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Author Topic: screen modulation again...  (Read 182962 times)
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N2DTS
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« on: November 03, 2014, 09:08:02 PM »

I ordered all the parts for John's (W2IMX) solid state screen modulator.
I have a bunch of rigs to try it on, but I have been thinking about the concept, plate voltage only goes as high as the power supply, there is no modulator and its power supply, and whatever the plate voltage is on the final tube is the most the rf deck will see, right?
So if it runs 2000 volts, a plate tuning cap can be 2500 volts and never arc over.
No mod iron, no modulator power supply, a VERY small circuit to drive the screens, wimpy rf deck components (voltage).

So that has me thinking about what tubes screen modulate the best.
I have 3 different ones to test the circuit on, 4d32, 4cx250, and 813's.

The 4cx250's are interesting (I have the 4x150a-7034 version, 250 watts of plate dissipation).
Small in size, cheap, easy on filament power.
Four tubes gives 1000 watts of dissipation, they are small, the rf deck parts could be reasonable at lower voltages.
I could use one to four tubes in a very compact rig.
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« Reply #1 on: November 16, 2014, 02:42:02 PM »

I got the thing built and as a test I patched it into my 2x4x150a rig.
I just pulled the screen feed and inserted the unit in line, voltage in (variac) out to screen, and ground.
It takes 600 ohm line level audio in.

Picture of it almost done:
http://n2dts.smugmug.com/Ham-radio/i-DdB6qD9/A

It does not seem to work very well, at least not yet.

The design is from John, W2IMX, and he sounds very good using it, I sound like a frog.
There are plenty of adjustments on the circuit, plus all the transmitter adjustments, the downside of screen modulation I guess, everything has to be set just right...
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« Reply #2 on: November 16, 2014, 10:42:49 PM »

Is there a link to the circuit diagram?  That may shed some light on the subject.
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« Reply #3 on: November 17, 2014, 11:33:00 AM »

It was on here someplace, in another thread, but I have not been able to find it.
In the thread of a super clean rig or something about a year ago.

I am sure its just adjustments and voltages, but at the moment, it makes old military screen modulated stuff (carbon mic) sound hifi...
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« Reply #4 on: November 17, 2014, 12:03:35 PM »

It was on here someplace, in another thread, but I have not been able to find it.
In the thread of a super clean rig or something about a year ago.


Brett, is this the thread and circuit you were trying to find?

Thread: http://amfone.net/Amforum/index.php?topic=35375.10

(Circuit is posted in reply number 11)

Circuit: http://amfone.net/Amforum/index.php?action=dlattach;topic=35375.0;attach=40659;image
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« Reply #5 on: November 17, 2014, 01:11:22 PM »

Yes, that is it.

John sounds great on his.
I just patched it in and it works, did not let any smoke out, but I need to play with it.
The mod transformer is still in line, so I will need to bypass that, and my screen voltage setup may not go high enough, its variac'd up to about 650 volts, John says you need 1.5 times the voltage.
I was getting up to 250 volts on the output if I adjust it that way, 40% modulation and downward carrier shift, along with a very muffled sound.

Pair of 4x150a (same as 4cx250b's) running 1300 volts, was tuned up to put out 300 watts carrier with the plate modulation. I just shut the modulator off and put the circuit in the screen circuit, I turned the screen dropping resistor to zero and adjusted the voltage to 600 volts.

Nothing was getting overly warm, but I also seemed to have little to no screen current.
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« Reply #6 on: November 17, 2014, 02:01:30 PM »

Yes, that is it.

John sounds great on his.
I just patched it in and it works, did not let any smoke out, but I need to play with it.
The mod transformer is still in line, so I will need to bypass that, and my screen voltage setup may not go high enough, its variac'd up to about 650 volts, John says you need 1.5 times the voltage.
I was getting up to 250 volts on the output if I adjust it that way, 40% modulation and downward carrier shift, along with a very muffled sound.

Pair of 4x150a (same as 4cx250b's) running 1300 volts, was tuned up to put out 300 watts carrier with the plate modulation. I just shut the modulator off and put the circuit in the screen circuit, I turned the screen dropping resistor to zero and adjusted the voltage to 600 volts.

Nothing was getting overly warm, but I also seemed to have little to no screen current.


I am glad that is the one you were looking for.

I have often thought of using my stock of 4X150D / 7035 tubes, (4X250A with 26.5 volt filaments) to build a G2DAF linear, in a very small package.   But current thinking is the G2DAF design is not that clean, so maybe a small self-contained transmitter for 75 and 40 would do well with four of these tubes.  I would be interesting to hear what parameters you choose when you get it fine-tuned to your satisfaction.  Thanks in advance!
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« Reply #7 on: November 17, 2014, 03:00:36 PM »

You might need a bit of mod on the control grid of the 4CX250 to reach 100% mod but be careful cause it is a bit fragile. 2 watt dissipation I believe.
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« Reply #8 on: November 17, 2014, 03:49:59 PM »

Four?
I think one will do.
I run two at low voltage and low current for 300 watts carrier out and they only get warm to the touch, about 107F with little air flow.
I am sure two would do 500 watts carrier no problem with 1700 volts and 400 mills or less.
They call them 4x150's but they have 250 watts of plate dissipation each.

Last year I worked someone running a single 4cx250 with screen modulation using a transformer in the screen circuit only. Sounded very good, looked good on the scope.
It seems like everyone but me can lash up any old thing and get screen modulation to work and sound good....lucky for me I can get plate modulation to work ok...







I am glad that is the one you were looking for.

I have often thought of using my stock of 4X150D / 7035 tubes, (4X250A with 26.5 volt filaments) to build a G2DAF linear, in a very small package.   But current thinking is the G2DAF design is not that clean, so maybe a small self-contained transmitter for 75 and 40 would do well with four of these tubes.  I would be interesting to hear what parameters you choose when you get it fine-tuned to your satisfaction.  Thanks in advance!
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« Reply #9 on: November 17, 2014, 04:12:27 PM »


Last year I worked someone running a single 4cx250 with screen modulation using a transformer in the screen circuit only. Sounded very good, looked good on the scope.
It seems like everyone but me can lash up any old thing and get screen modulation to work and sound good....lucky for me I can get plate modulation to work ok...

It may just take some time to fine tune all the parameters to get it to modulate cleanly.  It will be interesting to see if grid modulation is needed in addition to screen modulation with the 4X150.

I suppose one or two tubes will work well, but considering the low efficiency and large dissipation for "efficiency" screen modulation, I thought it might be prudent to spread the heat across several bottles.  I want to make the cooling system as quiet and unobtrusive as possible.  The 4CX250 has ceramic seals, where the 7034/7035 version of the 4X150 is rated at 250 watts with a glass seal, not ceramic.  So while testing, and determining the limits of this tube's capability, I really would not want to melt a pair of glass seals.

I just finished rebuilding a UTC MLF (Multiple Loop Feedback) amplifier to use as a speech amplifier for the PP 250TH modded by PP 810s.  The final and modulator are fully restored, and I am just finishing the power supplies and control circuits.  I am putting in a bit more protection, fusing, etc, than was in it when my grandfather started using it in 1937.  Right now I am kicking myself whether to go with an authentic bridge of 866A rectifiers, or cheat and use silicon rectifiers.  Getting the right combination of filament transformers, chokes, and oil caps can be a challenge!  Once this is done, I will get that receiver built, then work on the screen mod rig.
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« Reply #10 on: November 17, 2014, 06:21:30 PM »

Here is another design.

I haven't had a chance to prototype it yet so it might need some tweeking as well.


* 4D32 Screen Modulated.pdf (39.39 KB - downloaded 756 times.)
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« Reply #11 on: November 17, 2014, 07:35:27 PM »

Oh, you are going to do screen modulation also.
Yes, four would likely do 300 watts.
Its good they do not take a lot of filament power, four is reasonable for 1000 watts of plate dissipation.

John runs the circuit into a pair of 4-400's beat broadcast pullouts he says, and does not modulate the grid.

I should research it, maybe grid leak is a no no and I want fixed bias?
Well, something to fool with this winter.
I have the remains of my 4x 813 amplifier I ran with the flex stuff (for a very short time)  that might make a good screen mod deck.

Sounds like you have plenty to keep you busy as well.



It may just take some time to fine tune all the parameters to get it to modulate cleanly.  It will be interesting to see if grid modulation is needed in addition to screen modulation with the 4X150.

I suppose one or two tubes will work well, but considering the low efficiency and large dissipation for "efficiency" screen modulation, I thought it might be prudent to spread the heat across several bottles.  I want to make the cooling system as quiet and unobtrusive as possible.  The 4CX250 has ceramic seals, where the 7034/7035 version of the 4X150 is rated at 250 watts with a glass seal, not ceramic.  So while testing, and determining the limits of this tube's capability, I really would not want to melt a pair of glass seals.

I just finished rebuilding a UTC MLF (Multiple Loop Feedback) amplifier to use as a speech amplifier for the PP 250TH modded by PP 810s.  The final and modulator are fully restored, and I am just finishing the power supplies and control circuits.  I am putting in a bit more protection, fusing, etc, than was in it when my grandfather started using it in 1937.  Right now I am kicking myself whether to go with an authentic bridge of 866A rectifiers, or cheat and use silicon rectifiers.  Getting the right combination of filament transformers, chokes, and oil caps can be a challenge!  Once this is done, I will get that receiver built, then work on the screen mod rig.
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« Reply #12 on: November 17, 2014, 09:38:08 PM »

Oh, you are going to do screen modulation also.
Yes, four would likely do 300 watts.
Its good they do not take a lot of filament power, four is reasonable for 1000 watts of plate dissipation.

John runs the circuit into a pair of 4-400's beat broadcast pullouts he says, and does not modulate the grid.

I should research it, maybe grid leak is a no no and I want fixed bias?
Well, something to fool with this winter.
I have the remains of my 4x 813 amplifier I ran with the flex stuff (for a very short time)  that might make a good screen mod deck.

Sounds like you have plenty to keep you busy as well.

Winter time is great for building projects in the basement.  It always stays warm, and it is too cold to play outside with antennas.   

I built a 4-1000 screen modulated rig several years ago, with Frank's input.  I debated plate modulation, but did not want to rob the iron for the 304TL rig I am restoring, it is PP 304TLs modulated by a pair with a Stancor A3899 500 watt mod transformer. 

I thought for a long time about building a linear with those four 7035s, but never got started.  I am thinking that I could build a complete screen modulated transmitter, including vfo and power supply, in a new old stock HRO chassis and cabinet.  No, my dad never drilled hole one in that cabinet, it is still like new.  I was thinking of building it with the standard HRO PW dial for the VFO, and a panel layout similar to the HRO, meter location, etc.  Should be a conversation piece, and I already have the iron and caps for the power supply.  A  tube vfo and a buffer will likely be enough drive for the grids. 
 
I recall Frank stated that we should use fixed bias, no grid leak, in order to stabilize the desired bias voltage for screen grid modulation.  While he used a separate winding on the mod transformer for the control grid, it would be possible to capacitively couple the modulation to the control grid, using a mod choke to feed the bias.  A simple pot could be used to control the modulation level to the grid, and it could be adjusted separately from the DC grid bias.   Naturally I will test the solution before punching holes in the original HRO chassis and cabinet.

Let me know how you make out with your tests, and whether you find it necessary to modulate the control grid.   Hope the ideas above are helpful in your tests.
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« Reply #13 on: November 17, 2014, 11:07:03 PM »

Maybe that is the problem then, the grid leak resistor.
I have a big wirewound pot plus a variac bias supply, so all I have to do is adjust things and figure out where to set things so they work. Seems like a lot of fine tuning and adjustments, so sort of a pain in the butt, but I am learning.
I have some other things going on so I may not get to it right away, I got a deal on a flex 1500 I want to play with, I had the 5000 and 3000 but not the 1500.
I am also in the thick of cleaning out the basement and garage, junk just accumulates and its time to get rid of it.

Time to break out the Bill Orr books and read up on screen modulation..
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« Reply #14 on: November 18, 2014, 12:45:20 AM »

Doesn't grid leak rely on plate current to work?   And if we d op the screen voltage,  we will drop plate current,  causing our grid leak to drop as well....   Or am I off in left field?

I've got a single 250B here already set for 80-10 as an amp.   Was thinking about scream modulating it....   And if that works,  move it to the 4-1000 rig.

Maybe I should put up an antenna first.   

--Shane
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« Reply #15 on: November 18, 2014, 07:53:35 AM »

I think its grid drive driving the grid positive enough to draw current, you have grid current and voltage without any plate voltage or current at all.

I never looked up what you are supposed to do with the grid under screen modulation.
I know loading is important in most designs.
The way I tried it was to tune up the rig like normal, 300 watts of carrier, then insert the screen mod gizmo and adjust it for about 50 to 75 watts output.
At that point, I seem to get almost no screen current and the voltage is under 200 volts I think, grid drive was normal. I did have the mod iron in line, that has to hose things up some.



Doesn't grid leak rely on plate current to work?   And if we d op the screen voltage,  we will drop plate current,  causing our grid leak to drop as well....   Or am I off in left field?

I've got a single 250B here already set for 80-10 as an amp.   Was thinking about scream modulating it....   And if that works,  move it to the 4-1000 rig.

Maybe I should put up an antenna first.   

--Shane
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« Reply #16 on: November 18, 2014, 12:28:13 PM »

Here's the datasheet on the 4CX250 family.

Interesting read under the screen section.

Oh ya, 12W screen and 2 watt control grid dissipation.


* 4X150A_D.pdf (571.96 KB - downloaded 562 times.)
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« Reply #17 on: November 18, 2014, 01:39:26 PM »

Thin, 1/8 inch, plexiglass sheet can be found in the hardware store for replacement storm door glazing.  Smaller hardware stores like TruValue may offer custom cutting for small quantity/size.
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« Reply #18 on: November 18, 2014, 03:10:20 PM »

But I want to modulate the screen, not the plexiglass!
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« Reply #19 on: November 20, 2014, 09:04:47 PM »

Screen modulation can really sound amazing if done right.

First, grid leak bias is fine.  I've used it many times with screen modulation - no problems whatsoever.

The "problem" with screen modulation is that the load varies WIDELY over the audio cycle - from essentially infinity to some (not too high) finite value.

The modulator circuit shown isn't really that suitable for the job - not trying to be critical, but it's just the truth.  There's a resistive series element (the last transistor) in series with the non-linear load of the screen.  This will allow the capacitor across the transistor to charge more, with modulation, varying the DC screen voltage.  That's a problem.  Then the emitter follower transistor is there, but these types of transistors typically have a low beta, so the varying load will be reflected back to the resistance coupled stage, etc. etc. etc...

You would be MUCH better served using high voltage MOSFETs or IGBTs as the active elements.  A simple FET voltage amplifier (it can be resistance coupled) driving a FET source follower (with the source resistor going to a negative power supply to pull the screen negative for full modulation) would be just fine.

The great thing about MOSFETs is that they require NO gate current (other than that required to charge the gate capacitance) to turn them on, so looking into non-linear loads isn't a problem because the varying load is not reflected back to the previous amplifier stage.

Somewhere, I have a MOSFET screen modulator circuit.  If you're interested, I'll see if I can dig it out of the archives and post it.

I didn't look at the tube curves, but is 1200V really required for the screen?  Maybe it is - seems like a lot, but I don't know - didn't see the curves.  Looking at the 4-400A as an example, the screen voltage there is somewhere below 750 volts at full output.   So, you're going to be running a whole lot lower voltage than that to get linear modulation and leave plenty of headroom for positive peaks.  750V would be about the peak value.  If this is the case, you can use really inexpensive MOSFETs for the modulator.

Anyway, an idea!

Regards,  Steve
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« Reply #20 on: November 20, 2014, 11:38:03 PM »

Steve,

I'm considering doing it with one of my amplifiers,  I'd like to see your rendition of mosfet style screen modulator.

Thanks,  in advance.

--Shane
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« Reply #21 on: November 20, 2014, 11:55:56 PM »

Steve
et al.

I've been following this thread with interest. I have just one comment/question:

Isn't the only purpose of the third stage of the screen modulator to provide a fixed positive voltage (left-to-right) across the large value capacitor, which subtracts from the output voltage voltage of the 2nd stage (emitter follower)?

I have been assuming that... given the large value of the capacitor... if the potentiometers are adjusted properly, and if the modulating signal is a normal voice waveform, then the capacitor's voltage will hold fairly steady (i.e. very little change in this voltage, and the changes will occur at very low audio frequencies) as the modulated screen voltage is varying above and below its average (carrier level) value... even though the screen current is a non linear function of the screen voltage.

I believe that the objective is to adjust things so that:

A) The screen voltage is correct at carrier level

B) The fixed voltage across the capacitor is high enough so that the screen voltage is driven sufficiently negative on downward swings of the emitter follower's (always positive) output voltage. This fixed voltage is critical. If this fixed voltage is too low, then one will not be able to achieve sufficiently deep negative modulation peaks at the output of the modulated RF stage. If this fixed voltage is too high, the negative modulation peaks at the output of the modulated RF stage will be distorted. I.e. negative modulation swings will be compressed/clipped.

C) The emitter follower stage is biased so that its output voltage (at carrier level) is sufficiently large to accommodate the required downward excursion of the screen voltage.

Note: the impedance of the audio frequency load on the emitter follower is less than the value of the emitter resistor. Therefore the value of the output voltage of the emitter follower, at carrier level, must be larger than the desired downward swing.

So I think the issue is whether the 2nd stage (emitter follower) has a low enough output impedance to drive the time-varying load presented by the screen of the RF stage that is being modulated.

Stu


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« Reply #22 on: November 21, 2014, 08:12:42 AM »

After fooling around with my flex 1500 last nite I got the Bill Orr handbook out and read the bit on screen modulation and its circuits.

I got two things from the book, plate voltage on the tubes is better if its LOWER then normal.
And that loading is very important, the RF deck should be loaded more then when tuned up for plate modulation.

The Bill Orr handbook gave a good tube circuit and all the setup voltages and currents for an 813 tube.
One thing I am sure of is the circuit can work very well, I have heard it on the air when John uses it and it sounds great.

So I just need to set it up and then go through all the adjustments, starting off with the pots, a tone injected and an O scope on the output will show what is going on with the circuit, then adjustments of the transmitter.

According to Bill Orr, a pair of 813's will do 150 watts, one will do 75 watts, a good power level for 40 meters.
A 4x150 should be good for 100 watts, that could be a VERY small rig, 1000 volt power supply, small low voltage rf deck, and the screen modulator.
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« Reply #23 on: November 21, 2014, 09:36:18 AM »

I recall talking with Frank,  KB3AHE, when I built my screen modulated 4-1000 rig.  He said this tube likes to have the control grid and screen grid modulated simultaneously in order to get the best linearity and highest modulation percentage.   I do not know if this is also true for the 813 or other tetrodes and pentodes.  It would be interesting to see what effect this method would have on the 4x150 series of tubes. 

I know you prefer to have as quiet a rig as possible, with little to no blower noise.  But a screen modulated 4x150 will be running lots more dissipation than it would in class C operation, probably more than in linear operation as well.  So the 4X150 may need a good bit more air flow to make 100 watts screen modulated. 

I look forward to hearing more about your results as you fine tune the rig.
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« Reply #24 on: November 21, 2014, 10:52:15 AM »

Rick

The RF output stage will be operating in class C.

The reason the efficiency will be low (vs. plate modulation) is that (with screen modulation) the amplitude of the RF voltage from plate-to-ground will be less than half of the B+ most of the time.

For example, with screen modulation:

At carrier, the amplitude of the RF voltage from plate-to-ground must be adjusted (via the loading) to be less than (or equal to) 0.5 x the value of the B+ ... in order to provide enough headroom for 100% positive modulation peaks.

With plate modulation, the amplitude of the RF voltage from plate-to-ground, at carrier, can be adjusted (via the loading) to be 100% of the B+ ... because the B+ is being modulated to accommodate the changing (modulated) amplitude of the RF voltage from plate-to-ground.

This results in at least a factor of 2 reduction in RF output power (at carrier) with screen modulation (vs plate modulation)... given the same amount of electrical input power (at carrier) to the modulated RF stage.

Stu

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