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RE: Touche', sort of...

Posted by cpotl (A) on February 14, 2015 at 16:08:45

In Reply to: RE: Touche', sort of... posted by Ralph on February 13, 2015 at 14:18:40:

"In the case of the M-60 (with 1 amp of idle current) it remains (barely) in class A2 operation up to full output into an eight ohm non-inductive load (insofar as a 'scope on the cathode resistors is able to show anyway; cutoff does not occur until the amp is into clipping)."

Just for fun, I've been playing around with a Spice simulation of an M-60 this afternoon. Up to a point, I think I see now what you are saying. Namely, that the current in the bank of tubes that is heading towards non-conduction in a given half cycle of the audio signal goes to zero rather more slowly than one might perhaps have expected. ("Slowly" in the sense of requiring a more negative audio signal voltage to drive it into non-conduction than one might have expected.)

The quiescent current per tube bank was 533mA in my sim. I think this is roughly in line with what you were saying (about 66mA per triode section). At about 5W output into 8 ohms, I(max) through one bank of tubes is 1.27A, and I(min) is 0.125A (so not cut off yet). At about 10W output, I find I(max)= 1.65A and I(min)= 0.04A, so maybe one could still say it has not cut off. At about 17W output I get I(max)= 2.10A and I(min)= 1mA. Does one call this cut off or not? At an output of 20W, I get I(max)= 2.33A and I(min) really now shows as zero.

So, in summary, I suppose the question of what power level it transitions from class A to class AB really comes down to a matter of technical definition, and at what point one draws the line. If one were to say that it is class A if even one picoamp of current is flowing through the tube bank that is heading to cutoff, then I suppose one could claim that it *always* operates in class A. But this would certainly not be in the spirit of what class A is supposed to mean, I think!

It seems to me that the whole notion of class A for any kind of push-pull amplifier essentially assumes that the transfer functions for the two halves of the output stage are pretty much linear. If one is depending for calling it class A upon gross nonlinearities that mean the output tube banks are difficult to push into non-conduction, then it's not clear that there is any useful class A quality to the behaviour. It really seems to me that one would then be just claiming class A behaviour on a technicality of a definition pushed beyond its regime of sensible applicability.

Of course, the Spice simulation I did is principally limited by the accuracy of the tube models, but I think it has probably captured the essence of what one would find in an actual version of the amplifier.

Chris

Edits: 02/15/15 02/15/15

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