Correcting Plate Voltage

[JammyDodger]

Hey All,

I am working on an amp and my plate voltages on V1a and V1b are off more than I would like. They both have a 100k dropping resistor on them. How can I calculate what resistance I really need.

For example I am running 202V on Pin 1 with that 100k resistor. What can I change that to to get to 150V?

Cheers, Mike

[FYL]

Play with the resistor in the power supply node tied to this tube. Plate resistors condition tube working points and shouldn't be modified in order to lower supply voltage.

[Structo]

Yes the plate and cathode resistors set the bias of the tube, the plate resistor is loading the plate.
Trace back the wire or connection to the power supply.
There will be dropping resistors there, in order to lower the voltage to V1 you need to raise the value of dropping resistor.

I don't know the math as there are things that affect the loading of the circuit so I just experiment with different values until I get it close.

Be aware that even a different tube there can change the plate voltage by as much as 15v!
So maybe roll a few tubes through that socket before heating the iron.