• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

I suppose I am a little dense

In this article


I am trying to understand the transfer function graphs. Is he plotting the instantaneous DC grid voltage v.s. DC plate current?

It is an interesting approach I think. It might also explain why in different amplifiers different folks have come up with differing "optimal" bias point for VA stages using the same preamp tube.
Oh, BTW, in another article I read a claim that if the same tube is used with a CC followed by a CF using the same load and plate voltage that the distortion cancellation trick would also work in that case. I am not sure I understand how that could be since the degeneration in the CF should give it a different transfer function than the CC stage. Where am I getting off track?

Oh, Here it is...

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Well, it'd have been a lot easier if he had cared to label his axes ;-) Right now we can only guess.

My guess is that the horizontal axis represents the grid signal voltage (referenced to the bias voltage) and the vertical is plate current. Then again, the horizontal axis could be plate voltage referenced to the quiescent plate voltage... The gradient of the curve in Figure 1 indicates about 1600 ohm or 0.63 mS, which does not correspond to either Ri or S of the 300B?!?

When you balance off distortion against other distortion you are actually doing something not very different from negative feedback, except that NFB adjusts itself whereas manual balancing needs to be done manually. In both cases the result is a reduction in low order distortion and an increase in the relative effect of higher order distortion. The effect may depend on signal level.

He was talking about SE and evaluating valves on that basis. You need linearity for this. A push-pull amp can handle lots of even-order in the valves, as it cancels in the OPT. If you look at some of the popular PP valves they are not so good for SE.

Finally, he accepts that some people prefer distortion and some amps deliberately include it. If only everyone could be that honest we might make more progress in audio!