>2 wrote:
>> Gold sputtering is caused by vhf/uhf parasites.
>[...]
>>Overdriven at HF can not cause gold sputtering. AT UHF, maybe.
>
>How do you know that? If there is a difference due to frequency, what is
>the physical reason for it?
>
** 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
Mr. Richard L. Measures
6455 La Cumbre Road
Somis, CA 93066
Subject: 2 pcs returned 8875 S/N G8AD-241 and F8VD-428J, for evaluation.
Reference: Your letter dated 21 January 1986; EIMAC RPA #SC-2303.
Dear Richard:
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.
Though your tubes are quite old (mid-1978 vintage) we will replace them
in the interest of good user relations. We will scrap the two you sent in
here, as we have to be sure they are out of circulation when we replace.
Good luck in your continued operation.
Sincerely, WILLIS B. FOOTE, Chief Specifications Engineer, Power Grid
Division.
-------------------------------------------
>--
>73 from Ian G3SEK
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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