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.
First of all there is no commercially availalable 4500V 250pf air
variable with a minimum C under 30pf. 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.
The LK-550 used a 3KV cap, the same one in the SB-220, which has a
minimum C of 20pf. Even so with all the strays to contend with the L had
to be reduced considerably to obtain reasonance. The efficiency was only
around 50% on 10M with that setup.
The CB'ers use vacuum variables with minimum C around 7pf so they can get
reasonable efficiency...of course a few months of tube life is the norm
for them!
The 2 x 3-500 amp in recent ARRL Handbooks uses a modified commercial
240pf 4500V cap. It was converted to 2 sections. The rest of the article
leaves a bit to be desired but the only point I'm making is the
capacitor.
Look at the recent Alphas, they use split section caps. It costs a bit
more money to do it right.
73 Carl KM1H
>>
>>The CB crowd run 4, 6 and even more 3-500Z's on 27MHz !!!!!
>
>Apparentl;y, at least some of the CB crowd knows about hamdom's old
>wives' tale of not being able to tune a multi-tube amplifier at the
>high
>end of HF.
>Rich---
>
>R. L. Measures, 805-386-3734, AG6K
>
>
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