>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
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.
>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
Typically, vacuum variable capacitors with fixed vacuum padders for less
than approx. 4mHz, all of which are vertically-mounted as Jennings
R. L. Measures, 805-386-3734, AG6K
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