* * You need a tool like the 'Bias King' or a similar instrument to fit into the power tube plates and take voltage readings. You also need to know where the bias trim pots are and how to adjust them.
* * *I'm looking into this myself as I'm sick of paying big $$$ every time my Marshall has a problem. I'm going to get the book 'How to Service and Repair Your Own Tube Amp
* * * * * * * That said, you obviously don't want to go poking around a tube amp without knowing EVERY safety precaution to be taken. Tube amps
can kill you, even when they're not plugged in. If you even have the slightest doubt it's better to spend money than to conduct voltage with your body.
*Good luck and be SAFE!!!