Because it's a set once and forget forever issue with SS amps. Also, you aren't as much setting bias as DC offset.
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For the original question generally bias is set at the factory during the initial check and then again after a burn in period. Transistors and FETs don’t age the same as tubes and so while it may drift it normally wouldn’t be too much. It will drift more with temperature and line voltage than with age. Doesn’t mean it can’t be done or checked with a service manual and volt meter if one wanted to.
My experience has been that bias and DC offset are two separate adjustments. Bias sets the standing current through the output devices and somewhat the transition into class B. DC offset zeros out any DC potential between the two output terminals.
The main reason to increase the bias of a SS amp is to reduce crossover (i.e. notch) distortion, but it is not really something a consumer should do, and the benefits are rarely worth the risk to the transistors and power supply.
Where exactly to set the bias point for a given transistor is a matter of some art. Enough to eliminate gross distortion but not enough to warm the room.
SS bias need not ever (almost ever) be adjusted if no rail caps or output devices are replaced during the normal lifespan of an amplifier, or if there is no trim pot drift/failure.
Once upon a time you would find the bias procedure and values in the owner’s manual but today almost everything comes with a warranty and/or the manufacturer’s lawyers advise "No User Serviceable Parts Inside".
Because it's a set once and forget forever issue with SS amps. Also, you aren't as much setting bias as DC offsetNot true. Bias is applied to both sides, so current increases, but net voltage remains zero.
IMO, bias drift accounts for varying sonics in SS amplifiers. Bias drifts with temperature. Service manuals often have voltage spec, but no °C or how and where measured. Bias an amp that's been idling for 20 minutes and then measure it after Zepping for an hour and the values will be radically different. Add in one or two 10°C ambient changes for open top on a workbench in an air conditioned factory and a buttoned up amp in a rack and the delta is even larger.
This is why fuse changers hear such 'UGE differences. Neither their ears nor the electronics are in the same state.
Depending on where the feedback device is located, there may be a considerable time lag to the thermal compensation device. Consequently, many designs are under biased to preclude thermal runaway.
Transistors with on-die bias diode offer better performance at low signal levels, just where most of the music resides.
If your amplifier is in for service, the bias is a thing that should be checked by the service technician (and its very likley that the technician will have to check it anyway if the service has anything to do with a damaged output section). Its not hard to do, but isn't for the faint of heart as one slip with the probe of the meter and the entire output section could be destroyed.
If the amp has not been serviced and is 20 years or more old, I would have it tested by someone that knows what they are doing. While semiconductors are much more stable in terms of bias drift as opposed to tubes, its a simple fact that they do change over time and 20 years is plenty for that. In addition to the bias the DC offset should be checked as well but any competent technician will check that too.