DO NOT touch anything inside your amp chassis before doing this!
In your amplifier the filter capacitors (the large cans that stick on top of the chassis, sometimes the preamp one is board mounted) store voltages long after use, so this voltage needs to be drained, if not it can KILL.
This diagram shows the first preamp tube in a Marshall Plexi, in a normal Marshall Plexi it’s the one closest to the side edge of the chassis, and furthest from the power tubes.
Start by connecting an insulated crocodile clip to pin 1 of the tube socket, denoted by the grey line on the diagram. Connect the other end to a suitable grounding point, which in this case will be the chassis. Turn the mains and standby switches on. The voltage should drain in around a minute, but most people prefer to leave the crocodile clip connected to ground, as capacitors can re-grow voltage (like batteries)! Make sure you check that there’s no/negligible voltage left before you get to work on the amp.
Never short out the filter capacitors directly.
– Richard Baines