If it was a plate blocking capacitor the safety choke at the output would
have blasted open, if one was installed.
At 6M you dont need 2000pf for blocking, for an 8877 a pair of 200-500pf at
2X the plate voltage should give sufficient safety overhead. The 1000pf in
the small *58 size use an unstable X5V dielectric.
Most important is that RF rated caps with sufficient 6M current specs be
used, many out there are not RF rated. Ive no experience with the Ruskies.
Carl
KM1H
----- Original Message -----
From: "John Farber" <kg6i@hughes.net>
To: <amps@contesting.com>
Sent: Tuesday, November 16, 2010 10:59 PM
Subject: [Amps] Need advice on DCblockinig caps-My Text Body Didn't
ComeThrough!
> I have a 6 m amp, a HB model using a single 8877, a Drake L4 box modified
> by a guy "back east" that failed. Big arc in the RF deck, followed by a
> loud arc in the remote PSU. I replaced all the diodes, caps and equalizing
> resistors in the PSU with components at least as good as the existing
> components. A few of the old caps had rounded tops, and a few of the
> equalizing resistors were way off, and the diode bridge was toast. I saw
> no obvious arc paths or other telltale signs of problems in the RF deck.
> Powered up the amp, let it sit for perhaps 5 mins, let the 8877 3 min
> timer ckt kick in, etc. Applied a small amount of drive, got output,
> looked good on the scope, so I knew the tube was still good. Slowly
> increased drive pwr, up to about 1 kW out, when bang! arc in the rf deck,
> followed quickly by an arc in the PSU, and I quickly shut it down. All
> this in about 15-20 seconds. Let it sit for a couple years, as I have had
> some medical problems. Decided to pull the cover off the RF deck today and
> have a look-see. Again, nothing obvious, BUT upon closer examination of
> the DC blocking caps, the builder used two small doorknob caps in
> parallel, with an American brand name, 1000 pf 5 kV each. So I figure the
> two parallel caps is a good design, splitting RF current, and 1000 pf is
> about the right value. But It seems to me that the 5 kV rating is way too
> small. This amp runs 4 kV of B+. I seem to remember that the tank ckt can
> reach X 2 times the B+ value, which in this case would be 8 kV and way
> beyond the max rating for these little doorknobs. So I'm asking for advice
> on selecting suitable replacement caps. I found a site selling 970 pf, 15
> or 20 kV Ruskie doorknobs for a not-too-bad price, and I remember hearing
> folks remarking they were happy with the Rusky doorknobs. What do the
> esteemed elders of this Amp group recommend? And what other components
> should I be concerned with to beon the safe side when rebuilding this amp?
> My plan was to examine the rectifier/filer board for failed components and
> fix that. Then disconnect the B+ line to the tube and test the B+ by
> itself for an hour or more. If good, reconnect it and see what happens,
> and if all is quiet, apply rf and test for output. BUT that's what I did
> the first repair attempt, only to have things blowup, and it cost me about
> $100 in parts. So I figured I would ask for advice this time first.
> Thanks, John Farber, KG6I
>
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>
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