Hi John (& everyone) ...
John, I appreciated your recent posting which began ~as follows:
>From: John Lyles[SMTP:jtml@lanl.gov]
>Sent: Friday, December 12, 1997 11:47 AM
>To: amps@contesting.com
>Subject: [AMPS] various
(snip...)
>Having interaction with the Eimac engineers, marketing, and sales folks for...<
It was extremely interesting and completely consistent with
my own experience and judgment (which, however, result
from working on much less humongous stuff!) Your solid,
documented (or documentable) info was a pleasant change
from speculation, repetition, sniping, and setting up and
knocking over straw men.
Concerning the latter, seems to me the same poor old straw
guy has been hit over and over relative to the "BIG BANG."
In particular, one among us has repeatedly niggled over the
question of, more or less, "how can an arc occurring in the
near-vacuum of a tube make a BIG BANG, since sound isn't
well transmitted by a vacuum?" It's a non-issue that appears
to have resulted from not carefully reading what was said.
Having not checked the archives, I'm not sure who used the
expression first here. But I distinctly recall having mentioned
that I experienced BIG BANGs long ago, BEFORE discovering
the necessity for fault current limiting resistors in the HV
supply circuit. Most of the noise came from the literal explosion
of external circuit components - primarily the main anode RF
choke, occasionally a smaller series decoupling choke. It was
traumatic to push the HV ON switch with a new tube in-socket!
With 50 ohms or so in series with the HV plus and/or minus
lines, all the noise and fury disappeared. For the past 25 years
or so techs at ETO have referred to tube "thumps," rather
than bangs, because of the momentary (almost short circuit)
groan or thump that's often audible from the HV transformer
during the 20 ms or so it takes the primary relays to open. In
some cases the (dare I say it?) DC arc clears itself so quickly
that amp operation continues with only a barely audible thump
and without tripping the Ip overcurrent protection. I've manually
shorted cabinet-lid HV crowbars (directly shorting +HV to
chassis ground) dozens - probably hundreds - of times to test
or demo fault protection. Using a metal strap to directly short
out a 20 uf cap charged to 3500VDC and still connected to the
operating HV supply creates only a modest "snap" - PROVIDED
there's 50 ohms in series. ("But this demonstration is being
performed by PROFESSIONALS - don't try it at home!")
We've generally subscribed to the "barnacle" or sharp point
theory of what triggers such internal arcs in new tubes, mostly
because the phenomenon IS so much more common with new
tubes just put into service. I suppose that less-common
end-of-life arcing, though it generates symptoms similar to
new-tube arcing, is more likely gassy in origin. And of course
it ISN'T self-clearing.
Thanks for the interesting material, John.
Merry Christmas to all!
Dick Ehrhorn, W0ID
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