Just like cars, electronic equipment, especially that which deals with power output like amplifiers, needs a maintenance schedule to assure smooth and trouble free operation. An annual service will usually cover it for most players, unless they’re gigging every night at full volume!
Power valves take the majority of the strain in a valve amplifier and when they’re worn and it’s time to replace them, a re-bias is usually required to ensure the correct operating point is set for the new ones. This assures optimal sound versus lifetime. The bias itself can require resetting during the working life of the power valves due to their parameter drift and it’s a recommendation to have it checked every three to six months, particularly if you run your amp hard!
Component ratings will often drift over time and can eventually fall out of their acceptable tolerance range, which in turn can lead to stress in other parts of the circuit, causing further problems. Resistors change in value, capacitors become leaky and no longer efficiently block high DC voltages from sensitive parts of the circuit. Mechanical components like sockets, switches and potentiometers have a finite ‘number of operations’ rating. Eventually they can become lazy, scratchy or perhaps intermittent and so require cleaning or perhaps replacing.
Dry solder joints can adversely affect the sound quality of the signal path, the efficiency of the power supply, etc.
A service will ensure that these issues are ‘headed off at the pass’, keeping your amplifier running at the peak of its performance like it was at day one.