well.. your tubes are going to make a lil bit of audible noise if you tap the glass hard enough. be careful.. you dont want to break them.
but.. if youre tapping it really hard.. what could be happening is youre vibrating the heater and grid assembly and causing them to make contact momentarly..
NOT A GOOD THING TO DO.
youre causing a momentary short inside the tube.. and if youre not careful, that momentary short could stick a little TOO long and become a PERMANENT short..
.. and take your output transformer and about 1/3 of your power section with it.
not fun.
so be kind to your tubes.. and they'll be kind to you.
btw..
some rules of thumb to go by to know when you need new tubes:
1) they start to sound funky.. trust your ears!
2)they start to sound mushy and not as tight.
3) they get EXCESSIVELY blue inside.. thats caused by air leaking in through a break in glass somewhere... occasionally tubes can do this if theyre really old.. (or if you tap or bang on them too much.
)
4) one of them chernobyls (overheats and go -POOOF-) be careful of this one.. because it could either: a) take your cathode resistors, coupling caps, or output transformer with them.. or b) could be CAUSED by a faulty part in the power section.
basically..
trust your ears.. if they start to sound mushy or nasty.. or you play the amp often at loud volume.. a tube replacement might be in order. just remember that in a well set up amp, tube life is basically proportional to how much (and how loud) you play them..
..but if you DO decide to replace them, DONT FORGET to have the amp re-biased.
-T
