If I make the screen resistors higher....

[Chubsman]

If I make the screen resistors higher? Will that bias the tubes hotter, and drive them harder?

And is the opposite true also?

[BobD]

The screen grid determines what voltage will be required to bias the output tubes, and they have a small effect on the max plate voltage but they have nothing to do with running the tubes hotter or driving them harder. Their main function is to prevent over-dissipation of the screen grid.

You need to play with the cathode resistor to run the tubes hotter.

[kd]

If you increase the size of the screen grid resistor you'll lower the voltage on the screens, might make your tubes last a little longer. I wouldn't even think about running the bias on the el84's any hotter.

Ken

[zaphod_phil]

The Marshall 18W is already beating the EL84s to death - and the same goes for the 36W. They're biased above their maximum ratings and being slammed with excessive drive voltages, so that they're constantly operating on the brink of crossover distortion. Apart from changing to fixed bias, there's nothing more you could do with these amps, as every last drop is being squeezed out of the power tubes.