In the course of my most recent project, the radio under test started sounding weird, then went silent. I found an accidental short which apparently killed my only 2A5. I tested the tube and found all to be OK except it showed zero transconductance. I rechecked the settings, retested, then tested on an emissions tester. Same result. Zero emissions. So I set it aside.

In the next few days I acquired three used 2A5's. Two tested OK, but the third showed a heater/cathode short on the transconductance tester. Retest after a five-minute warm up gave the same result on both testers. So I set it aside.

Four days, before tossing the cathode/heater short tube, I decided to check it again. I put in the tester, turned it on, and forgot about it. A hour later I looked at it, and it showed no leak, no short, and excellent transconductance. So, it was a self-healing tube. Probably got something on the cathode or heater which took a while to burn off.

Then I thought, what the heck, I'll retest the dead tube. Son of a gun, it was normal too. This was good fortune, but also eerie. How did it come back from the dead?

https://farm5.staticflickr.com/4757/26002236328_9f96ec4d5b_z.jpg

Now that it's a Zombie Tube, how do I keep my other tubes safe?

A tube with low emission coming back to life after being heated for a while is quite common. It does seem to be some kind of contamination being cleaned off a long-dormant cathode coating. This is especially common with CRTs.

The H-K short going away is more unusual, and I would be worried about it coming back. Try banging the tube around a bit while testing to see if you have an intermittent.

I do rattle tubes when testing them. These two seemed OK, but I put them away with a note about their history.

Just keep it away from any fresh brains, you should be fine.

Just keep it away from any fresh brains, you should be fine.

I'm trying to remember, if Zombies are the ones that don't have a reflection.
Good old days of the Friday evening horror movies on TV. :scratch2:

It was Dracula that wouldn't have a reflection from a mirror. Also Dracula hissed and howled when a crucifix was shown in his face lol.