> If there is severe overshoot coming out of the exciter any
> amplifier is
> going to have problems with it causing distortion
> products.
A grid driven tetrode has the most severe problems with IM
perfromanc. Although they could be designed with controlled
negative feedback, most are not. That being the case many or
most are already poor as a general rule compared to cathode
driven PA's. Case in point, look at the grid driven
commercial amateur amps on the market recently.
Grid driven amps are especially intolerant of occasional
overshoots when they have loose goose bias and screen
supplies that have slow recovery times.
> With an AB1 amplifier with a stiff bias supply how do you
> keep from
> destroying the grid unless you NEVER bump into grid
> current? With a higher
> impedance bias circuit it is self limiting in grid
> protection and alc is
> easily developed as is done in the Collins 32S1 etc.
I think you mean the 30S1. The 30S1 is OK because the very
high amounts of negative feedback caused by cathode drive
overcome the poor design of loose screen and grid bias. The
NFB stabilizes the exciter load impedance and corrects some
of the distortion, so overall it is pretty clean for a
poorly regulated design. It was also a very good match for
the exciter power, and many of the rigs back then didn't
seem to overshoot as bad as they do today. It also had
pretty fast voltage recovery on the grid after being
overdriven.
Take away that negative feedback and the case changes.
Especially when the leading edge overshoot is handled by
allowing the grid bias to bump way up.
I'd be careful about using grid impedance to protect the PA.
As a matter of fact, I'd never consider it a good idea.
73 Tom
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