> I have a feeling that a lot depends on the tube(s) used: effectively, they
> have to be very linear as a screen modulated AM tube at the PEP output
> required.
> 73
> Peter G3RZP
Hi Peter,
There are a number of problems with shifting the operating point
around.
1.) Phase delay.
If you filter the rectifier that samples the input signal, phase delay
is introduced. There is no way around that problem. Phase delay
will cause envelope distortion, and normal feedback won't remove it.
2.) Erratic loading of the exciter.
The exciter is loaded heaviest on peaks, when it already has to
supply maximum driving power and is generally falling off.
Even the smallest amount of peak loading can cause significant
distortion. There wouldn't be a significant problem if the screen
drew 100 mW of load and the exciter was 100 watts swamped
through heavy resistive loading, but there is a big problem if the
screen draws 10 watts on peaks and the exciter is only 100 watts.
3.) Little control of operating point.
The operating point moves all over the place with tuning and
loading, as well as drive. Tetrode are already bad enough, because
of their inherent lack of negative feedback in most circuits.
73, Tom W8JI
w8ji@contesting.com
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