50W Screen Current

[ChrisM]

Anyone know what kinda current the screen's are drawing on the 50W ODS. Let's say biased to 60%-70% max dissipation, 440V on the plates.

Thanks...

[phsyconoodler]

And you need to know that because.................???
Likely the same as any other 6L6 amp.Minimum 470 ohm screen resistors required.

[ChrisM]

Designing an amp, trying to figure out if I can get away with the nice compact 50mA choke or do I have to use my bigger 90mA choke.

Was already going to use 470R 5W screen resistors. Looking for the current draw.

[Structo]

When I checked it out on my D'lite with RCA 6L6GC blackplates, it was only around 1 or 2ma if I recall so not much to even taking into consideration as far as biasing is concerned.
I use 560R 5w sand boxes.
The plate voltages are around 435v.

[phsyconoodler]

Take a look at a super reverb and a Twin.They both use a pair of 6L6's,or a quad of 6L6's and use a 4H-90MA choke.The Marshall JCM 800 uses a 40H-50MA choke.
And then a Vibrolux Reverb uses a pair of 6L6's and uses a 4H-50MA choke.
I am not sure why the differences with a very similar cicuit like the Super and Vibrolux.
All those amps have chokes in the same place in the circuit.
I would also like to know what the difference is.??

[ChrisM]

That must have what was flowing through the resistors?

That seems really low. Just thinking cause on Valve Wizard Merlin is designing an amp and specing the EL34 screens at 22mA or so. The EL34 has a pretty weak screen grid so I figured the 6L6GC would draw even more.

[Structo]

If I recall I measured the voltage drop across the resistor then divided that by the resistance which is 560R.
That's correct, right?

Maybe I did it wrong but I remember thinking that I didn't need to worry about that anymore since it was so small.

BTW, I used the 022699 4H 90ma choke on my 50w.

[ChrisM]

If I recall I measured the voltage drop across the resistor then divided that by the resistance which is 560R.
That's correct, right?

Maybe I did it wrong but I remember thinking that I didn't need to worry about that anymore since it was so small.

BTW, I used the 022699 4H 90ma choke on my 50w.

There is going to be a voltage drop across the choke as well though. You need to figure that out as well and add it to the "equation". I dont know how to do that though really, as the resistance of the choke is constantly changing I would think while the amp is operating.

[Structo]

Yeah so you would have to figure the inductive reactance and work that into it. And that will change with frequency and voltage.

Personally I feel that there have been so many amps built that use the PI filter that I just don't worry about it and bias my power tubes to what sounds good and ignore the screen current.
As long as you use screen resistors from 470R to 1K you should be fine.

[ChrisM]

Ya, I am more looking to see if I can save space and use my 50mA choke.

[Merlinb]

As a quick and dirty estimation you can expect the max average screen current to be about 10% of the plate current for beam tetrodes, or about 20% of the plate current for pentodes.