[Amps] Air Variable Spacing Question

[Bill Turner]

ORIGINAL MESSAGE: 
On Tue, 4 Dec 2012 10:33:47 -0700, 4cx250b at muohio.edu wrote:

>
>I'm trying to decide which air variable plate tuning cap to use on my
>forthcoming 160m monoband amplifier. The one I'd like to use has .080" plate
>spacing, and I've tested it and it works up to 3200 VDC on my tester before
>sparking across the gap. My question is whether that's enough of a safety
>margin to use with 2500V plate voltage on three GU-74Bs? Not that it matters
>particularly, but the plate impedance of the three tubes is about 740 ohms.
>There'll be no DC voltage on the cap.

REPLY: 
Quoting from the ARRL Handbook chapter on amplifiers:

"The peak RF voltage present across a properly loaded tank circuit, without
modulation, may be taken conservatively as being equal to the dc plate voltage."

 The problem is you have little margin for error, and errors do occur. If you
accidentally have a combination of being too lightly loaded and excess drive,
the RF voltage can rise to VERY high levels and your tune cap will arc. Once a
cap arcs, tiny blisters form at the point of arching and future arcing becomes
even easier. 

I like to be conservative with such things. For your amp I'd use at least a 10
kV rated capacitor. You could get by with less, but it would be risky. In my own
homebrew amps I use nothing but vacuum variable caps, both tune and load. 

Great email address by the way.  :-)

73, Bill W6WRT