[Amps] Reducing Ameritron AL-800H blower noise?
2 at vc.net
Mon Jan 27 07:29:24 EST 2003
>>** note: The "letter about parasitics" Mr. Foote refers to mentioned
>>that the resistance of the 8875s; VHF parasitic suppressor resistors had
>>changed from 100-ohms to over 400-ohms in a period so brief that the
>>outside of the resistors showed no sign of overheating. Richard Kaller,
>>W7MOI suggested that such a phenomenon might be caused by a brief VHF
>>parasitic oscillation because each resistor was virtually shorted at HF
>>by c. 0.07uH of L..
>> In a telephone conversation, Mr. Foote said that the gold sputtering
>>problem was encountered during the 8877's development. After autopsying
>>a leaky tube, the 8877 project engineers concluded that thin layers of
>>gold were evaporating during brief bursts of a high frequency oscillation
>>condition. I agree since I have seen this problem in a number of
>>autopsied 8877s. My guess is that the frequency of oscillation could
>>well be above 300MHz, where the 0.1pF of feedback is c. 53-ohms.
>> To address your question: Considering that the stored energy in a
>>typical amp's filter cap is c. 150-ohms, only a thin layer of gold atoms
>>could be boiled off during one event. At UHF, due to skin effect, gold
>>sputtering thin layers of gold atoms would seem to be do-able.
>> "VARIAN ElMAC, 301 Industrial Way
>>San Carlos California 94070 1 U.S.A. / Tel. (415) 592-1221 TWX 910
>>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
** In February, 1986, Eimac convinced me that the circumstantial
evidence I was observing was indeed due to an intermittent oscillation
>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
>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.
- 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?
- 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. On this side of the pond, such is
typically called "Not Invented Here Syndrome".
- 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.
- Most manufacturers loathe admitting mistakes and fixing problems.
In the last four years, this little problem cost Ford over $2 billion,
cost dozens of Ford drivers their lives -- and cost Ford's CEO his job.
Ford's habit of not protecting gasoline tanks from butt-ramming
continued from the Pinto to the Crown Victoria, a saga that covered a
quarter-century. GM pickups had a similar problem.
>73 from Ian G3SEK
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
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