There are three basic types of tubes used in guitar amps, rectifier tubes, preamp tubes, and output tubes.
Rectifier tubes are used in the power supply. Their function is to turn AC (alternating current) into DC (direct current). The DC is then used by the remaining tubes to amplify the guitar signal. Rectifier tubes generally have a very long life and rarely “wear out”. They typically work fine until failing at which time the amp will stop working and the fuse will probably blow. I don’t recommend replacing the rectifier tube until it fails which could be many years. No adjustment is usually necessary when it is replaced but it is a good idea to check the bias (see below) just to be sure. If the fuse blows, never replace it with a fuse that has a higher amp rating than specified. Note: Some amps do not use a rectifier tube and have diodes instead. Diodes are not user replaceable parts.
Preamp tubes are the smaller ones. Their function is to add gain or increase the guitar signal in the amp. Most common is the 12AX7, aka ECC83 and 7025. 12AT7 is also fairly common. Others are also used occasianally. Like the rectifier tubes, they have a very long life and rarely wear out. Sometimes they will stop working causing various symptoms. The most common failure is “noise”. You might hear static, hum, loud hiss, crackling, and “frying” sounds when they fail. Again, I don’t recommend replacement as preventative maintenance since new preamp tubes have a fairly high failure rate and new tubes might introduce new problems to your amp. Exceptions are the 12AT7 reverb driver in Fender amps and 12AU7 or 12DW7 driver in Ampeg SVT amps. These applications stress the tubes and I often fine weak drivers in heavily used amps. Replacement of preamp tubes normally requires no adjustment.
Output tubes are the big ones. They provide the power to drive the speakers. Since a lot of current passes through the output tubes, they do tend to wear down with usage. The lifespan depends on the tube and amp design but in general, 6L6, 5881WXT, EL34, 6V6 last about 1000 hours and EL84 last around 500 hours. By the time these limits are reached, the tube will loose the ability to produce maximum current and the amp will begin to loose tone. The bass frequencies will start to loose punch and start sounding a bit loose and flabby. There is also in increased chance of major failure of the tubes as they approach these limits. A major failure means the tube dies or shorts out which will usually blow a fuse. The output tubes should be replaced in matched sets. In most amps, the bias will need to be adjusted after output tube replacement.
Bias adjustment – Bias adjustment is necessary when replacing output tubes on most amps. The adjustment requires taking a measurement of the tubes’ current and making an adjustment (usually inside the amp). If the bias is adjusted too low, the tubes will run cool and the tone will suffer. If adjusted too high, the tubes will run too hot and fail much sooner than properly adjusted tubes. Please note that all Andrews amps have external test points and adjustments for bias. Instructions are included in the owners manual so that in the case of tube replacement, you won’t need to make a trip to the amp shop.