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Re: 'may be operated in triode mode at the same bias point'

Posted by Henry Pasternack on June 16, 2005 at 12:05:21

In Reply to: 'may be operated in triode mode at the same bias point' posted by JJ Triode on June 16, 2005 at 09:45:36:

IIRC, the most power you can get out of a quad of triodes in ideal Class A, push-pull operation without exceeding their ratings is the value of a single tube's plate dissipation. This would be 30W. It presumes the tubes are perfectly and ideally matched to their load, that the bias point is exactly right, and that the tubes are perfectly linear. in practice, the Class A power output will be lower. But there is a caveat. Especially if you run the tubes at high plate voltage and low bias current (as VTL does), you can get this situation where, due to nonlinearity at low currents, the tubes never quite cut off, no matter how hard you drive them. On the other hand, they don't contribute any useful power in this part of the operating region and may actually become net power sinks. But under the strict definition of the term, the output stage is still operating Class A.

I looked into this back around 1993 and discussed it on rec.audio.tubes. This was early in my tube education, when the "VTL Book" was one of my main sources of information and I was still trying to sort out a lot of new (to me) information. I was quite frustrated at the time because what I was seeing in the book didn't add up. Knowing that everything I say becomes part of the public record, I don't want to go so far as to say anyone is lying. But I will say that it appeared at the time that VTL was interpreting "Class A" in a way that contradicted my understanding of the term.

I propose that a practical and conservative definition of Class A requires not only that the tubes never cut off throughout the AC cycle, but that they remain in the substantially linear part of their characteristic curves, and deliver meaningful power to the load at all times.

-Henry

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