Step 4: DISCHARGE THOSE CAPS!!!!!
The 'filter' caps can store fatal amounts of electrical current. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.
If you're completely lost, and don't understand this, DON'T MODIFY YOUR AMP. You haven't enough knowledge to work on high voltage/current circuits safely...
Several ways to discharge caps:
FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)
--Take a screwdriver or a jumper and short the capacitors leads.
--OR jumper the power amp tube plate pin to GND for a minute or so (Class A, single power tube only.)
--OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here...
Some, or all of these methods may result in a spark... Obviously, your flesh can act as a jumper also.
It's NEVER a good idea to touch amp circuitry when it's switched on. If you're stupid enough to do it, NEVER USE BOTH HANDS--that way, there's at least a slim chance the current won't flow through your heart.
And don't work on amps in bare feet, on a damp basement floor. :-(
Don't even play your amp in this fashion...