Manual vs. Auto Bias

Not sure why the discussion of auto bias vs. fixed has come about since the OP is seeking info on auto vs. manual bias, where neither is fixed. Anyway, my manual bias amps also have the soft-start feature, which certainly is good to save tube life.

Manual bias adjustments here with a 2 minute soft start (Lamm ML2.1).

Need a decent true RMS voltage meter (I use a Fluke) and one has to make adjustments to both plate voltage and current on each monoblock. The measuring jacks and adjustment pots are right on the top, with the required values written adjacent - very easy access. It's a ten minute job and the drift over time is small. It's not a problem.

Regards,

Tube auto bias circuits are SO cool...I wish they had 'em in guitar amps. They get rid of an issue I've always had with high end tube amps.

Rockdanny time to go back to school, manual bias is technically referred to as fixed bias as the the bias is set and fixed; it does not float. Auto bias is referred to as cathode 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.