Amps
[Top] [All Lists]

## [AMPS] Phantom Capacitor

 To: [AMPS] Phantom Capacitor measures@vc.net (Rich Measures) Thu, 26 Mar 98 19:25:39 -0800
 ```>Hi Guys, > >Here is an engineering question. > >Preface > >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 example. >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. > agreed >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 recommends. > cheers, Jay cheers Rich... R. L. Measures, 805-386-3734, AG6K -- FAQ on WWW: http://www.contesting.com/ampfaq.html Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
 Current Thread [AMPS] Phantom Capacitor, W7CW Jay E Ostrem [AMPS] Phantom Capacitor, km1h @ juno.com [AMPS] Phantom Capacitor, Rich Measures <= [AMPS] Phantom Capacitor, Phil Clements