If the point of operation is set at -2 volts of grid bias or at -6 volts, we may never realize that the triode has a profound break in linearity. In other words, if we remain within a grid-voltage window of 0 to -4 volts, the tube's linearity is impeccable; the same holds true in the window of -4 to -8 grid volts. But at the -4 volts of grid bias point of operation, the tube's break in linearity is easily seen, as the triode offers a larger gain to positive going input signals than to negative going signals. In fact, the 2nd harmonic distortion is readily calculated to be 16.7% from the following formula:
       2nd Distortion = 0.5(Vmax + Vmin) - Vq
                                           Vmax - Vmin
(To some extent, all triodes resemble this imaginary triode in that their amplification factors are not constant, decreasing with increased plate voltage and low plate current.)
      Now let's imagine two of these imaginary, dual-mu triodes used in a line-stage amplifier. The first triode is used in a standard grounded-cathode configuration; the second, in a cathode follower configuration. Both triodes are biased at the -4 grid voltage point and both see the same cathode-top voltage and plate current. The first triode directly cascades into the second triode. How well will circuit made up of grossly non-linear triode work?

      To get a feel for instantaneous voltages, take a thick telephone book and draw a sine wave on the open edge. Each page holds only one small dot of ink in much the same way that one slice of time hold only one plate voltage. (Digital recordings simply take the instantaneous voltage value for any given time-slice and convert it to binary code.

       The next step to understanding how inverse distortion can lead to less distortion is to imagine a triode that displays an odd property: it has two sharply defined amplification factors. Form 0 to -4 grid volts, the mu is 40, but from -4 grid volts on, the mu is 20. And other than this break in a consistent mu, this triode is perfectly linear.

0   Vp     100            200            300             400

Vg              -10  -9  -8  -7  -6  -5  -4  -3  -2  -1   0

Constant-Current-Draw-Compound amplifier


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