[Amps] Plate Cap Quandary

Carl km1h at jeremy.mv.com
Wed Aug 3 17:31:26 PDT 2011

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>; "Radio WC6W" <wc6w_amps at yahoo.com>
Sent: Wednesday, August 03, 2011 5:38 PM
Subject: [Amps] Plate Cap Quandary

> Date: Wed, 3 Aug 2011 09:18:45 -0700 (PDT)
> From: Radio WC6W <wc6w_amps at yahoo.com>
> Subject: [Amps] Plate Cap Quandary
>    I'm contemplating a single 4CX350FJ (Cout = 7pF) amplifier for 250 
> watts on HF
>       where the plate impedance, at 2200 volts, figures to be 5000 ohms
>       and requires a (total) plate C circa 15pF on 10 meters,
>       which makes for a rather hard to find (air variable) plate cap.
>    So, I thought what if I take two MC325M, 1KV caps  (min C. 14pF)
>       with the contoured plates for nicer tuning, gang the shafts, and run 
> them in series
>       resulting in a theoretical min. C of 7 pF (plus strays naturally)
>    Does this make sense?   Has it ever been done?
>       Yes, I'm aware of the 1uH plate L network kluge.
>    Or should I just add a second tube, run twice the power at half the 
> impedance,
>        and sidestep the whole situation?
>    Constructive thoughts or tangential comments both welcome! :-)
> 73 & Good morning,
>   Marv WC6W
> ### either use 2 x tubes and double the plate current..and drop plate load 
> Z  in half ..OR... just use a small amount of L... like  .5 uh,
> between plate block cap  and C1  tune cap.  That alone will transform the 
> plate load Z  down to a low number... that the main PI net
> can now handle.    The bigger the extra L... the bigger the plate load Z 
> redux.    The extra L can also be installed between anode and
> plate block cap if you want.   A vac cap will also work.    Ur idea of 
> using 2 x air caps in series will also work..albeit as long as the
> sliding rotor  contacts are making contact.
> Later... Jim  VE7RF

Simply wind the 10M coil as per the software program using your favorite Q 
while ignoring any C. Then center tap that coil with the Tune C; it will be 
4X ratio so now you need 4X the C to resonate.

The effect carries thru  the other bands and rapidly becoming unoticable.

Its a method used for many decades (since the 40's at least) with big high C 
tubes in commercial shortwave and Ive been using since the 70's days of 
building many 2X  4-1000A's for customers.


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