>According to Bill Orr, in his article "Intermodulation
Distortion in Vacuum
>Tubes", "as far as IM distortion goes, (the perfect tube)
is a poor device
>to use in equipment designed for linear amplification".
However, he
>explains that the practical tube in fact will provide less
distortion.
>Speaking about the curve of the practical tube, he states
"the central
>portion is fairly linear and the lower portion resembles a
parabola". The
>G2DAF circuit simply extends the parabola. Compression at
the top end of
>the curve is the biggest cause of distortion, and the G2DAF
circuit may
>actually reduce this by increasing the screen voltage at
the peak.
>
Hmmmmm - IF (big if) the combination of tube characteristics
and drive related screen voltage resulted in an overall
characteristic that was second order (a square law parabola)
then the distortion products would fall at harmonic
frequencies and odd order IMD, the close in stuff, would be
low.
This might explain why the idea appears to work in some
versions of the amp, but not in others.
Steve
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