Sounds like a resonance in the RF Choke.
Have you checked for that?
Do you have a frequency counter to detect the oscillating frequency?
73
Don
N8DE
Quoting N1BUG <paul@n1bug.com>:
I'm trying to understand a problem with one of my amps. It is homebrew,
160-10 meters, using a 4CX1500B with screen grounded, grid RF grounded,
cathode drive. It uses a conventional tapped coil pi network with
vacuum variable capacitors.
The amp was going into oscillation when using it on 12 meters
(frequency of oscillation unknown, but not in-band). When it did this
it was also destroying the vacuum relay on the output. The relay would
fail with contacts stuck in the NO position (contact connected to amp
output).
While investigating I discovered the amp will always go into
oscillation as soon as operating bias is applied IF there is no load on
the output AND the LOAD capacitor is set to anything MORE than about
700 pF. Oscillation will stop if the load cap is reduced to something
under 700 pf. This happens regardless of band switch position or plate
tune setting.
On most bands the amp does not break into oscillation during normal
operation. That was only happening on 12 meters for reasons not
understood by me. I managed to get it to stop doing that by adjusting
the coil tap so that more output capacitance is needed. (That coil tap
needed to be optimized anyway, as it was never in the ideal spot)
Clearly this amp is not as stable as it should be. I am trying to
understand what is going on here. I am having trouble grasping why the
load cap seems to be the critical parameter, with coil tap and tune cap
having essentially no influence. Any suggestions?
Thanks,
Paul
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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