I tried to send this message yesterday...
2 Wrote:
>>>-----------------------------------------
>>> "VARIAN ElMAC, 301 Industrial Way
>>>San Carlos California 94070 1 U.S.A. / Tel. (415) 592-1221 TWX 910
>>>376-4893
>>>February 18, 1986
>>>Your letter about parasitics is quite interesting, and it appears your
>>>two tubes have had the same trouble. The emission was poor on test, and
>>>consequently other test results looked bad. The tube engineer then cut
>>>them both open for an internal examination.
>>>Both have been badly overheated internally, the apparent result of an
>>>oscillation condition. The grid in these tubes is gold plated and if
>>>overheated the gold vaporizes off, of course, and some of it inevitably
>>>lands on the oxide cathode, and that poisons emission.
>>
>>Thank you for the direct quote from EIMAC. From time to time it seems a
>>good idea to separate what EIMAC said from your own opinions about what
>>that implies.
>
>** In February, 1986, Eimac convinced me that the circumstantial
>evidence I was observing was indeed due to an intermittent oscillation
>condition.
See below...
>>
>>The part about vaporization due to the tube having been "badly
>>overheated internally" makes perfect sense, but it only says
>>"apparently" due to an oscillation condition. EIMAC cannot say for sure,
>>because EIMAC cannot know what happened - all they see is a tube that
>>has been overheated *somehow*.
>
>** You need to see a sputtered grid for yourself with a 30x microscope,
>Ian. The grounded end of the gold-plated grid often exhibits bare
>patches of molybdneum base-metal. The other end of the grid exhibits
>virtually no gold evaporation. To me, this seems similar to current
>distribution in a quarter-wave vertical antenna. This is what leads me
>to conclude that the current in a gold-sputtering episode is most
>probably UHF.
Sorry, but none of that theory ties-in with what VHF/UHF amplifier users
know about the same tubes.
The grid "monopole" is heavily capacitively loaded by the surrounding
anode, so the current distribution wouldn't be that of a free
quarter-wave resonator - it would be much more uniform. Also there is no
sign of a grid resonance as a "monopole" up to at least the
manufacturer's quoted maximum frequency for normal operation - which for
the tubes we're talking about can be 200-400MHz. Above that frequency
the gain drops off rapidly, so violent oscillation seems very unlikely.
I don't know why you observed what you did, but a quarter-wave resonance
does not seem a likely cause.
>>
>>Also EIMAC notably do *not* say anything about the frequencies involved.
>>
>** During our telephone conversation, I asked Mr. Foote about the
>possible frequency of the "oscillation condition". He speculated that it
>could be as high as UHF, but that the 8877 development team did not
>measure a frequency.
And weren't Mr Foote's spur-of-the-moment speculations later repudiated
by the development team?
>- The problem with a gold-sputtering event is that the window for freq.
>measurement remains open for perhaps a few milliseconds until the gold
>vapour cloud causes a +HV to grounded-grid flashover - which rather
>rudely shuts the window. This is why I look for post-mortem clues with a
>dipmeter, a saw and a 30x-microscope. Who know but that this is a
>fanily trait since my cousin Virginia is a county coroner?
Absolutely... but your cousin is probably more cautious about
speculating beyond the actual evidence! Given the difficulties of
post-mortems, "open verdict" still seems about as far as we can go.
>- Alpha currently admits that gold-sputtering is a problem, but that
>it's really and truly caused by the (idiot) consumer/operator driving the
>amplifier with too much HF power.
It's more likely a combination of several things. Very few users
actually know what their grid dissipation is (it is not simply V_bias *
Ig1). Many users cannot credit that a few seconds of overload while
tuning-up can do the damage. Many older amps do not have protection
against excess grid current. And also the tube manufacturer has a
problem: they have to decide on a grid dissipation rating very early in
the life of the product, and if long-term experience indicates that
rating was optimistically high, they would then find it very hard to
revise the rating downward.
>On this side of the pond, such is
>typically called "Not Invented Here Syndrome".
"Mutual Denial", more like.
>- One of the reasons that my October, 1988 article on parasites was
>published in *QST* was because a plethora of kaput TL-922 owners had been
>kvetching to Newington/HQ that Trio-Kenwood "Service" had been telling
>them that their 922s were damaged was because they were (stupidly)
>"bandswitching their amplifiers while transmitting at full power". HQ
>had apparently heard enough.
There was clearly a genuine problem with the VHF stability of that amp
(not to mention T-K's approach to customer service). You and ARRL both
did well to confront and solve it.
But not all amps are TL-922s...
>- Most manufacturers loathe admitting mistakes and fixing problems.
Certainly - but when a problem may be due to a complex combination of
the amplifier, the design of the amp and/or the way the customer uses
it, it can be hard to judge who should admit to how much share of the
responsibility. It's wrong to blame any party automatically.
And it's *very* wrong to transfer the same judgement from one case to
another!
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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