[Amps] Vacuum Variable Capacitors

[Roger Parsons]

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
VE3ZI