>
>On Fri, 26 Sep 97 10:04:12 -0800 Rich Measures <measures@vc.net> writes:
>
>>- With a 4kV supply, at 29MHz, for a tank Q of 12, a single 3-500Z
>>needs about 13 pF of C1 to tune the tank. Since one 3-500Z anode
>>contributes 5pF, a minimum C of 13pF -5pF = 8pF.is indicated. ... With two
>>3-500Zs, 26pF would be needed at 29MHz. Subtracting 2*5 = 10pF for two
anodes,
>>means that C1 needs a 16pF minimum. ... ... With three 3-500zs,
>>39pF would be needed to tune the tank to 29MHz. Subtracting 3*5 = 15pF,
>>indicates that 39 -15 = 24pF would be needed. With four tubes in
>>parallel, a minimum C capability of 32 pF is indicated.
>>
>>>A spit section air cap or a vacuum
>>>variable would do the job.
>>
>>It looks to me like any capacitor that is capable of tuning one tube
>>at 29MHz ought to capable of tuning three tubes at 29MHz.
>
>
>I dont know what brand of math you use Rich but that is way wrong.
"Way wrong"? The math checks, Carl.
>First of all there is no commercially availalable 4500V 250pf air
>variable with a minimum C under 30pf.
Did I say there was such a capacitor? As you suggested, a dual-section
cap is one way to go. Another way to go is a 16 - 150 pF, 4500v tune-C,
with fixed padders for 80m and 160m. The TL-922 uses the latter
solution.
>I use the Cardwell/Johnson 154-16
>with 241 to 32pf as a reference.
>Utilizing a more realistic plate voltage of 3500V and 400ma per tube we
>come up with a RL of close to 1600 Ohms. At 29 MHz that would require a
>tank C of about 35pf at some realistic Q close to 12. With 15pf in the
>tubes, 30pf in the cap and another 10pf in strays you cant get there from
>here as they say up in Maine.
- Which only proves that Carl's 'reference' capacitor is creating the
problem. The math still checks out. .. The old wives' tale doesn't.
- BTW, "10pF" in stray C seems a bit generous.
- Whatever tune C works ok for two tubes at 29MHz is also going to work
ok if another tube is added. The crunch when the third tube is added is
more likely to come on the lowest band -- not the highest band. For
example, with 0.4A*3=1.2A and a 4kV supply, RL=1670 ohms. For a Q of 12,
637pF would be needed at 1.8MHz.
Rich---
R. L. Measures, 805-386-3734, AG6K
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