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Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig




 
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Author Topic: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig  (Read 4579 times)
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K1JJ
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« on: August 31, 2020, 01:36:40 PM »

I'm in the thinking stage for a 20-35 watt carrier Pissweaker AM tube rig that is plate modulated.  Self contained with PS and VFO preferred. Chinese DDS VFO OK.   Super high fidelity mandatory.

This will probably be my last HB rig for a while.  I want to take my time and do it up right with newer parts and metalwork.  There is a lower power gap now with my smallest AM rig at about 125 watts carrier output.

Please post your thoughts and schematics. What smaller rig have you always wanted to build?  I am going to limit it to plate modulated using a conventional class C final and class B p-p modulators.  A minimum of iron; no audio driver xfmrs - just the mod xfmr and mod reactor (if needed) are preferred.  Power supply iron is OK and expected.

What little rig have you always wanted to build? What advantages, reasons to go this route?  Also looking for mod iron to match the project.

T
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« Reply #1 on: August 31, 2020, 02:58:09 PM »

Always a tube type "Part 15 AM broadcaster" in a rack mount chassis, with panel meters like any transmitter. Plate modulated and class C final amp. DDS would be OK or a crystal. Sort of a 100mW-input-rule set disguised as something of the power level you are considering. A stand-in for the missing AM Oldies station -gone several years now and I have a few nice old console radios on which I liked to listen to big band music. I could buy a high quality modern solid state synthesized unit for $400, but I'd like to try building it first. This is not near the top of priorities in my life though.
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Tom WA3KLR
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« Reply #2 on: August 31, 2020, 07:47:59 PM »

Hi Tom,

Didnít you inspire a bunch of 5 Watt piss-weak rigs about 10 -15 years ago and you had one?

Anyway, the DX-60ís with the WA1QIX? screen series modulator mod is very good.  Thatís my recommendation- a screen-modulated 6146.  You can build one or start with a DX-60.  I donít know if they have other weaknesses.  You can have audio response down to dc and since no modulation transformer, could implement an rf feedback audio if desired with no problem? But the feedback is not necessary but doable in this case.

GL OM.
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K1JJ
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« Reply #3 on: August 31, 2020, 09:20:51 PM »

Hi Tom,

Didnít you inspire a bunch of 5 Watt piss-weak rigs about 10 -15 years ago and you had one?

Anyway, the DX-60ís with the WA1QIX? screen series modulator mod is very good.  Thatís my recommendation- a screen-modulated 6146.  You can build one or start with a DX-60.  I donít know if they have other weaknesses.  You can have audio response down to dc and since no modulation transformer, could implement an rf feedback audio if desired with no problem? But the feedback is not necessary but doable in this case.

GL OM.


Interesting idea, Tom...

Frank/GFZ also suggested a screen modulated rig; a pair of 6146s screen modulated by a 11N90 MOSFET. Probably good for 25 watts out and audio DC to light. Though I'd like to stay with all tubes like the transformerless DX-60 circuit for example.

Yes, the 1994 Pissweakers net had 25 homebrew rigs check into the first net using 5-10 watt rigs. That started many guys homebrewing bigger rigs later on. I remember Rich/K1ETP went from his first plate modulated project, a 10 watt cakepan 6AQ5 -  to later building an 813 rig.  I still have my original 5 watt 6AQ5 series modulated rig, but want to do something else.

Steve/QIX's screen modulated circuit is quite appealing, no doubt.

T
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« Reply #4 on: August 31, 2020, 10:15:34 PM »

Yup that was Frank.There were a bunch of wacko rigs on. Series and Screen modulated 6AQ5's, 50C5's and so on.  I did up a suppressor grid modulated 6AG7 and then upgraded to an 837 Oscillator. 7W or so out. Just a Carbon mic and a tube.
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K1JJ
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« Reply #5 on: August 31, 2020, 10:24:22 PM »

Yup that was Frank. There were a bunch of wacko rigs on. Series and Screen modulated 6AQ5's, 50C5's and so on.  I did up a suppressor grid modulated 6AG7 and then upgraded to an 837 Oscillator. 7W or so out. Just a Carbon mic and a tube.

Is your avatar picture the PW rig you described?

Yes, I remember you running that rig. I have a folder of schematics the guys sent me back then. Maybe yours is in there too.

I have been getting messages saying the transformer-less screen modulated or series modulation approach is the way to go for a PW rig.  Just generating ideas right now. I've run both before and have had good luck.  As long as the series tube is a big mofo compared to the final... and the screen modulated rig is carefully tuned up with lots of loading, they are hard to beat. 20-30 watts output is not too much of a strain.  

The more I think about it, I already have three plate modulated rigs and no other types. I should build a PW series or screen modulated rig.

I have been hearing that a series modulated rig is less stable for RF than a screen modulated rig or plate modulated. What is it about instability with a series modulated rig?  The ones I built had some minor problems that I was able to correct...  These were the 6AQ5 series modulated and PDM tube rigs.

T
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« Reply #6 on: September 01, 2020, 12:09:38 PM »

  I second that notion: the QIX screen mod is fantastic, and I'm sure the concept could be easily applied to a HB rig. Matter of fact, I plan to do just that: maybe two 6146's instead of one - just to drive my amp a bit harder, or maybe a bit more voltage on the plate of a single - a better power supply than the stock DX-60; some better metering; stuff like that.
  There's little not to like: clean audio from 10 hz to 20 khz, no mod iron, and enough output to drive most amps up to a decent carrier. Audio reports have been better than I could have hoped for, especially given that I'm not blessed with an outstanding "broadcast voice", and watching the output on my REA mod monitor is almost a religious experience. Negative peaks limited, and those positive peaks!
  Efficiency becomes an issue, of course, if we're talking big rigs, but for PW/exciter class rigs, Steve's circuit has gotten me hooked on screen modulation.


************************

Anyway, the DX-60ís with the WA1QIX? screen series modulator mod is very good.  Thatís my recommendation- a screen-modulated 6146.  You can build one or start with a DX-60.  I donít know if they have other weaknesses.  You can have audio response down to dc and since no modulation transformer, could implement an rf feedback audio if desired with no problem? But the feedback is not necessary but doable in this case.

GL OM.

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K1JJ
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« Reply #7 on: September 01, 2020, 12:42:31 PM »

http://amfone.net/Amforum/index.php?topic=26603.0


Here's the QIX screen modulated circuit in case someone is interested.  I am thinking about it but with a bigger tube to make 25-30  watts output AM carrier.

If I do, I'll draw up a complete schematic and post it here for all to critique.  Maybe a self contained 4D32  (or two?)  screen modulated  -    using a DDS VFO would do the trick.  

I like the idea of using a negative peak limiter and really pile on the audio. I normally don't do this with the bigger rigs, but a PW rig could really play well with the extra audio.

Steve said: "The modulator features a negative peak limiter, set to approximately 93% negative.  When properly adjusted, the DX-60 with these modifications will modulate more than 150% positive and will give a really fine reproduction of the input signal.  Frequency response is quite flat from 5hZ to over 10kHz."

Here's an actual recording of the screen modulated modified rig:

http://amfone.net/Amforum/index.php?topic=26603.0


Hmmm... I'm getting more interested...


T
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« Reply #8 on: September 01, 2020, 01:17:05 PM »

N2DTS wrote:
"I just wanted to update it in case anyone else wanted to try the circuit, which works great within its limitations.

And I think some tubes just will not sound clean no matter how you modulate the screen.
I lump the 813 and the 4D32 into that category, the 6146 and the 4x150 seem good.
Maybe its tubes that use lower screen voltages that work best, and with this circuit, ones that do not pull much current on peaks and some tubes likely do not track plate current with screen voltage well.

From what I see, it seems like the output impedance is whatever the cathode resistor is?
In this case, 39k ohms?

In my tests, the 4D32 was flat topping easy unless run at very low power outputs (5 watts).
The solid state modulator worked fine with the 4D32's, even 3 of them at once.

I am sure Steve could design a simple mosfet circuit with a real low impedance that took little drive and was good for all sorts of voltage and current.
There are some amazing devices out there these days."



So maybe a MOSFET screen modulator would be a better choice for a 4D32.   I have a MOSFET circuit that Frank/GFZ designed... I may take a closer look at that.

T
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« Reply #9 on: September 01, 2020, 02:20:05 PM »

My audio chain is slightly asymmetrical in the positive direction, so I'm hitting 130% peaks to even 150% without pounding into the negative limiter; just tickling it a bit. Nice thing is that, in the absence of mod-iron, you can't hurt the thing. Aside from common courtesy issues, of course, you can pretty much lay into it and let the peaks fall where they may. As always, though - maybe even more so than with plate modulation - a scope or mod monitor is crucial, especially when loading. You can go from silky smooth audio to monkey-snot with a small turn of that loading knob.

http://amfone.net/Amforum/index.php?topic=26603.0


Here's the QIX screen modulated circuit in case someone is interested.  I am thinking about it but with a bigger tube to make 25-30  watts output AM carrier.

If I do, I'll draw up a complete schematic and post it here for all to critique.  Maybe a self contained 4D32  (or two?)  screen modulated  -    using a DDS VFO would do the trick.  

I like the idea of using a negative peak limiter and really pile on the audio. I normally don't do this with the bigger rigs, but a PW rig could really play well with the extra audio.

Steve said: "The modulator features a negative peak limiter, set to approximately 93% negative.  When properly adjusted, the DX-60 with these modifications will modulate more than 150% positive and will give a really fine reproduction of the input signal.  Frequency response is quite flat from 5hZ to over 10kHz."

Here's an actual recording of the screen modulated modified rig:

http://amfone.net/Amforum/index.php?topic=26603.0


Hmmm... I'm getting more interested...


T
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« Reply #10 on: September 01, 2020, 04:05:09 PM »

My audio chain is slightly asymmetrical in the positive direction, so I'm hitting 130% peaks to even 150% without pounding into the negative limiter; just tickling it a bit. Nice thing is that, in the absence of mod-iron, you can't hurt the thing. Aside from common courtesy issues, of course, you can pretty much lay into it and let the peaks fall where they may. As always, though - maybe even more so than with plate modulation - a scope or mod monitor is crucial, especially when loading. You can go from silky smooth audio to monkey-snot with a small turn of that loading knob.

A pair of 6146s might be another possibility cuz the tube circuit you guys are using is proven.  How much carrier out do you get with one 6146 at 150% positive peaks?  Yes, the loading is sensitive.

What maximum HV would you suggest?   25-30 watts out would be FB, but may be axing too much.  Maybe three 6146s.   I wonder if 1200 volts is about right... like a 600V plate modulated rig in a Ranger, but screen modulated.  400V dead carrier and 1200V peak under modulation.

T
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And, nothing like an old dog.
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« Reply #11 on: September 01, 2020, 05:09:51 PM »

  With my DX-60's single 6146 I can get a bit over 15 watts carrier, but when loaded for proper modulation the max carrier is about 12 watts. That's with the DX-60's stock power supply, which delivers 600 volts. The 6146B, as I remember, can handle 750 volts intermittently.
  So, I think I'll try pine-boarding something with a single 6146B at 750 volts, to start, since I'm only looking to get 15 to 20 watts for driving my amp. I'll make a few changes over the stock DX-60: a cleaner power supply (the DX-60's B+ winds up on the plate of the modulator/cathode follower as well as the 6146, so any ripple winds up in the audio); a nice clean regulated grid bias supply instead of grid-leak bias; better metering; and I think I'll set the bias to push the 6146 a bit further into class C than the DX-60 does now.

My audio chain is slightly asymmetrical in the positive direction, so I'm hitting 130% peaks to even 150% without pounding into the negative limiter; just tickling it a bit. Nice thing is that, in the absence of mod-iron, you can't hurt the thing. Aside from common courtesy issues, of course, you can pretty much lay into it and let the peaks fall where they may. As always, though - maybe even more so than with plate modulation - a scope or mod monitor is crucial, especially when loading. You can go from silky smooth audio to monkey-snot with a small turn of that loading knob.

A pair of 6146s might be another possibility cuz the tube circuit you guys are using is proven.  How much carrier out do you get with one 6146 at 150% positive peaks?  Yes, the loading is sensitive.

What maximum HV would you suggest?   25-30 watts out would be FB, but may be axing too much.  Maybe three 6146s.   I wonder if 1200 volts is about right... like a 600V plate modulated rig in a Ranger, but screen modulated.  400V dead carrier and 1200V peak under modulation.

T

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« Reply #12 on: September 01, 2020, 10:43:50 PM »

How homebrewed do you want it - starting from a bare chassis? Smiley

I have 80 and 40 meter "ARC-5" transmitters and built a screen modulator. There's a thread on AMfone a few years back (may be on the tech forum) with schematic, pics etc. The typical '60's speech amp circuit from the ARRL handbook, and a 6AQ5 for the modulator tube. The rig only puts out about 10W carrier but it looks good on the scope.
As already mentioned, loading adjustment is critical for a screen mod, and I'm glad I didn't put it on the air without a scope! Especially with the 4x4-125A amp  Cool
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« Reply #13 on: September 02, 2020, 08:37:32 AM »

The 6l6 series of tubes may be good in this spot, too.

A Trio of them would look cool as well as provide the power your looking for I think.  A quad may be better.

At 75 meters, the KT88 works fine business.  Heck, I've //gotten// a KT88 working on ten meters, but just barely.

Just a thought, there is life other than the 6146, if you want to be different 🤔

--Shane
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« Reply #14 on: September 02, 2020, 10:39:06 AM »

I've seen a lot of variations on ways to do 25w (more or less) suggested here, many of which will give a great sounding signal, I'm sure.  But I wonder if maybe you should be asking yourself, do I want something unique, something complicated, something maybe less stable, and so on, or do I want a back-to-basics simple rig?  The ideas of hifi audio and series mod and solid state all are great, don't get me wrong, but will you be happiest with that, or a simple little plate mod rig? A single 807 pentode can do 40+ watts, you like to run them up and turn them down in general use, if you want a triode, an 809, a little less common these days than an 807, good for even a little more maximum output. Find a 75 watt class mod transformer, build up a fully differential audio amp with feedback, and optimize the piss out of it, and you'll have a low parts count, classic, basic but great, 25 watter. But only if that's what you want.....either way, I'm watching, looking forward to whatever you share...

Ed

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« Reply #15 on: September 02, 2020, 01:45:28 PM »

  "Complicated", I think, is in the proverbial eye of the beholder, and I suspect that many of us are here - and into AM in general - because we like being technically creative. Tom is certainly one of the most ambitious builders around; he's probably built just about every sort of rig there is to build, and I assume he does it because he likes the technical challenge. It doesn't appear that he's gotten bored yet. Me, if I wanted perfect reliability and stability, I'd run a rice-box, but I'd soon be bored to death.
  To me, screen modulation is simpler than conventional plate modulation, and good mod-iron isn't getting any easier to find. Series mod can get complicated largely because you've got to isolate parts of your circuit, with high potentials floating around, otherwise it's pretty straightforward (check out the WB9ECK series mod rig, http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm).

I've seen a lot of variations on ways to do 25w (more or less) suggested here, many of which will give a great sounding signal, I'm sure.  But I wonder if maybe you should be asking yourself, do I want something unique, something complicated, something maybe less stable, and so on, or do I want a back-to-basics simple rig?  The ideas of hifi audio and series mod and solid state all are great, don't get me wrong, but will you be happiest with that, or a simple little plate mod rig? A single 807 pentode can do 40+ watts, you like to run them up and turn them down in general use, if you want a triode, an 809, a little less common these days than an 807, good for even a little more maximum output. Find a 75 watt class mod transformer, build up a fully differential audio amp with feedback, and optimize the piss out of it, and you'll have a low parts count, classic, basic but great, 25 watter. But only if that's what you want.....either way, I'm watching, looking forward to whatever you share...

Ed


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« Reply #16 on: September 02, 2020, 06:23:38 PM »

Thanks for the thoughts, guys.

Yes, there is life after 6146s.  They are a boring tube for sure.  But they are possibly the best performing tube in its class.

Building rigs and homebrewing: I like looking at and turning the knobs after they work well. I use slide-on enclosures so I can see the insides in seconds. Three rigs use Plexiglas covers for easy viewing.  I like looking at the work and the tubes themselves. Flipping on the switches and everything works right is fun.  It's fun just looking at the stuff in the open. In contrast, a rig hidden in a rack never gets seen for long periods of time.

I get no thrill looking at commercial gear like my FT-1000D or SDR receiver. But they are necessary and near impossible to homebrew.  The buildable homebrew gear is where it's at Daddio.

I've owned my share of boatanchors... probably every one made at one time. The transmitters were always breaking down. Rangers, Valiants, etc., gave me a lot of trouble. But my homebrew stuff, after working and tested well, rarely craps out. And if it ever does, I can fix it easily cuz I'm the one who designed and built it.  In contrast, when the FT-1000D craps out I feel panic coming on and 1/2 the time I can fix it and 1/2 the time I send it out to the Yaesu shop.

There is a slow rate of workmanship improvement with homebrew rigs that you can see comparing early rigs to later ones. We simply get better at it.

My first rig project was the 811A linear in the 1964 Handbook. I was 12 years old.  I didn't have any parts yet so started the chassis work. I used my metal Gilbert chemistry set as the chassis and a hammer and nail to cut the hole for the tube socket. The chassis collapsed from the pounding and that was the end of that project.  The next one was the swr bridge in the same handbook. I never got it working.  I then tried a one tube radio receiver. That never worked either. It was a rough start with no mentors.

The planning stage is probably the most fun...   Here's a strange looking tube that is plentiful on eBay; the 815 dual beam powered tetrode.  Two tubes strapped in parallel will do about 25 watts screen modulated. I am considering it with the other ideas.

https://tubedata.altanatubes.com.br/sheets/049/8/815.pdf

A spacesuit robot looking tube.

**BTW, a few months back I contacted Bob/ECK about his series modulated rig.  He said it later became unstable. He changed the series modulators to different tubes and never got it working to his satisfaction. I was thinking of something like that.    My 5 watt  6AQ5 series modulated rig could be scaled up to 30 watts.... that still works FB. As already said, the HV isolation and big heat from the series modulator is a factor.

http://www.amwindow.org/tech/htm/series.htm

T


* TUBE 815.jpg (169.14 KB, 1600x1200 - viewed 46 times.)
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« Reply #17 on: September 02, 2020, 10:58:54 PM »

I'm down to two designs:

I'm thinking I may build up the QIX screen modulated audio circuit. It is a proven design and if I get stuck I can always axe Steve.  I can build the audio and power supplies and leave the RF section clear for now until I decide on a 20-30 watt final(s). The equivalent of three 6146s putting out 30 watts carrier would be FB.

The second choice is the series modulated scaled up version of my 6AQ5 rig without the self-excited xtal circuit.  Either one will be very clean and have huge audio capability.

I'll start gathering general parts and decide on a chassis.  This rig should be lighter to carry around than the last three I've built.

Oh, another thing about the fun of homebrew rigs is hanging out on the air with a bunch of like-minded builders. Yesterday I was on 3885 trying out Baby Blue for the first time in a live QSO. On there with me was Frank/GFZ testing his new audio processor with his highly modified Valiant. Also was Rich/ETP another builder of a nice 813 rig. And then there was Bob/KBW talking about his class E rig. And Clark, another class E builder. The Tron checked in later on with his new pair of 813s in linear and was able to give me an audio report on the new rig. He said "don't touch a thang!"  

Everyone had a chance to talk/brag about their particular rig. Nowhere else in the "normal" world can you go and people are willing to listen and talk about their rigs.... :-) How many family members or regular non-ham friends would care about your new 813 rig?  Having a posse to get on with is so important.   The same benefit goes with using AMfone.net.

T
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« Reply #18 on: September 02, 2020, 11:55:44 PM »

I'm liking the KT88 more and more, as I look at it.  A pair, scream modulated would be a trifecta of 6146s.....

Or the 6550.

I like tubes that are still in production.

--Shane
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« Reply #19 on: September 03, 2020, 08:43:51 AM »

  That's too bad; that rig, along with a post by Steve, QIX, about series mod, inspired me to start building my own PW series mod rig (which hasn't yet gotten off the pine-board, mostly because I got distracted). I wonder what went wrong. Unstable in the RF section, or audio? Since he changed the modulator tubes, I'm suspecting audio. Series mod is cool, but I suspect there are a few more gremlins lurking there than in a screen mod rig.


**BTW, a few months back I contacted Bob/ECK about his series modulated rig.  He said it later became unstable. He changed the series modulators to different tubes and never got it working to his satisfaction. I was thinking of something like that.    My 5 watt  6AQ5 series modulated rig could be scaled up to 30 watts.... that still works FB. As already said, the HV isolation and big heat from the series modulator is a factor.

http://www.amwindow.org/tech/htm/series.htm

T
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« Reply #20 on: September 03, 2020, 10:13:23 AM »

A third choice, if you want to be bleeding-edge unique: heater modulation. Good for some really, really, low audio freqs!  Grin

I'm down to two designs:

I'm thinking I may build up the QIX screen modulated audio circuit. It is a proven design and if I get stuck I can always axe Steve.  I can build the audio and power supplies and leave the RF section clear for now until I decide on a 20-30 watt final(s). The equivalent of three 6146s putting out 30 watts carrier would be FB.

The second choice is the series modulated scaled up version of my 6AQ5 rig.  Either one will be very clean and have huge audio capability.



T

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« Reply #21 on: September 03, 2020, 12:29:55 PM »

I'm down to two designs:

I'm thinking I may build up the QIX screen modulated audio circuit. It is a proven design and if I get stuck I can always axe Steve.  I can build the audio and power supplies and leave the RF section clear for now until I decide on a 20-30 watt final(s). The equivalent of three 6146s putting out 30 watts carrier would be FB.

The second choice is the series modulated scaled up version of my 6AQ5 rig without the self-excited xtal circuit.  Either one will be very clean and have huge audio capability.  snip.....

Tom, whether you go screen modulated or series modulated, the overall dissipation power budget is simply moved between the RF final and the series modulator.  There are lots of screen mod rigs out there, but very few series mods.  Personally, I would trade the tuning and loading issues of a screen mod rig for the work in engineering a very linear series mod rig, and then operating and tuning is identical to the ease of a plate modulated rig.

If you place the series tube above the final, the only thing you need to isolate is the tube socket, and use a filament transformer with very good secondary insulation with high breakdown voltage.  Here are some ideas I might try:

Find an old HP13, SB-400, SB-401 junker and grab the power transformer.  That and another filament transformer is all the iron you need.  Using 1N5348 diodes and very inexpensive and small axial or radial caps under the chassis, and the dual-voltage power supply is finished.  250 or 300 volts for the exciter, ample bias, and 350/700 volts for the final.  While an RCA 808 would be pretty and retro, and the 6146 is sameo-sameo boring, perhaps an 807W/5933, or an earlier sweep tube for the final that works well at lower voltages would give it some snot.

I think it would be really cool to try a 4D32 for the series modulator, as it has great emission and would have minimal voltage drop on saturation.  What's more, they can easily handle the high voltage of a series modulator.  If you want great positive peaks, stack the 800 volt supply of the Heath transformer on top of (in series with) the 250/300 volt low  voltage output, and now you have tremendous positive peaks.  

The big issue is making a linear modulator for clean audio and minimal IMD using a single series tube.  Must stay in the linear area of the transfer characteristic.  In order to reach that goal with my bigger design, I am building just the power supply and modulator (breadboard style) and substituting a big fixed resistor for the final, with a value equal to the modulating impedance.  Then take a variable DC supply (maybe brute force with a variac) and vary the grid bias on the series modulator, through a resistor.  This is a cathode follower feeding this big dummy load resistor, so no grid current involved.   Now capacitively couple audio to the grid, and watch the signal on the dummy load, both for the DC simulated plate voltage potential as you vary the DC bias, and the linearity of the AC waveform as you increase or decrease the modulating signal. This will give you a good feeling for which tubes will play linearly with a series modulation circuit.

Once that is finished, you have the task of swinging the series modulator tube grid with bias and audio.  I plan to use a triode or tetrode, with the plate directly connected to the grid of the series tube, and a plate resistor connected directly to the HV B+.  This gives you the AC swing and the ability to set the DC operating point in a manner similar to the screen modulating circuits with which we are familiar.  This driver tube has to handle high voltage, but very little dissipation due to the low plate current.

If you can find a tube that qualifies for clean audio, good output voltage and current at the positive peaks, the rest is easy.  Very little iron, easy to tune, and no more heat than a screen modulated rig.  It seems practical for a PW rig, but only make sense for QRO in the dead of winter.

One final thought.  A sweep tube for a final on the low bands should not need neutralization, and with its high degree of emission needed for the TV pic to fill the screen, it should provide great output power and positive peaks without a great deal of modulated B+, probably much better than a 6146 or 807.  Do you have some left over from that 4x4 PDM rig that exited your shack?  You might even find a sweep tube works great for the series modulator as well, or maybe a pair in parallel?  GL and have fun with something COMPLETELY DIFFERENT!

Edit:  An afterthought-  If driving the series tube with bias and audio proves too complicated with it above the final, put the final directly below the power supply and modulate on the cathode side.   Return the final grid bias circuit to the cathode of the final, not the power supply ground, so that you have true series modulation and not cathode modulation - UGH!
This makes driving the grid of the series modulator simple and customary.  No need to float anything but the final tube and filament.  Capacitively couple the input and output RF, you just need caps that handle the high voltage, you have plenty.  Pi net output is standard, nothing out of the ordinary with the final on the top HV.  Grid circuit is similar too, with either grid-leak, or an isolated bias supply, which is not that much offset voltage for typical transformers to generate the fixed bias.  Have Fun!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.Ē   Ronald Reagan
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« Reply #22 on: September 03, 2020, 08:17:18 PM »

Hi Rick,

You have me more interested in series modulation. I like the idea that there are very few guys doing it, so it will be a novel project and take some experimentation to get it running right.  Not counting a few tube PDM rigs, I don't know of any except for W2DTC with his 3CX-3000A7 rig.

You said:
"If you place the series tube above the final, the only thing you need to isolate is the tube socket, and use a filament transformer with very good secondary insulation with high breakdown voltage."

What is meant by placing the series modulator "above the final?" Do you mean in the same place as a conventional mod xfmr?  If so the audio drive circuit would need to be insulated with fiber optics or equivalent to keep the HV off it, right?  Maybe you meant something else.

I have some 6LF6 sweep tubes and some 4D32s that could be a good start in the series modulator.  Yes, an 807 would be plenty enough as a class C RF final.  I would like to put as much HV on the rig as possible to have that headroom.  I had about 2200VDC on my little 4D32 PDM (series modulated) rig, so I know the 4D32s will take the voltage abuse.   That will be a little much for an 807.

** Rick:  Could you make a hand-drawn schematic of your best ideas for a 30 watt series modulated tube rig?  I think between the two of us we could come up with a workable design that could later be tried by others once the bugs are worked out.  There seems to be a lot of interest in a series modulation based on the long threads in the past.   30 watts carrier out or so would be quite buildable by most guys here.


T


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

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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« Reply #23 on: September 03, 2020, 08:41:59 PM »

I believe he's talking about the difference in putting the modulator tube between the plate and B+ supply, vs the cathode and B-.

--Shane
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« Reply #24 on: September 03, 2020, 08:44:08 PM »

Sweep tubes, too big!

I vote for paralleled 12AU6 or 6SL6, if you want lo mu. Parallel a few...
Feel free to improvise on the tube types.

Complete heresy.

                      _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
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