Series Modulated 813 Rig

[K1JJ]

Hola!

I need a 50 watt class hi-fi exciter to drive my pair of 4-1000As linear amplifier.  It needs about 15 watts of drive to do 400 w of carrier.  I figure a 50 watt, 200w pep AM exciter would make a very clean 15 w carrier at 200w pep. (> 200% modulation peak capability)

The series modulated design has my attention since it requires no mod iron, thus no phase shift.  I like the design of Bob WB9ECK's rig below. I also included my old 6AQ5 series modulated rig below that.  I still have this rig and works FB, but only about 5 w output.

I was thinking that a single 813 final series modulated by another 813 would do nicely with 2KV on the whole thing. In my 6AQ5 rig, I noticed the series modulator tube ran close to class A and dissipated much more power than the class C final, as expected.  I don't understand how the series tube can be smaller than the final, thus using a big tube for the modulator. (an 813)

Did I say that the 813 is one of my favourite tubes?  


Any comments or suggestions?

T



Bob / WB9ECK's design:
http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm

K1JJ 6AQ5 design:
http://www.amwindow.org/tech/htm/series.htm

[kb3ouk]

From some of the discussion I remember on here, you need a tube with a low mu and a low plate resistance that can handle the current that the final is drawing for series modulation like that to work properly, plus it needs to have enough dissipation. Kinda overkill for the power level you want to run, but an 833 sounds like a good choice. A better idea might be a GM70 or a pair of them, Russian triode that looks a lot like an 813 without a plate cap, and has specs similar to an 845 (higher dissipation for the GM70, 125 watts). I'd go for the graphite plate version for what you want (there's also a copper plate version that is hard to find and the audiophools desire it over the graphite GM70).

[N2DTS]

I guess the tube, or the filiment transformer has to withstand the high voltage in these designs.
Some tubes are not rated very high in the voltage the filiment to cathode can withstand, and some filiment transformers are not rated for 4-1000 kinds of voltages...

Another method is to screen modulate a tube directly with transistors, so it can 100% modulate the RF deck.


W2IMX sent me the design he is using on a pair of 4-400's, uses 3 transistors and takes line level audio in.
Everything is adjustable, so you can set it to 99% neg mod, and any positive mod you want, with no iron in the mix.

Note: these types of designs call for some method to limit the high frequency audio if you do not want a 40,000 Hz wide signal...

[K1JJ]

Thanks for the info, guys.

I plan to wind my own filament transformer on an old Variac core. It will be easy to isolate it for 5KV.  I've done this before.

That 4-400A final with transistor screen modulation sounds interesting. Could you post the schematic?  I would think some 11N90 900V MOSFETS would work FB and I have a bunch here.

The problem with screen modulation is the plate loading is critical and needs careful adjustment to be clean.  But so does a series modulated rig.

This exciter could also double as a low power rig running 50-100 watts when needed.  This is a step above a barefoot ricebox but not as much hassle as a big rig.

T

[N2DTS]

Sent it to you on email.
The transistors are horizontal output devices used in old TV sets.

On the air, it sounds really clean and the modulation looks good.

John runs it on a pair of 4-400 broadcast pull outs that have almost no emission left, at 200 watts carrier.

The interesting thing is that it will work on almost any rig, you just find the operating points, zero carrier, 25%, and 100% and adjust the pots on the board for the tube in use.

John reports it takes negitive voltage on the screens to cut most tubes off.

I heard it on the air and it works, looks good on the scope, and since John has limited or no audio processing, it goes out to 20Kc...

[kb3ouk]

Depending on how it's done, the series modulator should behave more like plate modulation, just applied to the negative side of the tube. The trick is to keep the grid to cathode voltage from changing with modulation, if it does then you are basically grid modulating the final. Floating the grid-cathode circuit along with the cathode circuit will do this (similar to a tube PWM final).
(maybe you already know this, I don't know. But those schematics almost look like that's kinda what's going on)

[K1JJ]

I received the circuit email.  Very cool, Brett, Tnx.

You said most tubes need some negative voltage to cut them off.  Does this mean that instead of grounding the circuit as shown on the schematic, it needs a negative supply to float this common lead above ground?

I'm thinking maybe a single 813 would do the trick.  I wonder what pep output a single conservatively screen modulated 813 will do with 2KV? 


The advantage of the screen modulated config over the series is it requires just one tube and the cathode / grids do not have to float at HV.



T

[N2DTS]

I was wondering about that, since John said most tubes need negitive voltage to cut them all the way off.
The design says to use 1.5 times the peak positive voltage needed on the screens, but I do not think it can go below zero volts...
But why set the resting power to 25%, 50% would be normal, no?


He also said the rig was 70% efficient, and I think it would do lots of power out with new tubes.

He was in the broadcast business in the NYC area, so some here likely know him.
His building of the unit looked top notch.

[K1JJ]

I would think the resting power at 25% would give the required 4X power for 100% modulation.  

He may be suggesting 150% screen voltage swing to allow for 150% modulation headroom, I dunno.

Maybe 70% efficient at 100% modulation, like a linear, but much lower at dead carrier conditions.  Still, the stage is in class C, which is good.   Some air with a chimney-blown 813 would allow for at least 175w of plate dissipation.

But whatever it works out to be, testing it on the bench with all sorts of parameter changes will find the best power and IMD sweet spots.

Thanks again for the schematic and I will definitely consider this method for a small 813 50 watt exciter.

Is it OK for me to post the schematic here for the other readers?

T

[N2DTS]

I do not see why not.
John seemed pleased anyone was interested in the sircuit and screen mod.
He used to work on big broadcast screen modulated rigs, and thinks they are great...

[W3GMS]

I would think the resting power at 25% would give the required 4X power for 100% modulation.  

He may be suggesting 150% screen voltage swing to allow for 150% modulation headroom, I dunno.

Maybe 70% efficient at 100% modulation, like a linear, but much lower at dead carrier conditions.  Still, the stage is in class C, which is good.   Some air with a chimney-blown 813 would allow for at least 175w of plate dissipation.

But whatever it works out to be, testing it on the bench with all sorts of parameter changes will find the best power and IMD sweet spots.

Thanks again for the schematic and I will definitely consider this method for a small 813 50 watt exciter.

Is it OK for me to post the schematic here for the other readers?

T

Tom,

You may want to see what Frank - KB3AHE did on his 4/1 final with his screen modulated scheme.  I seem to remember that he applied audio to the grid during modulation to improve its performance.  In his case he used I believe an ART-13 mod iron which had the second winding.  You could do exactly the same thing with a solid state solution so no transformers would be in the picture.  What you have is essential a series pass element and the output will follow which is on the gate or base depending if you use bipolar devices or FET's.  I would go with the FET's.   A high impedance bias network on the gate and that would then allow audio to be applied easily to modulate the bias voltage.  

Joe, GMS      

[K1JJ]

Joe,

Yes, I remember Frank adding some control grid monkey swing too.  I'll bet that looking at a THD test on the SDR would show when JUST the right amount of additional control grid swing is added.  That, proper drive, grid bias and proper plate loading should make the modulated output look sweet.


The Tron has a lot of BC experience with scream modulation too. I'll have get his take on it all.


Here's the solid state screen modulation circuit by W2IMX:

 

[N2DTS]

While I was in qso with him, I turned the screen voltage off on the 3x4D32 rig and still had some plate current, even though my bias is way high.
Not a lot of current, but some, with 1250 volts on the plate.
Every tube type will be different I suppose, I have not tried that with the 2x 4x150 rf deck yet.
The 4x150 / 4cx250b type tubes have some very different screen setup, in AB1 service, you can run 1000 to 2000 volts on the plates, keep the bias and screen voltage fixed, and have the same resting plate current.
I assume the screens have a lot of control over plate current.

I also have not tried it on the 813 rig, I may try it tonight.

[K1JJ]

Yep, I agree.

Coincidently, earlier I tried turning the screen voltage down to zero with my plate modulated 4-1000A rig.   The power dropped about to 1/3 and the plate current also dropped down.  But it will still needs more negative screen voltage and/ or grid voltage swing to cut the tube off.

So looks like we will need to add some negative supply voltage in the  control circuit.

Did you mean Tron?  What did he say about the subject?

T

[N2DTS]

No, Frank, W2IMX.
We had a nice qso about it for over an hour on 80 meters I think.

With the 3x4d32rig it was only 10 or 20 mills, and I am not sure I had any power out or not...

He was running 4000 volts on the 4-400's, he said he had to to get any power out of them, they had nothing left (emission).
I do not have any 4-400's or I would have sent him some.

He was talking like the circuit would go negative, but I do not see how it can. 

[K1JJ]

I was looking at Frank AHE's 4X1 screen circuit attached below.  His does not use a negative swing on the screen either.  Maybe he is making up for it with the grid swing from the transformer.

He doesn't use a grid leak resistor - full fixed -150V bias.   Interesting.

Back in the early 70's both Chuck and I tried that transformer circuit with a 4X1. We called it "composite grid and screen modulation."  


BTW, on the W2IMX circuit:  

A good friend sent me an email and suggested for improvements...  

"That circuit needs resistors Q1 base to ground and Q2 base to ground. Without them the circuit will be temperature sensitive so gain will change with temperature changes. Also the pot wiper leads could use some series resistors  so you can put too much current into the bases. FETs would be better."

Those changes would be easy enuff. I'll see what he suggests for values.

**  I do wonder what a negative screen current will do to the IMD. Maybe instead of negative screen, the control grid needs more negative swing to produce a cleaner cutoff without crossover distortion..



T

[N2DTS]

I did a test on the 2x 813 rig.
2000 volts on the plates, with no screen voltage, I get 120 ma resting current and 60 watts out.

[N2DTS]

Here is a reply I got from John:



Hello Brett,

Yes, most power tetrodes will produce RF output without screen voltage
as you saw for yourself in your test. The screen must be driven negative
towards the direction of cut-off :
 
"WITH RESPECT TO THE RESTING QUIESCENT VALUE OF 25 % POWER"

This resting voltage is always "POSITIVE"  So, when you begin to swing in the
negative direction "FROM THIS POINT", you're still in the "POSITIVE DOMAIN"
as far as the screen is concerned. It becomes a "BIASED" sinewave who's
zero crossing line is not zero but a positive voltage.

This part of the negative going voltage is derived from the audio voltage swing
at the output of Q2, emitter follower. Therefore, NO negative voltage supply required.

Regards,
John / W2IMX
 

 

[K1JJ]

Hi Brett,

Thanks for the info...

Well, the mystery still wanders thru my head - if the tube is still putting out power at zero screen volts, how can it be cut off when this circuit will not go below zero volts?  

I started getting parts together for an 813 version.  But then I started thinking about a simple class D 50 watt rig that could do 300 w pep.  I could use 11N90s in a band switched final and build a simple class A modulator with 11N90s too.

I know Jay / W1VD has a working homebrew class  D version - and aware of the class D "Senior" commercial rig.  That may be the easiest and cleanest approach for a small exciter for the 4X1 linear amp.

I would use my new 20 watt DDS VFO as a driver.  After having success with the 11N90 VFO IPA design, I am inspired to continue on in that solid state mode.

Still thinking.   Anyone have a simple, working, homebrew solid state class C or D and simple modulator circuit like I described?  Band switched 160 - 40M, 30 to 50 w carrier?

Here's Jay's version for a monoband RF deck - quite nice!

http://www.w1vd.com/375wattclassD.html

T

[W3GMS]

Tom,

I can see the temptation of going over to Jay's class D design with modulation generated with PDM and as we all know it sounds excellent.  

Sometimes the hurdles or question that remain unanswered can be a wonderful source of education.   So you may want to tackle those and continue while learning.  Any of my experience with screen modulation required a waveform with transverses below zero volts to achieve 100% modulation since zero volts on the screen produces some amount of output power, hence a sine wave approaching zero volts will not achieve 100% negative modulation.  Frank's transformer schemes does that due to the nature of the AC being superimposed on a DC voltage just like plate modulation does while using a modulation transformer.  To do that from a direct couple schemes require a that the AC swing goes below zero if the tube requires that to cease conduction.  

Remember that the screen is very non linear.  A low impedance driver that can deliver the current under modulation is required if one is to achieve clean audio at the screen during modulation.  Cathode Followers, Emitter Followers or Source Followers are capable of doing that very well.    

Joe - GMS    

[N2DTS]

Not sure how it works, but Franks circuit seems to modulate 100% in both directions, per what i saw on the scope, the the pictures of the waveform he sent.
That with a pair of 4-400's, which are likely no better at screen mod then any other tube.

It might be a good project for me this winter...

[IN3IEX]

http://www.ing.unitn.it/~fontana/IN3IEX%20AM%20MODULATOR.GIF

By combinig various circuits I developed a very effective screen grid modulator.
It works very well with my T4XB. I suppose that the voltage for the 6AU6 could be increased up to 500 V for tubes requiring 500V max for the screen grid. Update the zener diode voltage according to your supply voltage, maybe update the 6AU6 with a 6AQ5.
It can drive the screen grids of big tubes if the MOS and its heath sink can support them.
With a "touch" of controlled carrier the energy balance of the TX will improve....including that of the 10kW linear. 

Giorgio

[W3GMS]

Not sure how it works, but Franks circuit seems to modulate 100% in both directions, per what i saw on the scope, the the pictures of the waveform he sent.
That with a pair of 4-400's, which are likely no better at screen mod then any other tube.

It might be a good project for me this winter...

" Frank's transformer schemes does that due to the nature of the AC being superimposed on a DC voltage just like plate modulation does while using a modulation transformer." 

It works because of what I just said above.  I have cut and pasted if from my previous posting. 

Joe, GMS

[N2DTS]

I was talking about the other circuit from John with no transformer.

If you are going to use a transformer, it might as well be a mod transformer, no?

[K1JJ]

Yes, Frank/AHE is able to get a superimposed "negative" voltage by adding in the swing of the transformer.  I don't see how John is doing it with his circuit, however.


I'm still considering ideas here.

BTW, Frank/ WA1GFZ had an interesting suggestion. Why not screen modulate the 4X1 linear itself?

It's an interesting situation: My two 4-1000As GG linear amp uses a regulated 450v screen voltage, has diodes in the cathode for grid bias and the control grids themselves are bolted to ground.  Both the grids and screens are at RF ground.

Maybe I can add modulation to the screen regulator circuit. I imagine the proper amount of diodes in the cathode would push it into class C. (I already have a class C tap with the diodes)

The only "problem" is it would not hit -100% negative modulation since it is in GG and the cathode pass thru power is unmodulated. But that would make a great built-in negative peak limiter, right?  

I have 4KV on the 4X1s.    I'm not sure if there is any advantage to screen modulating it vs: driving it with an RF exciter other than the tube is in class C for better efficiency. But I can now run it biased into "linear" class C when on AM because of the carrier putting the tube into conduction. (Technique discussed in another thread)

Any thoughts?

Giorgio: That is a cool screen circuit for the Drake T-4 guys. Maybe Steve can archive it with the other T-4 circuit on the AM Window site.

T