[RodB]

It's one of the main reasons that testing tubes is a 50/50 deal. All it tells you is whether it conducts or not and if so is it enough or a little. When a tube is in the radio and warms up it could short, stop conducting, talk to It's neighbor, glow, squirm and some other things that will make your head spin. Even when you have spares there's still a good chance of goofy symptoms. LOVE it!

[Arran]

Well some tube testers will actually have a test for gas, but one way to tell if you have a gassy tube on a yester without a gas test is that the tube will test unbelievably good on quality, sometimes almost pegging the meter. Sometimes a gassy tube will also have a purple glow when the B+ is applied, though it is hard to see on some tubes, I had a #47 tube that was like that. Recently I found a 6F6 that had gas, or a secondary emission issue, in the set it works fine for about five minutes, then the audio became distorted, the same set has a different issue that I haven't sorted yet with some sort of distortion.
Regards
Arran