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Do you know or have an idea what causes various tubes to sound different? Emissivity is obvious; the higher the emissivity the lower the impedance. Interelectrode capacitance could affect highs, but really shouldn't be much of a factor in tetrodes and pentodes, especially at audio frequencies. An English guy measured the harmonic distortion of a bunch of tubes and determined that statistically black plate tubes have lower harmonic distortion than other tubes (but no explanation).

One thing you wrote I suspect is most of it: That different tubes sound better/worse in different amps. That suggests interaction between the tube and the circuit. Reactance, feedback, and power supply impedance (perhaps even as a function of frequency) come to mind.

So what's left? What are the physical reasons that tubes in good condition sound substantially different from each other?

I think that a perhaps a study of gain linearity and distortion as a function of signal level and complex load might be revealing. A good study would probably be at least a Masters thesis.



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