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@muohio.edu>
To: <Amps@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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