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
>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.
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*.
Also EIMAC notably do *not* say anything about the frequencies involved.
--
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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