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[AMPS] Phantom Capacitor

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Subject: [AMPS] Phantom Capacitor
From: (W7CW Jay E Ostrem)
Date: Fri, 27 Mar 1998 02:17:35 +0000
Hi Guys,

Here is an engineering question. 


I'm playing with a triode that has approximately 35pF of plate capacitance.
(3cpx5000a7 equivalent) The tube has a resonant load impedance of about
1800 ohms.  A PI network appears to be very feasible on 160 through 40
meters, but begins to get difficult on 20 through 10 meters.  The plate
tuning capacitor has a minimum of 75pF (air variable 75-500pF at 8kv),
which complicates things.  ARRL Handbook charts indicate a maximum of 35pF
is needed on 20 through 10 meters.  

I considered a few ideas,

1.  Using fixed plate capacitors, and tuning the L.  In this case the tube
capacitance of 35pF would act as the plate capacitor on higher bands.
Apparently this will give the blocking capacitor a workout.  It would also
require a hefty tuned inductor.

2.  Switch to a vacuum capacitor.  This appears to have promise, but
minimum capacitances may still be too high, along with the cost of a cap
that can handle 6Kv.

3.  Use a small inductor .5 to 1 uH in series with the plate lead to lower
the impedance the PI network sees.  I'm not sure if this will cause any
added strain on the blocking capacitor, but it appears to raise the minimum
capacitance required for plate cap.

The question.

Which of these methods, or other methods would constitute good engineering
practice for building a band switchable network?  (how are the big boys
doing it?)

                              73 Jay W7CW

                                 1AB Ranch
                                 Gillette, Wyoming

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