>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.
>
Zzzz, Zzzzz, Zzzzz.
>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.
>
Since you are not the only person who has experienced a big bang, it
remains an issue.
>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!
>
However, numerous 'big bangs' reportedly did not result in the
destruction of such chokes. Numerous 'big bang' reports I have heard of
occurred when the amplifier was more than a year old.
>With 50 ohms or so in series with the HV plus and/or minus
>lines, all the noise and fury disappeared.
Resistance limits current. Yawn.
>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!")
>
- I'm certainly not one of the "experts", and I do it. Does it really
take "PROFESSIONALS" to short something out? With reasonable care,
virtually anybody can do such a test.
>We've generally subscribed to the "barnacle" or sharp point
>theory of what triggers such internal arcs in new tubes, ... ...
A theory to be sure, and one that seems not to be supported by the
evidence from kaput tubes. Roughly 2/3 of the oxide-cathode (8877 et
cetera) tubes I open have gold sputtering damage - a condition which can
cause arcing across the anode insulator -- between the anode and the grid
collet. I have never seen an arc mark on the grid itself. . . . It
seems that the lack of evidence indicates that the "barnacle" theory is a
"sea story".
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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