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I am not a tube guy!

All active devices have a non-linear transfer characteristic from Vin to Iout - for tubes and FETs this the predominantly square law and for bipolar transistors (BJTs) it is predominantly exponential. This is the primary cause of distortion.

For transistors, the change in transfer characteristic caused by changing the voltage across the device (drain-source or collector-emitter) is modeled by a term called the Early Voltage that acts as a resistor in parallel to the transistor and defines its inherent output resistance. But, this is a linear effect so does not contribute to distortion. The linearity of the output followers in my hypothetical amplifier example will not change as the supply voltage goes from +/-30V to +/60V. There could be second order effects if that change mandated picking different transistors to handle the additional voltage. Higher voltage handling FETs will have lower transconductance and higher voltage BJTs will likely have lower current gain that could increase distortion but, again, this would be a second or third order effect.

I am not a tube guy but I would expect them to behave in a broadly similar way (though I don't think anyone makes a tube power amp with cathode follower outputs). Maybe the tube equivalent of the early voltage is a non-linear effect?

Regards
13DoW

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