Different types of amp bias

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Adjustable "Fixed" Bias

Bias exceptions

Cathode Bias (Self Adjusting)

Non-adjustable "Fixed" Bias

Just like the idle of your car may need adjusting when a major change is done to the engine, new power tubes need their idle (the "bias") adjusted to make sure the amp sounds as good as possible. Some amps do this automatically, but many do not - particularly in the world of guitar amps. Many tube amp owners are unaware of this simple regular service for their amp that will keep it sounding its absolute best. The bias methods of amplifiers fall into three basic categories:

Non-Adjustable Fixed Bias amplifiers (no bias adjustment potentiometer)

Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the "fixed" bias point)

Cathode Bias amplifiers (also commonly referrred to as self-biasing)

Each of these types is unique.