Recapping amps

Sean's advice is good...Two additional points from my experience.
1. After waiting for the power supply capacitors to discharge, before you touch anything short the capacitors with your screwdriver. It there is still juice there it's better to put it through the screwdriver than through your finger.
2. With a regulated power supply, adding extra capacitance can screw things up.

Also, Sean has focused on power supply capacitors. In the case of tube amps, where the signal passes from one amplification stage to the next by "coupling" capacitors, these will degrade. Since they are directly in the signal path, replacing them was a fairly routine procedure with tube amps.

Here is a good article on capacitors, measuring them, discharging them etc. - with a good suggestion for a more graceful method of discharging them than the possibly quite startling "screwdriver arcwelding method."

http://www.repairfaq.org/REPAIR/F_captest.html

I know there are several types of capacitors, in terms of the materials used to make them. Are some types more stable than others? Are some types not a problem at all, regarding time, disuse, heat, etc (i.e. they are very stable over many decades)? I am asking because I will be ressurrecting a late 1970's Quatre preamp, and the friend who will be putting it together for me says that some of the caps will be good and stable despite no use for several decades (depending on the type of cap).