There are many websites out there describing modifications that can be done to popular tube amplifiers. While amp modding can be a lot of fun — and can let you custom tweak your amp to sound exactly the way you want it to, be aware that this can be a dangerous pastime if you’re not careful!
The power supply filter capacitors must be drained of the voltage they hold before you do any work on an amplifier. These caps can hold hundreds of volts of juice; if, while working on the amp, you manage to form a path for the caps to discharge through your body, the current will pass through your heart and could kill you.
Note: working on an amp with one hand in your pocket or waving in the air behind you does not necessarily protect you – current can still pass from anywhere that’s wired to the cap’s positive plate through your body to ground.
Note 2: leaving the amp turned off/unplugged for a day or a week or two does not guarantee that the filter capacitors do not contain lethal voltages. They must be drained for safety’s sake.
Note 3: using a rubber-handled screwdriver across the cap terminals may drain the voltage, but it may also result in a screwdriver fused to the capacitor.
Note 4: the popular “trick” of turning the amp up, playing a loud chord, and pulling the power cord out of the wall is not a reliable way to discharge filter caps.
Please consult a qualified tech if you don’t know how to drain power supply filter caps or don’t know how to measure the amount of juice each cap contains.