A few months I took up vacuum tube regeneration following the procedures recommended by Lane S. Upton, published by the California Historical Radio Society in 1978. I had a few "bad" tubes which I would have eventually discarded, but I never got around to it. I'm sure most of you are familiar with this--the rejuvenation process differs for different types of filaments and involves a voltages ranging from 100% to 300% of normal and times ranging from a few seconds to several hours. I accidentally killed one low-value tube, but had some success with several others. Notably, I moved a 45 from "bad" to "useful" according to transconductance.
After screwing up a few times, I put together a rejuvenation kit to use with my Heathkit TT-1. First, I made separate templates for each common filament voltage to show switch positions for the recommended rejuvenation voltages.
https://flic.kr/p/2haGbJb
I bought an automatic shut-off countdown timer because I'm forgetful. Also, for tubes (like this '01A) for which plate and grid voltage is not wanted, I made reusable filament pin extenders.
https://flic.kr/p/2e1Lp6V
My greatest difficulty is determining what type of filament each tube had. Upton gives a few examples and describes what the different filaments look like at rated voltage--"white" for pure tungsten, "orange-to-yellow" for thoriated, and "dull red" for oxide emitters. I can ignore the pure tungsten description, because I don't got any of those. But I'm never sure about the appearance of the thoriated and oxide filaments. Also many tubes, like all of my '99s, and so getter-coated that I can't see the filament. Here's a '26 which should be a "dull red" oxide emitter:
https://flic.kr/p/2dWa6J5
I would call that a "non-dull orange-red."
So, putting together all the information I could find--published lists/examples and tube data sheets--I made a chart.
PURE TUNGSTEN: '01, 200
THORIATED: '01A, '20, '99, 200A, 210
OXIDE: '26, '27, 41, 42, '71A, 75, 78, 80, 7A4
These I presume are oxide emitters: '24, 57, 58, 76, 77, 84, 89.
Does this look accurate? I'd love to see a definitive list or filament reference for all pre-WWII globes and STs, but this is the best I cab do so far. Any other suggestions for being sure of filament type?
One other question: When a data sheet says "coated filament," does that mean oxide-coated?
Thanks,
Henry, The Eternal Novice.