6CB5A Fixed Bias G2 Limits

[tunertom]

I have seen cathode-biased 6CB5A SE amps (G2 connected to the plate via 100R) designed with

Va= 400V
Vk = 75V

which exceed the maximum G2 plate rating of 220V apparently without ill effect/failure. I would like to try a fixed bias (power drive) version. Should I limit the Va to 325 volts and call it good enough, or could I go higher to pick up a little more output power? I don't really understand how much gyrating the plate/G2 voltage swings around while the tube operates under various loads and bias schemes. OPT is big Hammond 1628SEA if that makes any matter.

Thanks.

 

[kodabmx]

Because the plate is the most positive element when using a dropping resistor from G2 to plate it usually works fine, however Tubelab has said he build an amp with similar tubes (6LW6) and it worked for weeks or months, and then a tube would run away and die. He suggests keeping it under 375V.

Also I connect G2 to plate with a 220R.

 

[45]

It depends on the actual tube and how it is used. There are many examples of pentodes that have g2 max voltage lower than max plate but can use higher voltage when triode strapped. For example the 6F6 that has a 285V G2 limit but can work pretty fine at 350V in triode mode without any resistor between g2 and the plate!

Anyway that 400V on the plate and 75V at the cathode reminds me the 6CB5a amp built by VinylSavor which is a pretty reliable amplifier. So the short response is: yes, it is possible. Higher is probably asking too much...