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EIA 312 tubes.
Slyvania manufactured their own electron guns in Ottawa, Ohio. Their guns were different from RCA and Zenith's.
Slyvania used what is called a "soft" cathode coating, designed for fast and easy processing on Delta and 36mm inline guns. They were not designed for long life.
The Zenith and RCA electron guns used a "hard" 4% tungston cathode coating which required better processing techniuqes.
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