Screen voltage higher than plate voltage

[EETStudent]

Finally got my 5f2 completed, and it sounds very nice. I was taking some final voltage measurements to confirm everything was as it should be, and I noticed pin 4 on my 6v6 was about 20V higher than pin 3. Is this something to be concerned of?

[MWJB]

5F2 (as opposed to 5F2A) taps both the screen & B+ supply from the same point (like 5E# Fenders), rather than after a dropping resistor. The plate draws more current than the screen so the voltage will be higher...which is probably the case for all 5E# Fenders & even tolex champs & Princetons. 20v sounds a lot though? What is your B+ and plate current?

Have good listen to it, then add a dropping resistor from the B+ rail to pin 4, to drop 20v you might need a 10K resistor and this sounds excessive to me. I'd try a couple of kohms up to whatever gets your screen voltage just under the plate voltage, you could add another filter node & dropping resistor between plate & screen supply.

[tubeswell]

Maybe your OT primary DC resistance is unusually high? (in which case you might want to even things up a little with a 470R-4k7 3W-5W screen dropping resistor as MWJB suggests. But when you do this, bear in mind that if the screen dropping resistor is not 'decoupled' at the screen end of the resistor (with a hi-voltage cap to ground), the screen voltage will change with the signal, and the tube current won't be as constant as if could otherwise be, so your power output will be a little lower in general.

[loudthud]

Look closely at the attached graph. It shows the plate current as the plate voltage is swept from 0 to about 600V while the screen voltage is held constant. There is a different line for different values of screen voltage. The grid voltage is zero volts. Note how after the plate voltage gets to about 50V, the plate current stays relatively constant. It's no big deal if the screen voltage is slightly higher than the plate voltage.

However, if there is ripple on the screen voltage, it will significantly modulate the plate current and get to the speaker.

[pdf64]

Is there a way of assessing (ideally a rule of thumb, but also maybe the full formula) how much ripple on the screen is significant?
Like what's the gm of the tube at g2, compared to g1 (in the context of regular guitar amp circuits)?
Will the ratio be fairly common across all power tube types, or are pentodes different to beam tetrodes, or are they all individual?

[Steve Conner]

Multi-grid tubes should have an "inner mu" quoted on the datasheet. If I remember right, you can take the ripple voltage at the screen grid, and divide it by this figure, and that's the ripple voltage at the control grid that would have the same effect.

In a 50 watt amp, the voltage gain from a power tube control grid to the 16 ohm speaker output is about 1. Less for smaller amps, and presumably cut in half for SE. So you can relate control grid ripple voltage to noise voltage/ power at the speaker that way.