Thomas
Tubes in AGC & other circuits which don't draw much plate current lose emission. This can often be forced back in coated cathode types by making a tube draw high plate current.
Increaee filament and positive grid #1 voltage helps draw more plate current. In some cases this takes less than a minute to bring up emission.
Norm
:Here's something to try (perhaps others have tried it before). Normally, when I try to rejuvenate oxide coated cathodes, I'll first bombard them to shake off some crap, and then I'll run them at a low current for a long period of time. However, recently I decided to take advantage of something new. Tubes will act like a very small battery when operated without any external power sources (save the heater). If you connect a meter between the cathode and the plate (remember to connect grids to the plate), you'll see a very small voltage when the tube is lit. I decided to take a weak tube and rejuvenate it by using this phenomenon. I've heard that allowing a tube to run idle for long periods of time is bad for it (all elements left unconnected to a power source). I'm not sure why, but apparently this is true, since tubes will often be weak when found in a circuit that isn't used much (say, in a multi-function radio). However, I decided to connect all of the elements together when I rejuvenated my tube. By connecting the elements together, the natural battery action of the tube causes current to flow from cathode to plate, and then from plate back to cathode, since I have all of the elements connected together externally. I let the weak tube run this way for about 15 minutes, and then set it up for testing. I got a fairly stable good reading. Then I took another tube that was a real dud and tried to do the same thing, but it wouldn't rejuvenate. What I did then is turn the filament voltage up a bit more (from 2.5 to 3.3) so that the cathode would operate well. First I tested it to confirm better operation due to the hotter cathode, and then I connected all of the elements together and let the tube run like this without external plate current for about an hour. Then I returned the filament voltage back to normal and continued rejuvenation for another hour. The tube now tests good. I don't know how long this rejuvenation will last, but it is interesting to take note of. Sometime I might try rejuvenating a tube for a day, if I have a day to sit at home (I wouldn't want to leave my tester running unattended....this is dangerous, since testers aren't really meant to be run for a long time). I will also take note of how long my current rejuvenations last.
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:Thomas