What appears on the scope's screen tells all. If the amplifier operates in Class A, the waveform will match the waveform across the 8 ohm load, save for amplitude. If the trace looks very distorted and in fact resembles a half wave rectifier circuit, then the amplifier is something less than Class A. Really, it is that simple.

Since the amplifier runs in Class A2, thus all that has been written about current and Class A operation and all the math does not apply to the evaluation of the Class A nature of the amplifier.

    Sorry, but it does, for Class A2, which means that the amplifier experiences positive grid voltages, points in the wrong direction. The issue is not whether conduction can be further increased by driving the grids positive, but, rather, if the tube stops doing any meaningful work during any part of the waveform. Tube cutoff is the focal point, not positive grid current.

   Well, no it isn't. You see this only means that the tubes used are grossly non-linear in their cutoff region and may actually never cut off due to poor design or manufacture; for example, remote cutoff pentodes and variable mu triodes such as the 6ES8 are made with a variable pitch in the grid wire spacing, which results in a path for some electrons to flow through constantly and makes for a terrible tube for audio use.

Triodes that never turn off

Triodes that do turn off






64 w

   Well, is this not a testament to the  genius of designer of the amplifier, as he purposely choose to use spongy tubes so he could make an amplifier that broke the laws of physics and put out gobs of Class A watts without having to pay the price of high idle current and dissipation? Brilliant, no?
   Actually, no. The problem with this approach is that the desirable attributes of the triode and Class A operation in general melt away in the sponginess region. Not all conduction is equal.
    Imagine if a buddy of yours boasted that he had more money in his wallet than you did in yours as the two of you walk into a casino in Vegas. You are puzzled, as you saw him pull only $100 out of the ATM just 10 minutes ago and you have $500 in your wallet.


-110                -55                 0          +30   Vg

Cathode current vs. grid voltage

Since the flat portion of the waveform across the 1 ohm resistor is not sharply flat, but softly rounded, therefore it must be Class A as the tube never stops conducting. There, see, the amplifier is Class A just as I said it was!


pg. 18


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