In an electronic circuit the bias voltage (or bias current) serves to control the internal DC conditions necessary for proper operation .
Most high-powered guitar amps employ push-pull class AB circuitry. In this configuration the power tubes do not operate continuously but are alternately switched on and off. This causes a very ugly glitch to occur known as crossover distortion. Ideally, the output stage bias voltage is set to provide enough current flow to eliminate this crossover distortion. Too little current will cause your sound to be distorted and lacking in sustain, too much current will cause the plates to overheat.
When the bias voltage is applied directly to the input grids this is called fixed
biasing. Usually you will have an adjustable control to set the proper voltage. Even though you can vary the negative bias this is still known as fixed bias because once it is set it does not change. If this voltage disappears, so do your tubes.... they will get red hot and incinerate. If there is too much of it your sound will be cold, choked, and with no sustain.
A more dynamic method known as self-biasing
sacrifices some of the output power to provide negative feedback voltage. This technique is used in smaller amps and sounds softer when the amp is heavily clipped. The Vox AC30 is a prime example. A self biased amp is more forgiving than a fixed biased one when installing new tubes, but in extreme cases needs adjustment as well.
Used to be that you could replace your output tubes with new ones from your original amp manufacturer and not have to worry about setting the bias voltage. Tubes were selected to conform to tight specifications and some amps had no adjustable bias control as this was not really needed. Today, however, manufactures no longer reject any tubes no matter how their specs differ from the original design. Instead they now market them as having "early distortion" or "late distortion".
With the wide range of tubes available, it is necessary that the bias be checked when changing output tubes. Even matched pairs of the same rating as your original tubes can be appreciably different from each other. I have measured "matched tubes" that differ by 50%.
Unmatched sets vary as much as 1000% (10 times the current in one tube as opposed to the other) or more! Still, some people question the need for matched tubes! Each tube must be carefully measured before biasing.
The plate voltage of the amp must be taken into account when setting the proper bias level. There is no way to properly bias a '74, 100 watt Marshall operating at 560 volts and to eliminate all the crossover distortion when it is driven to clipping. 50 watt Marshalls can vary from 325 volts to 450 volts and this must be taken into account when biasing the amp.
The quality and brand of the output tubes are naturally extremely important. If you have cheaper brands they won't hold out and you must compromise on the bias setting and therefore on the sound quality of your amp.
Amps are often played in conditions which are far from ideal and are subject to poor ventilation, improper and fluctuating mains voltages, sudden surges, etc. All of this must be taken into account or your amp will blow right in the middle of your big solo.
I strongly suggest that all biasing be done by a skilled and experienced technician, preferably one who will test the amp at full power with a guitar and not with a signal generator.
The preamplifier stages of an amp tend to be "self-biasing" and generally you never need to adjust the current in a 12AX7, for example. This does not mean, however, that you can substitute another type of preamp tube such as a 12aT7 or a 12AU7 and expect your amp to work properly. If you wish to try lower gain preamp tubes you must change all the other components that set up the operating conditions as well and redesign the circuit for the new tube that you are using, otherwise distortion will occur.
Generally, though, this means that you can replace the preamp tubes of your amp and not have to bring the amp in to a tech for a bias adjustment.