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Arny Krueger wrote:

Actually, there are two effects

(1) Modulation distortion in the form of modulation of an audio signal being
amplified at the same time.

(2) Noise caused by modulation of the bias voltages present in the tube.

Wouldn't a low-impedance bias driver help reduce the levels of (2)
above? (Granted I don't know amplifier design theory, so I'm not sure
that the bais point CAN be low impedance. But it would seem to me if
the current flowing through the bias point was substantial (10x)
compared to that flowing(leaking) into the tube bias, that the bias
point voltage would therefore not change much as a function of the
leakage through the tube grid.

Or mechanical vibration propigated through furniture and room structure.

of course; I assumed physical structure isolation, but did not mention
it.

and how much is due to internal system issues in-time with the signal
being amplified?

Now we're back to the more common form of distortion due to the nonlinearity
of the tube.

Ah. Very good. yes.

...dane