1955 Gibson GA-30

[boroman]

Cap value doesn't matter as a cap doesn't pass DC (except if it's leaky).

What does the cathode resistor measure?
What is cathode voltage?

255 Ohm measure without cap
16.5 V across it

[Helmholtz]

Means a plate dissipation of 8.8W per tube (assuming a matched pair) or 73% with a 12W tube like an original 6L6GT.
If you think you need higher PD, lower the cathode resistor to 220R or 180R.

[boroman]

Means a plate dissipation of 8.8W per tube (assuming a matched pair) or 73% with a 12W tube like an original 6L6GT.
If you think you need higher PD, lower the cathode resistor to 220R or 180R.

I know changing cathode resistor will change dissipation (but will also affect PV, right?) - question is, was the amp designed to run close to 100% like most of other cathode biased amps - please educate me here...

[Helmholtz]

What we do know is that the amp was designed with a 250R cathode resistor.
Cathode resistor sets the cathode current.
Changing its value means redesigning the amp.

[glebert]

"Amp sounds great" but yet you are considering making changes? If PD was too high I could see it, but for what you described I would leave it aloooone....

[g1]

was the amp designed to run close to 100% like most of other cathode biased amps - please educate me here...

'Most' cathode biased amps are not running near 100%. Unless they are single-ended (one power tube).

[pdf64]

My thinking is that anode dissipation often ends up necessarily high, near the limiting value, due to anode voltage being up near its limit. ‘Necessarily’ because otherwise there can be bias shift (cathode voltage increases) at high power output, resulting in the amp’s sound degrading a bit.
But as your anode voltage is well below the limit, it may be things panned out (eg with the load impedance) such that the idle dissipation could also be well below the limit but still sound good.

[boroman]

'Most' cathode biased amps are not running near 100%. Unless they are single-ended (one power tube).

So, is GA30 a Class A or shifts more towards AB1?

[Helmholtz]

So, is GA30 a Class A or shifts more towards AB1?

Not possible to tell from just looking at the schematic.
But you can easily find out: If cathode voltage increases (by more than say 10%) at large output, it's class AB.

In fact I've not yet seen a PP guitar amp running in pure class A (this includes the Vox AC30).