[Amps] Air Variable Spacing Question

[Carl]

The Johnson 154-9 used in the SB-220 has .075 spacing and they rate it at 
3000V yet they have a history of spitting at 2500. Johnson/Cardwell rate 
.080 at 3500V but Id go for .100 if you can find one and make up any C with 
a fixed padder.
With the OEP caps you can build to suit with whatever spacing you want

Carl
KM1H


----- Original Message ----- 
From: "Jim Garland" <4cx250b at muohio.edu>
To: <Amps at contesting.com>
Sent: Tuesday, December 04, 2012 12:33 PM
Subject: [Amps] Air Variable Spacing Question


> Hi All,
>
> 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.
>
>
>
> I also have several air variables with .075 in spacing, and one with .140"
> spacing. The latter I've tested to 6kV, but want to save it for a 
> different
> project. In principle, I'd like to use one of the .075" caps for my
> monobander, because of their convenient size, but I think that's cutting 
> it
> a bit close for 2500V. Whaddya think?
>
> 73,
>
> Jim W8ZR
>
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