I’m surrounded by tubes - many hundreds of them - in gear and in boxes awaiting use. I build/repair tube guitar amps as a hobby, and have built tube mono block audio amps, and repaired tube audio preamps. So, while I’m not an EE or professional technician or designer, I’ve worked with tubes for a long time, so I’ll offer my comments.
I agree with Don Sachs’ comments. In addition, the equipment design is very important. For example, those that run very high plate voltage will wear out tubes quickly. Those with incorrectly high heater voltage will do the same. Preamp tubes will last a long time if they are not run at the upper end of their plate voltage capacity, and if biased properly, will still provide good sound. If biased too cool, distortion will result and if biased too hot, there will be too much current through the tube, increasing heat and reducing life. Some audio designers like to run preamp tubes at their limit in an attempt to maximize gain and headroom, and in the case of many guitar amps, the preamp and output tubes are run well beyond the tube’s specified limit.
NOS and vintage tubes will eventually lose emissions, become gassy, develop leakage, or other problems, even when not in use. I have had to discard many NOS tubes that test fine until placed into service in equipment that pushes them hard.
Pulling tubes, as Don said, is potentially a huge problem. Tubes draw substantial current through the heaters and in passing signals. A power supply designed for all tubes drawing current will, when tubes are pulled, potentially exceed the voltage rating of capacitors or other components, and put more voltage into the remaining tubes. Audible Illusions, as just one example, warns against that practice. I am not a fan of leaving tube gear on all the time, and don’t like equipment that keeps the heaters powered up all the time either. Properly designed circuits together with proper turnon/turnoff procedures will keep your tubes happy.
New preamp tubes are not that expensive, and should last a long time. When they start to fail or emissions fall, replace them with other good quality tubes. You may decide to do some "tube rolling" to hear how different tubes make your preamp sound; in that case, you won’t worry about lifespan in your preamp because you’ll have other tubes. Have fun!
