Manual vs. Auto Bias

Viridian, the term 'auto-bias' refers to a form of fixed bias operation, wherein the bias current is monitored in some form by either an analog or digital servo circuit which allows the amp to react to changes which may come from the tubes or perhaps line voltage.

The term 'cathode bias' or 'self bias' refers to an amplifier that is biased by a resistor in the cathode of the power tube circuit. Now it is true that such an amplifier does not need adjustment, but how that differs from an auto-bias amplifier is that the bias in an autobias amp is applied to the grid and is not derived from the cathode circuit.

This issue is indeed confusing!

The reason that autobias arose is there are those that think that self bias does not sound as good. Self-bias also requires higher power supply voltages in order to get the same amount of power.

Yes, the whole thing is confusing!

'Fixed bias' refers to any method of biasing the amp where the changes to the bias are affected by adjusting the voltage on the grid. This voltage 'fixes' the bias.

'Self bias' (also sometimes called cathode bias) refers to the use of a cathode resistor to bias the tube, the grid being tied to ground through a high resistance.

'Manual bias' is a method of fixed bias that is user adjustable.

'Autobias' is a method of fixed bias that is adjusted by a circuit (servo) in the amp and does not need user attention unless a tube has failed.

The two former terms are considered engineering terms. The two latter terms might be considered marketing terms.