[AMPS] Re: New G2DAF amplifier information

[Tom Rauch]

> 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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