A WORD ABOUT HIGH-VOLTAGE SAFETY
Many people don't know that working in tube amps can be extremely dangerous. There are LETHAL voltages inside these things when they are powered up, and for a time after they are powered down, even days. It is entirely up to you if you wish to risk your life working inside one of these, otherwise refer to a qualified and experienced Amp technician.
This is not a hobby for the faint of heart, but if you commit a few safety rules to memory, then you will live a long healthy life without getting electrocuted or bitten by a nasty jolt due to carelessness:
* ALWAYS WORK ONE-HANDED whenever working inside an amplifier or other tube-based electronics. This does not matter if it is powered up or not. The Capacitors can hold a charge for a long time, unless discharged properly. Once you can confirm that your Volt-Ohm-Meter measures 0 Volts DC from the B+ to Ground, then it is safe to work with both hands after unplugging. Otherwise, shove your non-dominant hand into your back pocket, and leave it there.
* NEVER WORK TWO-HANDED if you are not sure if your amp still has a charge. Since if you make a circuit between your hands, the current will flow straight across your heart. I can't reiterate this point enough.
* OBSERVE PROPER POLARITIES of components, otherwise you risk fires or or exploding capacitors in your hardware...And you will weep if you destroy an expensive piece of Iron (transformers) or pop expensive tubes.
* CAREFULLY CALCULATE YOUR MOVES AND ACTIONS before attempting to probe a hot circuit. Don't be careless. Use an insulated probe like a glass rod, fiberglass rod or dry wooden chopstick.
* A "BLEEDER RESISTOR" CAN SAVE YOUR LIFE. This is simply a 100KΩ 1-watt resistor attached to the High-Voltage B+ rail at the first stage (the Plate of Tube 'V1'), and grounded to the input jack's ground. After you power down your amp and unplug it, this will slowly bleed all the remaining current from the capacitors, typically over a period of 2-10 minutes. Confirm afterward by measuring the B+ rail to ground that you get 0 Volts DC before attempting to work inside of it.
(In progress, stay tuned for updates to this post)