Instead of lowering Cmin at Ctune, you can insert a small L between the tube
and the blocking cap. This has the effect of raising the plate load
resistance and increasing the minimum Ctune required at higher frequencies.
The effect reduces as frequency goes down of course.
I did this on a plasma heater I converted into a ham amp. It uses a 5CX1500
which has a pretty big Cout. Adding a small coil (.3uH or .5uH, I don't
recall) in series with the anode did the trick. You will have to calculate
the effective plate load resistance for each band to determine the Ctune L
Cload values.
Also did this with a 2X4-400 rig to accommodate a vac variable I had on hand
for Ctune.
I used Ian White's (GW3SEK) "Netcalc" program to calculate the plate load
with various values series L, then plug the effective plate load into your
favorite PI net calculator. NETCALC here:
http://www.ifwtech.co.uk/g3sek/netcalc/netcalc.htm Thanks Ian!
73, Bill N2BC
----- 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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