A Simple Plate Modulator

[KX5JT]

This months Electric Radio has an article with some relevance to something I might want to try.  Seems very simple and maybe too good to be true.  AB7YD has written an article based on a plate modulator utilizing half of a 6de7 driving a triode connected 6146 utilizing a 'bias shift or sliding bias' design into a Heising modulator choke. 

As someone interested in getting some real plate modulation at the 15 to 25 watt carrier range, this looks very intriguing.  The circuit is very similar to a controlled carrier circuit but on the audio not the rf.  How wonder if the audio is really linear from this arrangement or is there some distortion.

I also wonder about the 5 Henry choke value.  I recall from Heising modulators the value for that choke being more like 30 or 40 H.   

Mostly just wanted to open a dialog with other interested parties about the article.

JT

[KM1H]

The bias shift first appeared in the 30's in an obscure magazine article but it was Bill Orr who popularized it in a 50's CQ article. It was used with a 304TL (or TH, not sure without looking) as a way to reduce the very high AB1 idle current. He did state that it could only be pushed to moderate power before distortion became objectionable.

I believe I mentioned it on here last year.

Carl

[KX5JT]

I just read through Orr's 1959 WCH 15th ed on his High-Level Bias Shift Modulation section.  His example there shows a pair of 304-TL in parallel.  He also note's that it is not possible to reach 100% modulation but greater than 90% is reachable. 

The ER article does seem pretty attractive to plate modulate a lower power transmitter.  I may have to play around with one if I can do it seperately and tap into my DX-60 via the back connector.

[WQ9E]

JT,

I also have a 1955 Radio and Television News article, "A Reference-Shift Modulator" written by one of the Collins engineers.  PM me your email if you would like a scan of this article.

[KX5JT]

My interest is to plate modulate something in the 20 to 25 watt range.  I've been checking outdifferent schemes.   I kinda like the idea of a single-ended class A (I know the efficiency will suck but this is not huge power) tube, maybe an 811A that'll give me about 15 watts in Class A and a Heising modulation scheme with an 807 or better yet a 6146 loafing.  Two seperate power supplies makes it a bit more complicated but what the heck.  I want a really hi-fi plate modulated transmitter to drive my Henry 2K4. 

I am revisiting the 15th Edition West Coast handbook and I see a bias-shift modulator there that claims it runs class A when audio is applied but shifts the bias down close to cutoff under no signal conditions.  The ER article is based on this idea. 

But then I'm kinda thinking why not just use a classic Heising-modulation scheme with Class A at these lower power levels. Surely that's gotta be some sweet hi-fi capability without crossover distortion.

Also, what the heck is "Class A2".  When I googled "811A single ended class A " I'm seeing quite a bit of Class A2 (not AB2).

[W2PFY]

I am revisiting the 15th Edition West Coast handbook and I see a bias-shift modulator there that claims it runs class A when audio is applied but shifts the bias down close to cutoff under no signal conditions.  The ER article is based on this idea. 

It seems to me that a zener diode placed in series with an 811A tube cathode would make sense. Perhaps it would run at zero negative bias when in class A. A relay could short out the zener when you push to talk. Perhaps two zeners would be in order.One to bias it properly. One would be shorted out while you are talking and then both open when you not.  Is this a good idea? Maybe a 812A would also work as a modulator?   

[w4bfs]

Also, what the heck is "Class A2".  When I googled "811A single ended class A " I'm seeing quite a bit of Class A2 (not AB2).

class A2 operation is still class A as evidenced by 360 degree plate conduction angle .... the 2 suffix indicates that grid current is being drawn whereas in usual class A (or A1) no grid current is drawn ... this is of course the usual grid driven topology ...I'm not sure of other schemes without dealing with each .... class A2 can be more efficient than class A but not necessarily so

[W9RAN]

JT - this "Golden Gate Modulator" has intrigued me ever since I ran across this short article about it:  http://picasaweb.google.com/RANickels/GoldenGateModulator#

Haven't tried it yet, but want to.

73, Bob W9RAN

[WD5JKO]

JT,

   I don't have much experience with the DX60, but going from screen modulation to plate modulation on any rig will increase the output power by about 6 db depending on the levels of modulation before/after. This means the plate HV circuitry will see about twice the voltage as before. The first problem I think of is the plate tuning capacitor might flash over on modulation peaks. Another area is the HV wiring just being in a harness might need to come out and be better insulated/separated.

  Your idea about a class A Heising system has merit, especially at this power level. An example would be what I did to my Gonset G50 using a P-P audio output transformer with a single ended modulator. The concept gets around the  classic Heising issue where the choke takes the combined DC current (RF + Mod) such that core saturation limits the low audio frequency response. The P-P transformer puts the mod B+ on one side, and the modulator on the other such that if equal, the currents cancel (RF - Mod). This approach got my G50 to modulate clean to 50hz whereas before < 500hz was a big problem since the modulator choke was saturating.

Jim
WD5JKO

[Opcom]

I think it's a fine idea, and it seems to work in a rig similar in power to a DX-60, why not try it?
Please post any waveforms and results!
One helpful thing present here is the zener diode in the cathode to keep the modulator mostly cut off.

[k7yoo]

What about scaling this up to using a single 4-1000 modulating ??
Not that I need to, but it might be interesting
Why not?
Skip

[WD5JKO]

What about scaling this up to using a single 4-1000 modulating ??
Not that I need to, but it might be interesting
Why not?
Skip

Hi Skip,

   Do you mean scale up a class A Heising modulation system to use a 4-1000? If so, with a 1KW DC input to the modulator you might get 200 watts of audio. Sounds like a nice wintertime shack heater to me. 

It's your idea, so try it and let us know how it works...

Jim
WD5JKO

[ke7trp]

Why not just modulate the Henry?  Serious.. You could build a nice Triode modulator, Use the DX60 to Kick the henry up to about 400 watts.  Then modulate it. Its not like it has not been done before. Couple HV connectors on the back of the henry to run it out to the modulator.

What I suggest you do is to modulate the Little DX60 outboard with a HiFI audio amp. The trans and parts are cheap and it would seem to me that you could do this in an afternoon.  Then you have a good solid and powerfull modulator to kick the DX60 up to hifi status.

C

[KX5JT]

Why not just modulate the Henry?  Serious.. You could build a nice Triode modulator, Use the DX60 to Kick the henry up to about 400 watts.  Then modulate it. Its not like it has not been done before. Couple HV connectors on the back of the henry to run it out to the modulator.

What I suggest you do is to modulate the Little DX60 outboard with a HiFI audio amp. The trans and parts are cheap and it would seem to me that you could do this in an afternoon.  Then you have a good solid and powerfull modulator to kick the DX60 up to hifi status.

C

Are you suggesting plate modulating?  That would require me to move the bias to class C.  Cathode modulation is more practical with a linear but .... NAHHHH.....

[k4kyv]

As someone interested in getting some real plate modulation at the 15 to 25 watt carrier range, this looks very intriguing.  The circuit is very similar to a controlled carrier circuit but on the audio not the rf.  How wonder if the audio is really linear from this arrangement or is there some distortion.

At the 25 watt power level, you need only about 20 watts of audio to modulate a class C final.  Why not just use a pair of 6L6s or similar tubes in push pull AB1, driven by a phase inverter? That would give less distortion than the bias-shift circuit and  would be a lot easier to get running.

If you can't find a small modulation transformer, use a push-pull audio output transformer rated for about the same power level, as a mid-tapped choke.  Some people have even used the midtapped HV winding of a small power transformer with success.  This would be a "push-pull Heising" arrangement.  In order to achieve 100% modulation, as in the case of conventional single ended Heising, you would have to run higher voltage on the modulator than on the final.  One way would be the 1920's-30's era trick of using a dropping resistor, by-passed with several mfd of capacitance, in series with the HV plate lead to the  final.

The push-pull Heising circuit would work much better if you used a separate modulation reactor, since most push-pull audio transformers are not designed to carry DC through one of their windings, except for modulation transformers that have a gap in the core.  In that case, you could use a separate power supply at  higher voltage than what you run on the  final, since the coupling capacitor isolates the audio power supply  from the rf plate supply.

PM me if you are interested, and I'll look in my junk collection.  I might even have a small modulation transformer that would do the job.  Then you wouldn't have to  worry about a choke, bias shift, and all the other complications.

[KX5JT]

This was meant for pvt message...