In particular any paper capactitors made by Hunts are suspect. These are tubular in shape and either brown or black in colour and were used extensively in Britsh equipment of the fifties and sixties.9 out of 10 will be leaky causing general deterioration of performance, and extreme cases, such as the grid coupling cap to an output valve, damage to other components.

The FM950 pictured elsewhere on this site I have had for about 4 years. It was fitted with several Hunts capacitors and although it originally performed well, over time its performance was falling right off. Removing all the Hunts components brought it back to life. Interestingly in the most critical positions Erie disc types were used, making me wonder if they had problems with leakage from the word go. I have heard before that Hunts destroyed the British consumer electronics industry. Personally I think this is overstating it, but it didn't help.
 

One problem with capacitor replacement for valve equipment is sometimes locating components with a high enough voltage rating. In the UK maplin (www.maplin.co.uk) is one source, but for the FM950 I needed values they didn't have and in the end ordered from RS components (rswww.com) via the web. Another good UK source is Farnell (www.farnell.co.uk).

Carbon resistors having values above 100k or so will sometimes rise in value to many meg. Again replacement is the only answer.

Valves that are more prone to failure are those handling appreciable power, such as output pentodes, and those used in circuits having a large heater to cathode potential such as cathode followers and cascodes.