Good Day Gentlemen,
Well, I've absolutely had my fill (and then some!) with fixed ceramic / mica
transmitting capacitors in final pi-tank networks...! Please bear with me as I
explain --- your comments / feedback are not only welcomed here, I'm soliciting
them...specifically:
I have a homebrewed 2 x 813 grounded-grid linear amplifier here, and I LOVE
operating on 160-meters. Because the air variable plate tuning capacitor has a
maximum value of 100-pfd., I've had to switch-in fixed padding capacitors in
parallel with this, in order to operate on Top Band. Until last season, this
padding capacitance consisted of a large 5 kv Sangamo mica block, which was
mounted in parallel with a four-piece 100-pfd. "circuit" of ceramic door knobs
arranged in series / parallel...the final total fixed capacitance in parallel
with the air variable capacitor thus was 350-pfd., giving me a grand total of
450-pfd. at my disposal (range of 450- to 350-pfd.).
All went well, until near the end of this past winter: I noticed that the plate
tuning increasingly became a sort of a "...hit and miss" affair. I'd peak the
linear on the band, and during the course of a night's operating, the overall
plate tuning capacitance seemed to diminish, and I would have to compensate by
adding in more, with the air variable, before shutting-down for bed. By the end
of the season, however, I would have to compensate with the plate tuning
capacitor after just one brief transmission! Finally, the cap would be set on
the maximum position, output would (of course) fall some 300%, and that was
that...
I resolved to replace the entire padding network with fresh capacitors,
avoiding the use of any of the older surplus rectangular "mica blocks" (which
I've had fail on me in other applications in the past, as well) --- and
yesterday I did just that, with another parallel / series combination of six
door knobs, two of which were rated at 20 Kv! Anyway, as I bench-tested the new
arrangement: the tuning changed again, ever so slowly, as if by some unseen
hand --- once more I had to compensate with more capacitance from the air
variable until it, again, ended-up with all its plates meshed, & the RF output
down to some 200-watts...
I ripped these padders out of the circuit, and temporarily jerry-rigged a
Jennings 250-pfd. fixed vacuum capacitor, in parallel with a 100-pfd. 3.5 Kv.
air variable --- these, in turn, were paralleled with the air variable. Eureka!
Stability and smooth tuning at last...all I have to do next is figure-out a way
to squeeze these rascals into my amplifier's enclosure...!
But to cut to the chase: I have NEVER, EVER employed vacuum-type capacitors of
any kind in all of my 35 years in Ham radio. From your personal experience, are
they electrically stable...? Might I expect this unit to last longer than any
ceramic / mica capacitor (short of dropping it on the floor, of course!)...?
How can I tell if / when the vacuum has become (or is becoming) depleted...?
I'm definitely looking for long-term, maintenance-free stability --- I do NOT
relish "...getting into" the amplifier on a semi-regular basis!
I sincerely look forward to receiving any & all feedback on fixed transmitting
capacitors --- especially those of the vacuum variety --- with my thanks in
advance...
~73!~ Eddy VE3CUI - VE3XZ
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