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Generally speaking there are two types of tube transfer characteristics:

The optimum characteristic can be described as two straight lines: constant transconductance at high input levels, and a second straight line of transconductance going from zero to the large signal value at low input levels.? When biased at the half max transconductance point ideal class AB operation is obtained: for small signals there is half transconductance for both positive and negative going inputs, and for large signal there is twice transconductance for positive going signals and cutoff for negative, i.e. the same time-average.? Triodes often have this characteristic, as the decreasing plate voltage keeps the gm (transconductance) from rising at high drive levels as the load line decreases the instantaneous plate voltage.

A non-optimum characteristic would be one where the transconductance is constantly changing, with no large constant regions.? This typical of many tetrodes, for instance, where the screen allows the gm to keep rising at hight currents and the device is more constant current.

In the latter case feedback can be very helpful in improving distortion, and without it IM tends to be awful no matter the bias point.? But in the former case the biggest factor is the bias point, and there will be a distinct dip in IM as the bias is adjusted.? The 3-500Z is the former type, and you can see from the data sheets of various manufacturers that IM of -40dB or better can be obtained if the bias point is optimum (the 1500V zero bias case and the 3500V -15V bias case).? An old-school analysis of the the tube transfer curves leads to the same conclusion.? So for the 3-500Z family choosing the optimum cathode bias voltage can be more effective than feedback in optimizing IM.

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