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RE: Class A Operation/Bias Points with PP 6B4G

The class of operation refers to the tube's duty cycle. Class "A" operation means the tube is conducting 100% of the time. Class "B" operation means the tube is conducting 50% of the time.

For max. power O/P, Class "A" requires the tube be biased exactly 0.5 way to cut off. The theoretical max. plate efficiency for Class "A" is 50%. In the real world, Class "A" triodes (SE or PP) are good for approx. 20% plate efficiency.

Class "B" operation implies the tubes are biased exactly to cut off. The theoretical max. plate efficiency for Class "B" is 78.5%.

FWIW, I like to set PP "finals" up in "shallow" Class "A". Under that regime, both tubes are conducting MOST of the time. Only when peak power O/P is demanded, are the tubes alternately cut off. The small amount of crossover/notching distortion is hidden in the loud music.

Eli D.

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