[Amps] Reducing Ameritron AL-800H blower noise?

2 2 at vc.net
Mon Jan 27 07:29:24 EST 2003

>2 Wrote:
>>**  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 
>that implies.

**  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 
probably UHF.
>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.

cheerz, Ian
>73 from Ian G3SEK   

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 

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