Manual vs. Auto Bias

Autobias has a main advantage over fixed bias that tube life is much longer than in fixed bias. During powering up an auto-bias self-adjusts the quiescent current thus giving a tube a 'soft' start.
Sonically It's also more beneficial since stabilization of quiescent current decreases distortion.

Marakanetz,
Thanks for bringing up that point. The builder of my Coincident amp as I recall did mention soft startup for the tubes in this design, and said tube life is very long.

More over I'd say that if I ever go back to tubes, I'll make sure I'll buy an auto-bias amp or if price is convincably low for fixed-bias unit, I'll DIY implement it(most-likely with SS parts for less space and money).

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,