Rich:
>Does it make sense that the IMD products generated by a grid-driven
>4-250A would DECREASE as the grid is driven from the AB1 region of zero
>grid current into the AB2 region of grid current.? If this were true,
>then it would be "EXACTLY correct" that the G2DAF circuit is pristine
>clean. :)
Have a look in The care and Feeding of Power Grid tubes: there's a set
of curves of IMD against input as you get into grid current.
Provided that the grid (and screen, if applicable) stay at constant
volts, and that the increased loading caused by grid current doesn't
screw up the cleanliness of the driver, why should grid current cause
distortion? (Here we are assuming the correct plate load impedance, of
course) After all, there have been lots of Class AB AF amplifiers built
where the tubes ran grid current............they needed a low impedance
supply so that the drive volts didn't sag, if distortion was to be
avoided.
After all, 3-500Z s in grounded grid draw grid current, without being
disastrous on linearity. I suspect that the G2DAF problems came from
poor drive regulation, insufficient screen volts, and being touchy -
Dick Thornley certainly produced some good looking pictures of two tone
tests, and I seem to remember some guys at the Marconi Company getting
good IMD on the spectrum analyser using that circuit. Somehow, I was
never too impressed - seemed to me to rely too heavily on getting the
ratios of voltages, drive etc just right.
73
G3RZP
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