Here is one alternative to the problem, Scott.!! a few years back I bought a
"kit" capacitor which had a total capacity of about 500pf, and I needed the
lower min.c for 10 meters. I cut the stator rods at about 4 or 5 plates
from the rear , inserted a couple of nylon spacers, which I had ,of the
right size, cut threads in them ( 10/32 ) and assembled the cap .!! I can't
remember the exact capacity of the 10 mtr. section, but when paralleled for
the lower bands it was about 500 pf. and using only the smaller section on
15 and 10 it worked just great. I don't know if that "kit" is still
available but it worked fine for 4 811A's and also for a pair of 572B's . A
little effort there was well worth my time. Someone out there now has that
cap in a HB amp.
Have a good day all.
----- Original Message -----
From: "Scott Townley" <nx7u@arrl.net>
To: <amps@contesting.com>
Sent: Sunday, October 02, 2005 12:02 PM
Subject: [Amps] air variable min C?
> Regarding air-variable capacitors:
> Most folks refer to air-variable caps by their max C value, and sometimes
> include the plate gap and/or rated working voltage (particularly when
> selling them!)
> But much less frequently mentioned is the minimum C value, which of course
> is quite important in Ctune applications.
> It's surprising (to me, the uninitiated) how few "vintage" caps (Johnson,
> Millen, etc.) are suitable for say a single 3-500z amp without
compromising
> the loaded Q at 28MHz. Or perhaps I am missing something. (I am
purposely
> excluding vacuum variables here). To me "suitable..." means around
> 10-130pF at > 4kV.
> So a few questions:
> 1. What are the primary driving factors behind achieving small minC in an
> air-variable? Say I would homebrew one...what are the mechanical/layout
> considerations in the capacitor itself that tend towards small (c. 10pF)
min C?
> 2. Are there other sources/manufacturers beyond those I've previously
> named? I have several old Johnson catalogs and it seems that only 4 of
the
> many, many caps they make/made even come close.
> 3. Or is padding for the low bands more common than I think? There are a
> few more Johnson caps that would work if you padded both 40m and 80m.
> Thanks,
>
> Scott Townley NX7U
> Gilbert, AZ DM43di
> http://members.cox.net/nx7u
>
>
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>
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