What the heck is bias anyway?


I'm getting a new tube amp that will need to be biased. The process of doing it seems fairly straight forward, but I am curious: what are you actually doing when you bias an amp? In english, for us technical Ludites.
grimace
Kirkus, where the analogy of idle speed falls apart is with an engine, regardless of whether I set the idle speed at 500 (for the sake of example - too low), 800 (recommended), or 1650 (too high) RPM, performance will be identical at 2500, 3500, or 6000 RPM.

With a tube amplifier biased at 20 mA (again, for the sake of example - too low), 50 mA (recommended), or 75 mA (too high), its performance will be different in readily apparent ways when playing music.
My injectors are class D but I only run them in class A. I generally bias with IAC adjustments but sometimes fuel pressure.
Mitch4t, this depends on the amp. Some amplifiers have very stable bias settings and can be checked maybe once every 6 months. Others seem to need constant observation. So I would go with the manufacturer's recommendations.
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Thanks for the reply Atmasphere. I really would like to try tube amps, but I'm not mechanically inclined at all. I'm a plug and play kind of guy, which is why I have solid state amps. I'd consider tube amps as long as they don't require much fiddling around with. I think I read somewhere on Audiogon about self-biasing amps. If that technology exists, why don't all tube mfgs use it. Is self-biasing expensive to implement? Is it as accurate as a manually biased amp?
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