What does Eimac say about parasitic oscillations?
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.