The harder the modulated tube is driven into class-C conditions and
saturation, the more linear does the plate modulation become.

Operation of the tube becomes independent of curvature in the tube's
characteristic. The plate current operating angle is small.
It behaves more like an on/off switch.

The more non-linear it is, the smaller the operating angle, the more
linear is the modulation.
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Reg, G4FGQ.