When it sounds like a tube amp needs rejuvenating,

Power & output tubes going bad tend to snap crackle & pop,literally...
Small signal tubes going bad are sneaky,usually only displaying lack of volume until going dark...JME,YMMV...
PS:Figure around 5000 hours for small signal & between 2000 & 3500 for power/output depending on bias...

thank you - to the point - I appreciate your experience. 

- john

oh also - when a tube can no longer hold bias, it's time to replace it., right ?  

-john