Eddy,
The experience you were experiencing were changes in the fixed caps value
due to heating of the components. Even though your doorknobs were rated at
20KV it is the current flow through them that heats the units.
I had run into this same situation modifying commercial / military equipment
for top band, usually just a couple hundred pf on the tuning and loading
sides would be all that was needed to slide coverage from 2.0 to 1.8 MHz.
After putting the hammer down, tuning would drift. Installing caps of
higher current rating solved the issue.
You must look at the current capability of your components, those mica
blocks usually won't hold up and TV style doorknobs are from HV not high
current circuits, not able to handle the current flow in tuner or loading
duty.
You also need to pay attention as to where your cooling air is headed,
having the hot air off the tubes blowing across your caps even if they are
NPO units will effect their capacitance and add to their eventual break
down.
Vacuum caps no matter what the size usually have enough current rating at
ham power levels. They are very stable and a great replacement for both
your bread slicers and the fixed units in an amplifier of reasonable price.
HOWEVER at 813 power levels and price / value considerations using a matched
value pair of quality 7500V sized ceramic transmitting fixed caps in
parallel to pad your tuner and loading slicers should be all you need to
distribute the current across all the caps and get you back on the air with
stabile tuning.
Sincerely,
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Ed Swynar
Sent: Wednesday, September 27, 2006 6:29 AM
To: amps@contesting.com
Subject: [Amps] Need Some Experienced Feedback Here...
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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