06-13-2014, 12:22 PM
Here is one method of tube rejuvenation. This is mainly for triodes with a thoriated filament.
Early tubes that do not respond well to this method of rejuvenation: 11, 12, 00, 26, 27, 45, 50, 80, 81, 71A
Another method is to "flash" HV across the tube filament. While I am not exactly sure how this works, I have a Knight tube tester that has been converted into this kind of rejuvenator. With it I have had success bringing back many varieties of tubes, including 75s (triode section), 78s, 80s, 45s, 6A3s, and 6L6s. FYI the 6A3 went from 1300uMho to 4600uMho, as measured on my Weston 981-3.
