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[AMPS] Phantom Capacitor

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Subject: [AMPS] Phantom Capacitor
From: (Rich Measures)
Date: Thu, 26 Mar 98 19:25:39 -0800
>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.  

I calculate  RL as 6500V/(2*2.5A) = 1300 ohms.  For a Q of 13, XC1 = 
1300/13 = 100 ohms - which is approx. 54pF at 29.7MHz.   It appears that 
a variable Tune C with a minimum of roughly 15pF is called for.   A 
vacuum variable C would seemingly fill the bill.  

>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.

not enuff on C on10m, unless a lower Q is used than I used in the above 

>Apparently this will give the blocking capacitor a workout.  It would also
>require a hefty tuned inductor.

The current through the L is roughly Q*Ianode.  The blocking C always 
gets a workout at 28MHz.  
>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.

..... not according to the Jennings catalogue.  
>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, 

Doubtful it would any strain.  

>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?  

All seem good to me.  

>(how are the big boys
>doing it?)
Typically, vacuum variable capacitors with fixed vacuum padders for less 
than approx. 4mHz, all of which are vertically-mounted as Jennings 
cheers, Jay


R. L. Measures, 805-386-3734, AG6K   

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