Q: "How often should I replace the tubes in my amp?"

A: This question comes up quite a bit, and has been discussed before (feel free to search the TTOTD archives).

The difficulty is there's no concrete answer to this question, though there are some guidelines. Tubes will exhibit vastly different characteristics, including longevity, depending upon how they are used. This refers to he type of circuit they are in, how that circuit is set up (tweaked), and how hard the amp is pushed how often. Of course, different types of tubes have significantly different life spans as well. Practically speaking, you should only replace tubes in an audio amplifier when you start to notice changes in the sound quality. Usually the tone will become "dull", and transients will seem to be blunted. Also, the gain can decrease noticeably. If you're using relatively current tube equipment it's a safe bet you can get some specific guidelines from the manufacturer. If you have some ancient equipment you probably should seek out a technician who's familiar with older gear. If you get lucky you may even be able to find a tech who has access to a device known as a "mutual conductance tester." These devices aren't that common, primarily because tubes aren't used in electronics anywhere near as much as they were 20 to 50 years ago. Nevertheless this is the only reliable way to determine if a tube must be replaced. Since most people don't have access to such a device the general rule of thumb that gets put into practice is, "when in doubt, replace the tube." But remember, this only applies to old gear. Many devices built in the last 15 or 20 years are designed to have much longer tube life, especially things like preamps and compressors. You should really contact the manufacturer. Also, don't forget, when you change tubes you usually should re-bias the tube circuit as well.