[Argus25]

Joe,

Interesting the idea of putting resistors in series with the screen grid. I'm not 100% sure of all the effects of this, but depending on how large the R value, this could be three. (Maybe more could be added to the list ? )

1) It could lower the gain a little or signal level at the plate.

2) with the screen moving around with signal voltage, there will be Miller effects at the grid (g1) so the apparrent input capacitance of the valve will probably increase, lowering the frequency response at radio frequencies, maybe why it helps with HF stability.

3) maybe suppresses Barkhausen oscillations of electrons around the screen grid.

What other effects I wonder ?

In some vintage TV's I have seen using EL33 output tubes I have seen 47R resistors placed in series with the plate. Presumably this was to suppress oscillations due to the capacitance and the primary leakage reactance of the transformer with the secondary loaded by the speaker.

But with small class A output stages at least, I have never had one yet that has not been tamed with the screen bypassed to AC directly to the cathode with a combination of a large value for low audio frequencies and a good 0.1uF cap and a large enough series g1 resistor. I have not had one where the anode resistor is definitely required, though I have fitted one as a link across socket pins for wiring convenience, but it was all for radio, not HiFi.