• Recapping an amp

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.