Cathode follower distortion: It was been called the “penultimate” distortion – the positive half of the signal clips first, and the negative half much later. This is typical cathode follower distortion, which sounds creamy and smooth and wonderful. Many “boutique” amp makers try to achieve this in their circuitry.
This example is the output signal from Fred Shuman’s 1967 Princeton Reverb. The PR uses a cathodyne phase inverter which is part of what makes the amp sound so good. Here the amp volume control is at 4.5, and we can see the cathodyne distortion; the top peak is clipping and the bottom peak is just starting to clip. What a great amp!
If you look closely you can see the “crossover notch”; the little bump between the top and bottoms of the signal. This is crossover distortion. The “notch” is also used by some amp techs to set the idle bias of the amp – you want to set the bias so the notch disappears. In Fred’s amp, the crossover notch is absent when the amp is at idle, but as with all AB amps, it will show up with the rest of the distortion when the amp is pushed a bit, as here. Crossover distortion sounds different, but still good.