> >I'm learning a lot here, ...snip,,,
>
> Yes, but does it wash? When a tube with a high output-R (100k ohm) bias
> supply is temporarily overdriven by a voice peak, the negative grid bias
> voltage temporarily increases due to grid-rectification and the resulting
> E=I*R drop across the 100k resistance, so the grid becomes more negative,
> so grid current tends to decrease, which limits IMD-genenrating grid
> current. OTOH, with a stiff grid bias supply, overdrive produces high
> grid current. ....... Does increasing grid current in Class AB1 increase
> or decrease IMD?
You idea does not wash at all Rich.
IMD is caused by plate current saturation, as well as non-linearity
in the grid.
Of these two effects, plate current saturation by far dominates the
IMD. We know this because we run, on a daily basis, grid current in
linear amplifiers with very good (sometimes better than -40 dB) IMD
results.
The reason grid current caused the problem is, as Ian correctly
pointed out, the grid spikes up to a high value. This moves the tube
into cutoff, and reduces the output capability during peaks when you
need it most!!!!
If you ever watched an AB1 PA on a spectrum analyzer, while
over driving the grid slightly, you see Ians postulation is EXACTLY
correct. A stiffer grid supply reduces distortion.... just the
opposite of your claim.
73, Tom W8JI
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