How do they leave thee? Let me count the ways: (1) The estimated life of some tubes is based on the heater element(just like a bulb filament). They burn out the same way. (2) Then there is the gettering- The shiny stuff either on the top or bottom(on older tubes). It gets thin and see-throughish as the tube ages. That will weaken it's output. (3) Repeated turn-on voltages applied before the heater has created an electron cloud around the cathode will cause it to "strip", also weakening the tube. That's usually only a problem if the operating voltage for the power tubes is brought up too fast(for instance by replacing the tube rectifier with a solid state unit). (4) I've had tubes that made snap-crackle and pop noises during play, which(to me)indicated a bad internal weld(connection). Popping them with my finger would bring them back to normal for a while, while I secured replacements(most of those were small signal tubes like 7199s, 6DJ8s, etc). (5) I brought an amp into the house once from the cold of a winter night, and couldn't wait for everything to reach room temp before turning it on. YEP- The glass cracked on a NOS EL-34. (6) Sometimes the tube envelope will leak of it's own accord though, and the tube will get gassy and burn out(hopefully you'll notice the color change in it's glow, and get it out of there first). (7) Then there are the miscellaneous internal shorts that may take out a resistor or cap, if the tubes are not individually fused. Sometimes you'll hear a pop, sometimes not. (8) If the bias is user adjustable- Some run it high and shorten the output tubes' lives that way. I've never had a really dramatic tube failure in the past 28 years, but- I'm certain they do happen. Capacitors exploding occasionally is about as exciting as it's ever gotten for me. I've probably missed some of the potential failure modes, but these will give you something to look forward to.
noob toob question
well i guess technically not a noob, i have owned a few tube amps/pre's before. but i haven't had this happen yet (but it's inevitable i suppose). what happens when you blow a tube? i would suppose any well made peice of equipment would be protected from a total disaster, but how bad is it? horrible loud speaker shredding sounds? supernova burst of light followed by walls of flames? difficult to tell anything happened at all? what are the worst case scenarios...blah blah i think you get the point.