> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Tue, 22 Jul 97 03:19:59 +0000
> > it ain't grid current that's the defining
> >characteristic! it's the linearity! the "transfer characteristic"
> >should be a straight line. if it bends, then it ain't linear.
>
> Agreed. With grid current, the line bends.
No, that is not true. The line can curve without IMD.
The slope of the curve affects the level and amplitude of
various IMD products, and if the curve is sloped correctly no IMD
products are produced despite the fact the slope changes.
Anyone who thinks a straight line is required for proper IMD
performance needs to read Bruene's "Single Sideband Principles and
Circuits", or look on page 68 of Eimac's Care and Feeding of Power
Grid tubes.
A sloped line, with a since shape, does NOT produce harmful third
order products. A Fourier analysis shows there are NO odd order
products in such a curve, so the harmful third, fifth, seventh, and
so on products causing splatter are not present.
The ideal plate grid transfer characteristic on page 69 of Care and
Feeding shows just such an ideal curve.
To be a good mixer and produce strong third order products, you need
a sharp knee in the transfer characteristics, not a curve.
Some intentional distortion in the grid can be used to cancel other
distortion in the output. There is NOTHING special about this
technique. Anyone who has done IMD testing has certainly seen this
effect. I've seen many PA's get cleaner as grid current appears.
An 8877, drawing grid current, can easily be in the mid forty to
fifty dB down IMD range, on the other hand a 4CX250B operated in
class AB with ZERO grid current might only be 20 to 30 dB down for
IMD.
Saying grid current always produces more IMD is a good example of
distortion......... a distortion of fact.
73, Tom W8JI
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