more than enough filter caps to do the job.  I actually can get my mitts on an ART-13 mod iron which has the additional screen winding.

Running an 813 at 1500 VDC and a modulator at either 1250 or 1500 VDC using separate supplies and the ART-13 mod iron sounds ideal.  Specs say 250 to 300 watts input under those conditions would be perfect.  I'm not restricted as far as RF components are concerned.  I've been collecting parts for the better part of 15 years in order to put together ultimate AM coolness!

[W9ZSL]

Gotcha Tim.  If I use the VM-3, 1250 volts to the 811A's makes perfect sense.  Likewise, 1500 to the 813 does the job. When and if I can score a 200 watt mod iron, all I would have to do is make a couple of changes to jack the final up to 2 KW and the mod to 1500 and drop in the larger mod transformer.  To heck with the ART-13 mod iron.  I know it can do about 150 watts, but I'd rather plan on 200 watts with 350 or so for the 813.  Sounds like a plan.  The best part is the two-supply system was option A to begin with.

Time to get started.

[W9ZSL]

I'm still wondering if I can use the DX-60B for an exciter if I use it in the AM mode only considering the carrier is so low when unmodulated.  I have the matching VFO.  Regarding the DX-20 as an exciter, I'm not sure how to wind a 50 ohm matching load and still prevent overload of the final since the 20 puts out at least 25-30 watts.

[AB2EZ]

You will have a problem running a DX-20 for long periods at "key down"... as an exciter for an AM transmitter.

The plate voltage and the screen voltage on the 6CL6 crystal oscillator are too high to allow "key down" operation for extended periods of time. [Too much dissipation]

Separately, as you point out, a DX-20 produces more output power than you need.

To fix these problems, you might consider the following:

Modify the HV power supply to reduce the B+ applied to the plate of the 6CL6 (via its output tuned circuit) and the plate of the 6DQ6 RF output tube (via its plate choke) to around 300V.

Continue to use the existing 50k ohm voltage dropping resistor (between the B+ and the 6DQ6's screen) to provide screen voltage to the 6DQ6 RF output tube.

Use a 10k ohm 5W voltage dropping resistor in combination with a 150V 5W Zener diode to produce a regulated 150V screen supply for the 6CL6.

These mods will allow both tubes to run key down for extended periods, and will also reduce the RF output power by a factor of about 4.

To drive the grid of your grounded cathode 813 RF amplifier:


According to the RCA specification sheet for the 813... if it is operated as a plate modulated, class C RF amplifier...and if you have 1600 VDC of plate voltage (at carrier), 300 VDC of screen voltage (at carrier), and -160V of DC grid bias... then you need an RF grid drive signal whose peak amplitude is 250V. Since grid current will flow on RF peaks of the grid drive signal, there is 2.7W of drive power required under these conditions.

Let's assume that the driver transmitter will produce 7.5 watts into a nominal 50 ohm resistive load. Note that if the driver is a DX-20, then the output pi network can match into loads that are significantly different from 50 ohms, and which can have a reactive component... but let's assume that we want the input impedance looking into the 813 amplifier to be in the ballpark of 50 ohms (resistive).

7.5 watts into a 50 ohm load corresponds to a peak (not rms) voltage of: [the square root of (7.5 watts x 50 ohms x 2)] = 27 volts.

So the input matching network must do several things.

1. It must convert the impedance looking into the 813 (i.e. the impedance from grid-to-ground), from whatever it is, to something in the ballpark of 50 ohms (resistive)

2. It must step up the voltage produced across this 50 ohm input impedance from around 27 volts to 250 volts (a 10:1 voltage step up)

[Of note: the DX-20's RF output tube operates, intrinsically, as an RF source with around 300V amplitude (in the reduced power version)... driving into an RF load impedance of around 6000 ohms. The pi network in the DX-20 transforms the tube's output into an RF source with around 27 volt amplitude... when feeding into a 50 ohm coaxial cable with a 50 ohm load at its other end. At the other end of the coaxial cable, the 813 amplifier's input matching network transforms the output of the coaxial cable into a an RF source, whose amplitude is 250V. Therefore the DX-20's output pi network, and the 813 amplifier's input matching network, in effect, cancel each other out... for the sake of allowing a 50 ohm coaxial cable to be placed between them. In fact, the 813's input matching network could be a reverse-connected pi-network]

The input matching network of the 813 amplifier could be a reverse-connected, link-coupled, output network. It would include two coupled inductors (possibly in the form of one inductor wound around the outside of another inductor) with a turns ratio of around 1:10 (to produce a transformer with the needed 10:1 voltage step up ratio). In addition, the coil that is connected between the grid of the 813 and ground (with a capacitor in series to block DC) must have sufficient inductance to resonate with the grid-to-ground capacitance (including stray capacitance and wiring capacitance) of the 813. The resulting parallel resonant circuit must have an impedance across it that is greater than 5000 ohms. That will ensure that the impedance looking into the side of the matching network... that includes the coil with the smaller number of turns... is greater than 50 ohms. [The impedance looking into a transformer with a 1:10 turns ratio is 1/100th the impedance of the load on the output side]

For example:

According to the specification sheet, an 813 has a grid-to- (cathode/filament + screen grid + suppressor grid) capacitance of 16.3pF. If one rounds that up to 25pF to include wiring capacitance and other stray capacitance, then the grid-to-ground impedance, due to this capacitance, at 3.885MHz is: -j1639 ohms. That is too low an impedance, across the output of the matching network, to produce the desired impedance looking into the matching network. One needs a parallel inductance, from the 813 grid to ground, provided by the input matching network, whose impedance at 3.885 MHz is approximately j1639 ohms. I.e. one needs an inductor whose value is approximately 67uH. Alternatively, if one adds some additional parallel capacitance (in the form of a variable capacitor whose range is, for example 25-250pF), then one can use a smaller value of inductance to form the parallel resonant circuit.

Finally, one may need to add some "swamping resistance" across the input of the matching network to produce the desired 50 ohm resistive component of the input impedance. Note that there will be some resistive component, looking into the input of the matching network, that is associated with the power dissipated when the 813 draws grid current, and possibly associated with the grid biasing circuitry.
 

Stu

[AB2EZ]

Note that I significantly revised the portion of my post (immediately above) that describes how one might match a 50 ohm RF source (like a transceiver or a modified DX-20) to the input of a tube like an 813... that requires an RF grid drive signal having an amplitude of 250V.

Stu

[W9ZSL]

I don't want to modify the DX-20.  I'm thinking that I'd rather build a 6AG7 crystal oscillator which I can also drive with my Heathkit HG-10B VFO and use a 6V6 or 6F6 as a buffer/doubler.  Output would be a simple L/C tuned circuit with an inductive coupling to the input of the PA.  Restricting operation to 75 and 40 makes sense.  Shoot, if there isn't much AM activity on 40 or 20, I might as well make this beast a monobander.  I have a 3885 rock, and if I dialed up 3725 on the Heath, I could run the 6AG7 straight through.  So how much AM activity is on 40?  I also have a rock for the main AM calling freq on 40 so I could run the 6AG7 straight into the buffer/exciter.

Then again, recall how I mentioned that I might be able to use the DX-60B for an exciter since the AM carrier level at rest is quite low.  Couldn't I just excite the 813 with the DX-60 unmodulated?  Of course, I have one 813 "multi-purpose" schematic which can act as a linear.

In any case, for now I'm shooting for 1500 VDC on the 813 and 1250 on P/P 811A modulators.  Classic AM combo.

[W4AAB]

Lots of AM activity on 7290 and 7295 kHz. I just got back on AMfone here after a couple of difficult years with lots of medical issues and job loss, but I am retired medically at the end of August.I hope that we can get some activity going again on 7160.You are headed in the right direction.
                                               Joe W4AAB