Tom has some very good points. Personally, the G2DAF is not an approach I
would use, but it can be made to work. My feeling is that some tubes work
better in that circuit than others; the screen grid time constant needs
careful consideration, and the effects of the rectifier on the effective
grid loading are not negligible. I suspect atube that was very low
distortion in screen modulated AM would be the best performer. Whether the
approach can, under optimum conditions, be made to work adequately well for
today's bands is another matter. I am certainly unconvinced that any tetrode
can be just stuffed in and produce acceptable or better results.
I cannot accept that grid current is always, under any and every condition,
tube and circuit, is of necessity going to produce too high a level of
distortion.
>The operating point moves all over the place with tuning and
>loading, as well as drive.
This is where the G2MA circuit, which effectively used a clamp tube to
modulate the screen, won out; the variable RF loading was reduced to merely
a control voltage. However, I still believe that a tetrode with suitably
regulated screen and bias supplies, operated with a low grid RF source
impedance, and RF negative feedback, in a conventional circuit, is a
superior product to a G2DAF approach.
I figure that Rich and Tom will probably agree there!
73
Peter G3RZP
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