When doing the tap test on pre tubes...

[Vogelsong]

Is it possible to tap a tube and have it not do anything at first. And then about 5-10 seconds later it starts singing and humming like a banchee?

Or do they usually freak out right away if they're bad?

 

[Triple Cheeseburger]

i always though right away, but regardless it sounds like its cooked.

i found though depending on which tube and which channel, results can vary, for instance if i tapped V1 whilst on channel 3 in would make more noise because its amplified by 4 gain stages where as channel 1 could be next to silent. i only tried this for the first time the other day so excuse the n00b talk.