[Amps] Plate Cap Quandary

[Jim Thomson]

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