Champ screen grid voltage (rel to plate)

[Groover]

I have noticed that the 5F1/2/a amps run the screen at a much potential than the BF Champs do, based on the filter string resistors.

Which started me wondering... what happens to current flow and sound/response as one makes the screen voltage lower relative to the plate?

[MWJB]

Less clean headroom.

[Groover]

So I can raise the screen voltage by changing the first resistor in the filter screen to a lower value, correct?

I guess I should add the difference to the 2nd filter stage to keep the voltage the same to the preamp tubes, correct?

[MWJB]

Yes, I'd stay >2K & add a discrete 470ohm 5W screen grid resistor between pins 4 & 6.

Preamp tubes don't draw a lot of current, so changing say a 10K screen supply resistor to 2.7K & keeping a 22K preamp dropper isn't going to have a huge effect on the preamp voltage (e.g. typically to halve voltages at the preamp you might have to use a resistor x10 the original value), if you are raising the screen voltage then you will be expecting better clean headroom...raised preamp voltages will do the same thing. <250vdc on the 12AX7 plate might be OK?

[Groover]

What is the reason for adding a discrete screen grid resistor as opposed to just having the extra resistance in the filter string resistor itself? What is gained in this approach?

[MWJB]

Good practice. Is it 100% necessary...? You'll find it on high end repros.

[Groover]

buy why is it a good idea? What is gained? Is it like using a grid stopper? (something else I don't entirely understand)

[MWJB]

The original schems often didn't include grid stoppers & screen grid resistors (shielded grid wires, 68k input resistors mounted on jacks, buss wires, twisted pair heaters & other things that you should do), but they were built to a cost & a proven layout. If one had a significant fault, it went back to the test shop...if it was liveable with, like some hum, it was punted out & the customer told "it's normal"...come to think of it on that front the typical amp hasn't evolved much!) You're building one amp and you are expecting to work right off the bat, experience has found these things to be useful safety measures, so put them in, they won't kill your tone but they might save you some head scratching & extra work in the future. Of course they don't make the build "bullet proof" so google some actual pics of working amps & take note of wire routings, grounding etc.

[Groover]

I understand that quality-build, over-build philosophy.

What I am asking about is the electronics of why to use a discrete screen grid resistor. What does this do electronically?

[Bruce / Mission Amps]

I understand that quality-build, over-build philosophy.

What I am asking about is the electronics of why to use a discrete screen grid resistor. What does this do electronically?

There are some impedance issues with these screen grids and resistor but probably not worth talking about right now.
Read up a little more on what the screen grid does with respect to the plate.
However, the thing that MWJB is talking about is that separate screen resistor creates a way for there to be an independent voltage drop for the screen alone.
One of the basics of Ohm's Law is that current through a resistor creates a voltage drop.
If the screen grid draws a lot of current, the voltage drops to the screen.... and if there is way too much current flowing in the screen grid, hopefully the resistor will also burn open, which completely shuts off the voltage to the screen.

[Groover]

If the screen grid draws a lot of current, the voltage drops to the screen.... and if there is way too much current flowing in the screen grid, hopefully the resistor will also burn open, which completely shuts off the voltage to the screen.

This would argue for using a somewhat low pwer resistor in this application, no? If you want it to act as a fuse you don't want too robust a component in there, no?

[defaced]

Correct, and there are guys here who use the appropriate sized resistor so that it will burn open if the screen current runs away.

[Merlinb]

This would argue for using a somewhat low pwer resistor in this application, no? If you want it to act as a fuse you don't want too robust a component in there, no?

I would say no. If the screen grid burns up and the screen is left floating, the tube usually redplates itself to death. The screen resistor limits the screen current when the valve is being overdriven, and prevents the screen from melting. Whatever happens you don't want that resistor to fail. (I suppose you could parallel it with a much larger resistor, like 100k, as a fail safe)

[Bruce / Mission Amps]

I would say no. If the screen grid burns up and the screen is left floating, the tube usually redplates itself to death. The screen resistor limits the screen current when the valve is being overdriven, and prevents the screen from melting. Whatever happens you don't want that resistor to fail. (I suppose you could parallel it with a much larger resistor, like 100k, as a fail safe)

Not arguing the comment but if that is the case, I wonder why I've read of people using on-off screen voltage (or at least really really low G2 vdc) as a standby switch and or reducing the screen voltage to push the tube near cutoff.
Is that a fallacy?

[Merlinb]

Not arguing the comment but if that is the case, I wonder why I've read of people using on-off screen voltage (or at least really really low G2 vdc) as a standby switch and or reducing the screen voltage to push the tube near cutoff.
Is that a fallacy?

I believe they ground the screen, rather than simply leave it floating.