Thanks for the explanation.
I wonder if tipically in a modern tube amp(as PrimaLuna etc) the Automatic-Bias system constantly adjust the tubes (say thousands times per second) or it adjust the tubes some times per minute??
Sorry if looks a dumb question.
Automatic/self is DC bias and continuously adjusts bias, at any instant in time.
Fixed bias (output stage, tube) typically allows for greater output power because the full B+ is applied across plate to cathode. The cathode is typically grounded (or close) and negative voltage is applied to the grid. Thus maximum plate voltage swing is achieved for a given B+.
Cathode bias, on the other hand has the grid grounded, with bias voltage applied from cathode to ground. Thus optimum use of B+ is not achieved since the bias voltage is subtracted from B+ at any given instant, thus the tube sees less plate to cathode voltage.
There are variations such as cathode bypass caps vs no bypass caps, which affects the output power.
Hope this is clear; it is simplified.
Cheers.