Lowering plate current in Marshall 1974X Reissue Combo Amp

[nevetslab]

Is it odd that the plate voltage did not rise from 321v when you increased the cathode resistor?

Yes. It DID indeed rise. I have it on the bench now, opened up, and just using DC Voltmeters for the measurements instead of the Bias Probes. The plate voltage at Pin 7 of either EL84 tube sockets, I have 340.8VDC. I have 12.09VDC across the 120 ohm cathode resistor, and 0.937VDC across the 100 ohm screen resistor. So, calculating the idle current....12.09VDC / 120 ohm = 101mA, and having seen previous balance current thru the two new tubes, that gives us 50.5mA. Screen current is 0.937V/100 ohms for 9.37mA total, or 4.69mA per tube. So, Plate current per tube would be Ip - Is, or 50.5mA - 4.69mA = 45.7mA. Plate voltage is 340.8V - 12.09V = 329VDC, and Pwr Diss = Vp x Ip = 329VDC x 45.7mA = 15.0W

I haven't made any measurements under drive conditions. Plate voltage would come down, though the plate current would increase. and no doubt the plate dissipation.

[Helmholtz]

With cathode bias plate dissipation typically lowers with signal, because increasing cathode voltage shifts the operating point to colder operation.