from
http://www.drifteramps.com/safety.html
RETUBING -- Its ALWAYS better to change your power tubes before they go on you. When a power tube fails, a lot of times it takes other components out with it. How often should you retube? There is no set-in-stone amount of time they last, since there are many factors affecting tube life. A rule of thumb, however is if you gig more than a few nights a week, every year you should retube at least. If your amp doesn't seem to have the power that it used to have or just sounds dull and lifeless, chances are its time to retube. Preamp tubes tend to last much longer. In fact, there are amps out there today that have been used heavily for 20 years or more with the original preamp tubes. ALWAYS BIAS YOUR NEW TUBES! IF YOU CAN'T DO IT YOURSELF, TAKE THE AMP TO A QUALIFIED TECH!