• Recapping an amp
The capacitors in a amp are a large part of its sound. They serve several functions.
Filter caps
These are part of the power supply. If they get leaky, hum may increase, and bass response may change. These are the caps that store a charge when the amp is turned off and uplugged, so you must discharge them before working with them or they could kill you.
The 3 caps on the right side with the orange ends show signs of leaking--that little blister. These should be replaced for reliability reasons as well as tone. It seems like if you buy replacement caps of recent vintage, they will not need to be formed by slowly bring the voltage up over time. You can substitute them and power up.
These are electrolytic caps, and have a positive and negative end. Pay attention to which way you hook them up or they explode.
(I need to mention that I found these 2 photos on websites, they were not taken by me, but I've lost track of where they came from and can't credit them. Let me know if you have info on who took them)
These caps have high voltage ratings--several hundreds of volts. You can always use caps with a higher voltage rating, but not the other way around. Also, if you have a tube rectifier, you should match the values to that of the caps you're replacing. If you increase the capacitance that a tube rectifier sees outside of specified limits for that tube, you'll shorten that tube's life.
If the amp has a fixed bias supply, this supply will also have a electrolytic filter cap. It's voltage rating will be lower, typically 100 volts or less. It's a good idea to replace this as normal maintenance. As it ages it can cause hum in the amp. I often upgrade the voltage rating on these, some Fender amps use voltage ratings that were close to the voltage being developed, and I bump up the rating to about twice the bias voltage.
Cathode bypass caps
These may also be electrolytic, but are low voltage, about 25-50 volts. Electrolytics will dry out with age. These are inexpensive, and replacing old ones can have a large effect on tone.
Coupling caps
These are high voltage (400-600v), small value (0.001 to 0.47 uf) caps, but are not electrolytic. In Fender amps, the brown "chocolate drops" are not regarded highly, but blue Mallory and Sprague Orange drops please a lot of ears. Upgrading these caps may be another area that will increase an amp's tonality.
Tone circuit caps
The tone circuit often uses several caps, and sometimes there are caps bridging resistors. Ceramic disks may often be used for this purpose. While not high-quality caps, some argue that they are part of the vintage sound. Silver mica is considered an upgrade for this cap.
I have put silver mica caps in my amps, and not noticed too much difference. However, my silverface Deluxe Reverb used a ceramic cap on the input to the phase inverter, and this is one that I think should be changed to a Sprague or Mallory. All the sound of the amp is going to pass through this cap, so I wouldn't skimp on this one.