On Wed, 27 Aug 1997 00:37:58 GMT wrt@eskimo.com (Bill Turner) writes:
>A question about possible VHF parasitic oscillations in a VHF
>amplifier:
>
>Using a grounded-grid 8877 on 50 MHz, with the tube output capacitance
>serving as the only tank capacitance, is there a need for a parasitic
>suppressor? The tank circuit would be comprised of only the tube
>capacitance and a coil connected directly from anode to B+, with the
>usual bypass capacitor, of course. Tuning would be done by
>compressing the coil, and output coupling would be a tuned link as was
>typical before pi-networks became common.
Although that is a workable solution it may very well not do a good job
on harmonics.
Actually, it is very easy to Pi Network tune a 8877 on 6M without
resorting to mechanical complexity. With an output C of 10pf it is
almost identical to a pair of 3-500Z's.
A low minimum C cap can be made from a larger cap by removing plates. The
key is to start off with a cap sufficiently long so that you wind up with
an inch or so spacing from the plates to the end frames. Minimum C of
<6pf is easily attainable.
Low C vacuums are also available and they are what Henry uses in their
industrial 30-70MHz 8877 amps.
I even use a 2-15pf vacuum in my 2M 3CX1000A7 amp where the experts say
it couldnt be done. Runs 1500W + out for years now.
>It would seem that the VHF resonant circuit present at HF using
>a pi-network simply is not there and no suppressor would be needed.
>If I were to use a separate tuning capacitor, then the unintentional
>VHF circuit would appear again, right? Or am I making a fundamental
>error here?
The key with that tube is grid grounding. Do not simply rely on grounding
the socket pins. Invest in the grounding ring available thru RF Parts,
etc. Properly built there should be no need for a parasitic suppressor on
6M. I have converted numerous industrial amps and never had any stability
problems.
73....Carl KM1H
>
>73, Bill W7TI
>
>
>--
>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
>
--
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
|