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I see that really Paul almost did the wrong thing.

Paul did a fine job in his article you showed us. Basically his 11 ohm result is what I also got. And he also explains that this number is for one condition and that it moves from there.

The output impedance change is directly related to the lack of class A operation going on. At 0.6 amp quiescent bias, the amp operates class A to 1.44 watts into 8 ohms (0.6 amp peak current into 8 ohms). During this time, there are two tube "banks" (phases) driving the output impedance, even if Paul is not fully convinced.

When at max power, 60 watts into 8 ohms, the peak current has to be 2.74 amps for one phase and 0 for the other. When operating in this outside zone, only one tube bank contributes and the output impedance now comes from the cathode follower impedance of one phase bank of parallel 6AS7s. I don't have that formula right now, but it's easy to look up.

The 6AS7 is not fully linear, and so when one bank cuts off then the Zout is moving with the changing plate curve point for it. And this might help it or hurt it, depending. But for sure it has left the help of the other phase.

So the output impedance slides continuously over many changing values instantaneously during a full sinewave signal cycle.

There is instantaneous and average full cycle output impedance. Most people measure average on meters, few will do the complete job and measure the full plot of instantaneous output impedance over phase and over varying loads. This is the problem you get into when you try to get one number with highly assuming assumptions about the measurement that is not fully understood. Small signal offers a decent complete number but large signal is nonlinear in many ways, especially in this case, and there is no real good measurement technique for that, although Paul tried hard.

I hope this helps explain things.





Edits: 08/31/11 08/31/11

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