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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