Hi Kevin

Thank you for your comments, we would only recommend this is carried out by an engineer.

Regards

Marshall Support

Kevin, you won't get the schematic here :)

Origin 20 is cathode biased and no bias adjustement is needed. This is the one I have.

Yours is 50W - it is not a cathode biased amp, it is very seldom to see 50W amps being cathode biased. Yours is fixed (aka "adjustable" ;) ) bias.

So we need some comon sense and widely available knowledge to bias your amp.

When you open it there should be pads on the PCB marked CN32 - this where you measure voltage drop across 1 Ohm biasing resistor.

There should also be TRIM1 trimmer to adjust it.

As the resistor is 1 Ohm the voltage you measure = current *both* tubes consume.

Knowing your plate voltage, plate to cathode and this current through 1Ohm resistor you can calculate your tube dissipation and adjust it to using the trimmer.

Excellent info and calculators here:

https://robrobinette.com/Tube_Bias_Calculator.htm