Manual vs. Auto Bias

My bad. I ASSuMEd auto-bias meant auto-adjusted by the amp itself, fixed-biased meant there was no adjustment knob/screw to change the bias, and manual-bias meant the adjustment could be made manually by adjusting a control knob/screw.

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.

The same amp that has manual bias can be upgraded to have an autobias by quite a few examples from simple shared cathode resistor to a complexed current mirror (using transistors, zenner, shottke diodes). After such upgrade the sound does not degrade at all in the same amp. The cost of labor increases and probably steals the manufacturer's profit, hence there's a reason to say that it can degrade the sound.

Do you know that 2 matched new tubes will age differently and even if you check bias of your amp often, you might still miss the moment when tube's going out of bias beyond its operational tolerance even before reaching its half-life? I'm very savyy with DMM of any kind, but learned to appreciate tube amps with autobias implemented.