What are filter caps?
Filter caps are electrolytic capacitors (see above) found in most power supplies. They smooth, or filter out, the ripples left after rectification of the alternating mains voltage, producing the clean, steady DC voltage that most equipment requires: hence they are called filter or smoothing caps. Valve amps require a very high DC voltage (up to 700v) to operate, so the filter caps used in these amps have to be capable of tolerating these high voltages. All capacitors store energy within them, and filter caps are designed to store a lot of energy which they use to maintain the high voltages found in valve amps. They are easily capable of storing enough energy to maintain lethal voltages even after the equipment is switched off. If they are not safely discharged they can deliver fatal shocks. If in any doubt do not touch them!
