[Amps] Plate Cap Quandary
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
> 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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