ORIGINAL MESSAGE:
On Sat, 16 May 2009 17:38:44 +0200, Ulf Tjerneld <star@frizon.org>
wrote:
>
>2, I have a perfect capacitor for the Pi-tank in the above example.
>But is rated exactly 2500 volts (Johnson something...). Would that be
>to little for an unloaded plate voltage of 2500 volts?
REPLY:
The short answer is no, that is not enough voltage rating and here's
why: When everything is tuned up correctly and not overdriven, the
peak RF voltage will be almost exactly equal to the DC voltage. The
problem is, there will be times when it is not quite tuned right, i.e.
there is too much drive for the amount of loading, or you might QSY
and forget to retune, or you accidentally select the wrong antenna,
etc, etc. If any of those things happen, the capacitor will arc and
very likely will be permanently damaged. You didn't say whether the
cap is an air or vacuum variable but it doesn't really matter. Once a
capacitor arcs, the surfaces become pitted and become even more prone
to arc the next time. I would never use a cap rated less than twice
the DC voltage, and more is better. In my current amp with 2900 VDC
under load, I have a 15 kV cap. Some would say that is overkill, but
it has never arced and probably never will, no matter what dumb thing
I do.
73, Bill W6WRT
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