The AM Forum

I came across a very simple screen-mod circuit.
-one dual triode, no iron, also doubles as a screen protective circuit (if you loose grid drive, it will cut out the screen V to some low level to prevent damage).

Check it out!  I'm thinking that this circuit and a beefy tube (6080) will screen mod my 4-400 deck and provide some protection to boot.

Does look simple, As a cathode follower, voltage gain less then 1, I think  its gonna take alot of audio
voltage so that you get decent swing on the screen of the final.

True that.  to be honest I don't know what the audio drive requirements will be, yet.

-and that 68k screen to ground resistor will have to be a high wattage unit too.

You also need to know the voltage drop across the 2nd triode and what screen voltage is desired.  Perhaps 225 screen voltage, 225 drop across the resistor split from 450?

Yeah basically you run 1/2 screen voltage then normal class c operation, so 225 volts sounds
about right for a 4-400, you will then need 225 volts audio signal going into the circuit
above. Not realkly a big deal, but not as simple as just feeding it line level. Looks like 68k, should be at least 2 watts..
Hope ytou bulkd it Ed and give us report, I got a bunch of 6080s here, and may do
something like that..

BTw maybe some of the guys that actually have built/use screen modulators, can chime in here, but I think in order to get 100 negatve (cut final
off) you need negatvie screen voltage, the circuit above won't do that, but get close perhaps, nice thing about low mu triode..

Of course if you need negative screen voltage to ensure cutoff, all you have to do is return the 68K cathode resistor to a small negative voltage rather than ground. It's basically the same circuit as a cathode follower driver for class B grids.

I was originally going to use the same tube lineup for a cathode modulation setup.  After talking with Steve_QIX who recomended the cathode follower to screen modulation idea this is the circuit I came up with after researching. 

Overall it will end up a little more complicated than I intended (bigger audio drive required now) as the original circuit only used two tubes (6SN7 audio amp and 6080 as modulator) to cathode modulate the final.

I know few of the "efficiency" modulation setups will get 100% modulation, but it seems to me that we go through lengths to limit the negative peaks to <100% anyway.

just a couple of things ...

the output section of the 6080 has high cathode voltage .... it will need separate fil voltage source ... this will exceed the spec on the other half ....better to use the 6080 sections in parallel and gain another 13 Watts plate dissipation (as I recall) ...check 6as7 spec

just about any low mu triode should work for input stage .... 6c4, 6s4, etc
could be fun ...

I have something fairly simple using MOSFETs if you're interested.  Do you know how much negative voltage is required to cut the tube off (virtually all screen grid tubes require negative voltage to achieve 100% negative modulation)?

An effective screen modulator would be reasonably easy to construct from MOSFETs, and no fil xformers or anything like to worry about.  The audio from this circuit will also be supurb.

Let me know !

Regards,

Steve

A 6080 has its voltage limits but a 6883 will take 400V and a bit more. If you can confidently rely on a MOSFET, that is a fine thing. I really like MOSFETs for such 'switching' functions, but I am still afraid of a spike popping one and taking the rig out of service. Ever since I put a 450V one in the socket of a 6L6 single ended audio amp... then another, and another.. I ran out of free ones. That was 20+ years ago, I suppose I could do better now. I'd like to see a proven circuit for a screen mod/clamp using a FET. Maybe I learn some new tricks.

I was originally going to use the 6080 with both triodes in parallel for the Cathode Mod circuit, since that thing could handle obscene current for that application. I was worried about the voltage limit in this circuit but mostly I was going to use the 6080 because I have one in the junk box, not because it was the ideal component. 
The FET's have advantages, and I'm not stuck on any one device type (hollow or sand state). 
 I'd like to see the FET circuit too. 

One reason I like this technique is the clamping function in case of loss of excitation. This circuit will pull the screen down if the grid drive goes away, keeping the screen from overloading and going POOF.  I can also use the pot to reduce output for tuning etc. another nice feature for so few parts.

It is easy to put the clamp function into the FET screen modulator.  A trigger off the negative grid bias voltage developed across the grid leak resistor.  If the voltage rises (from the negative) over a certain point - goose !  No screen V  8)

Ok, this looks simple, but will it work? 

I don't know what the bias should be for the fet, or what kind of audio volts it would take to drive it.  This would be a 900v 8A Fet I found at Mouser.com.   I figure that would have plenty of headroom.  Go with a discrete grid bias supply.

Did you ever get an answer to that question?

I'm interested in using that circuit as a screen regulator.  Would make adjusting screen bias with an inexpensive pot VERY easy!


--Shane

Well, you've got a source follower going  ;)   You need to be able to drive the screen negatively with respect to the cathode - AND - you need a voltage amplifier ahead of the follower gate.   Here's a concept schematic.  A real implementation could be greatly improved and provide things like negative feedback, voltage stabilization and an easier adjustment.  

There is a way to completely eliminate the negative power supply, and still drive the screen negatively using a zener string and a capacitor.  This method works if you have a higher voltage power supply available than you actually need.

This one shows the necessary parts and the concepts, but is not a good implementation.

I post something more practical when I get a chance to do the complete design 8)

(http://www.classeradio.com/screen_modulator.jpg)