We get asked how often do I need to change my valves? This is subject to personal taste really as long as they've not shorted or failed otherwise physically.
When valves age they tend to give a softer sound sounding like the speaker is wrapped in carpet slightly distorted and muffled. We generally say if you play every day for a few hours then 2 years is pretty good. If you are a bedroom player never thrashing it you could quite happily get 4-5 years without issue.
Replacing the valves is straightforward, relatively... the smaller tubes often marked 12AX7 or ECC81/3 can simply be swapped out without any worry or need to rebias. It's important to notice where the missing pin is as this is the alignment to ensure the right pin is doing the right function. The output valves often the larger ones that generally come in sets of 2,4,6 should be replaced in associated quantities as they work together to push and pull like a tug of war. In most amps they need to be biased which involves measuring the high voltage often abbreviated to HT, measuring of the idle cathode current draw and some ohms law to work out the current, with the known value of resistance of the cathode shunt resistor, and voltage.
We have tables that show the dissipating current curves vers plate voltages and work out the current from here to set the biasing. It's not something we recommend to do outside of a controlled workshop you can easily be exposed to voltages in excess of 500V
We also fit an electronic automatic valve protection system and self biasing adjustment known as Tubesync you can read more about it in the link.
For the technically minded bias is usually measured as current (I) with the usual unit of mA (milliAmp). This measurement is taken from each amp in it's idle period i.e. no sound and is the normal current needed to keep the valve in it's most linear part of amplification.
Some manufacturers and forums say values of -48V, a) this is expressed a Voltage (V) and b is a minus figure, how does that work? Well this is the bias voltage applied to the signal side of the power tube, setting this to a 'preset' value is of no use as it has no meaningful stance as it doesn't tell you how much the valve is taking and conducting, it simple tells you how much your putting into it. Measuring the current is by far the best way to work out exactly what a valve is doing.
Using ohms law we use P=V.I rearranged to give us the current - therefore I=P/V
Power (P) is a known quantity of the valve as different valves have different typical power outputs for example a EL34 is happy around 15W, EL84=7.2W 6L6=18W with a safe window plus and minus. We measure the HT power entering the power tubes, this can usually be measured form the tapping of the output transformer and may range from 250 - 550V so care must be taken to avoid danger.
Using the measured values V and the value P that can be derived from the valve data sheet we are able to calculate I which is the current that needs to be set either by adjustment or replacement of resistors.
The above example of how biasing is worked out is to be taken as a guide for educational theoretical purposes, we do not recommend opening amps and touching parts due to potentially lethal voltages. These calculations may or may not be correct for all amplifiers and in particular Class A amplifiers operate completely differently. No liability will be held for anybody using this guide.