Great idea, Jim. I think Ameritron did this on some of their amps. I saw
a picture of one recently that had a spark gap on the tune cap.
I was recently bringing a 30L-1 back to life and was having problems
with arcing in the tune capacitor. The rotor plates were a little off
center, which made the problem worse. After adjustment the capacitor
breaks down around 3200V which is reasonably close to what you would
expect with 45 mil plate spacing. Still it would be nice to have a spark
gap in there.
73, Mike W4EF................
On 8/19/2024 11:28 AM, jim.thom jim.thom@telus.net wrote:
Date: Mon, 19 Aug 2024 07:17:31 -0600
From: Tom Hellem <tom.hellem@gmail.com>
To: Mark - N5OT <r-emails@n5ot.com>
Cc: Amps <amps@contesting.com>
Subject: Re: [Amps] I like to think there are no stupid questions
<My LC800 was arcing and tripping out.
<Turned out to be the plate tuning cap.
<Once they start arcing it's tough to fix em. Tried filing and cleaning up
the carbon tracks, <but eventually ended up replacing it with a vac
variable.
<That amp uses 3 x 3cx800A7's.
<I figured it was abused by the previous owner.
<I realize this doesn't exactly answer your question, but might save you
some grief in case <you have the same thing going on.
<Tom
<K0SN
This is why I advocate installing a simple, adjustable spark gap across the
tune cap...
(which also protects the bandswitch) and a 2nd spark gap across the load
cap. These are easily made from brass 6-32 / 8-32 / 10/32 / 1/4-20
hardware. I use brass acorn nuts, which face each other. The acorn nuts
are semi domed....and easily replaced if they get trashed. Brass nuts /
flat washer's etc are used to lock the gap.... sorta like the adjustments
on a vibroplex keyer paddles. Box store items.
Then the gap is set such that the gap will arc before the tune and load
caps arc. Either feeler gauges used and / or a hi-pot tester.
I also install a a pair of adjustable spark gaps on my hb amps, that use
vac tune and vac load caps. If an issue with say a TR relay, connector,
wrong ant, no ant, etc, you don't want 1.5 kw going into a wide open =
infinite swr = load cap welded shut asap. If you think trying to fix an
air variable cap is a bitch, try fixing an air variable load cap. A
bandswitch is also a bitch to replace.
Save yourself some grief.
Jim VE7RF
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