[Amps] Tank circuit arcing

[John Becker]

I recently bought a used Henry 3K Premier. Something arcs when I tune it 
up only on 12M and only with the HV switch in the SSB position. I 
removed the cover to take a look and found that the rotary inductor was 
exactly one turn from minimum. This placed the rotary contact in close 
proximity to the termination at the end of the winding. I tried running 
the amp briefly with the cover removed to visually confirm that this was 
where it was arcing. Unfortunately, removing the cover affected the 
tuning, and when retuned there was no arcing but neither was the rotary 
contact in proximity to the end contact anymore.

Unless someone has a better suggestion, I'm going to apply an insulating 
coating to the termination area at the end of the inductor to see if 
that fixes the problem. I can use either "Q Dope" or "Corona Dope". "Q 
Dope" is intended for RF circuits and shouldn't have any effect on the 
inductance or Q of the circuit, but may not have sufficient insulating 
capabilities. "Corona Dope" is intended for high voltage circuitry in 
television receivers, but might not be as good a choice for an RF 
circuit. My gut feeling is that I should use the "Corona Dope" because 
it will be applied to less than one inch of the inductor and therefore 
should have a minimal effect on the inductance or Q. On the other hand, 
the increase in breakdown voltage required to fix the problem may not be 
very much and the "Q Dope" might be adequate. Opinions, anyone? Thanks!

73,

John, K9MM