when to change the tubes?

[hpm-1000]

I have a tube pre-amp, ming da mc7 that uses one par 12ax7 and one par 12au7. i dont know how old they are, the tubes were allready in the amp when i bought it. here and there i heard some pops from the speakers i havent heard before. is this a sign the tubes are getting old??

i also notice the tubes start making some noises zzzzzzz, is it time to change?

thank you

 

[ferninando]

Id get a tubetester and test the tubes first.

 

[Kegger]

Probably, but if there the original Chinese branded tubes it was time when it was new.

Almost any old stock US or European branded tubes will sound better then the originals.