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[Amps] Vacuum Variable Capacitors

To: Amps <>
Subject: [Amps] Vacuum Variable Capacitors
From: Roger Parsons via Amps <>
Reply-to: Roger Parsons <>
Date: Mon, 18 Mar 2024 20:22:40 +0000 (UTC)
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Please excuse what may be a dumb question. What does the voltage rating of a 
vacuum variable mean?

The reason I ask is that I have had one fail due to arcing whilst being 
operated at well under what I thought was its rating.

I run my amplifier in class AB1, but for simplicity let me assume class C. In 
that case I believe that the plate output voltage swing is between near 0V and 
near the plate DC supply voltage. The flywheel effect of the tuned circuit 
means that the rf voltage swing (after the coupling capacitor) is plus and 
minus (nearly) the plate voltage. Whilst simplified, I think that is correct. 
It also assumes that the amplifier is working into a matched load.

So, there is no DC voltage on the tuning capacitor, and the rf voltage swing is 
as above. However, it is never more than the plate voltage in either a positive 
or negative direction, and it is obviously only in one of those conditions at 
any one time. So if the voltage rating of the capacitor is identical to the 
plate voltage it should be sufficient, although with no margin? Or should the 
voltage rating be the peak to peak voltage? Or should the voltage rating be the 
rms voltage?

I assume that my capacitor has lost all or part of its vacuum, although there 
is no obvious sign of damage.

Comments welcome.

73 Roger

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