Matthews Guitars
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Normally when tubes get old and worn out they just fade away. They become less efficient, their transconductance drops, you get less gain and less output, and they just fade away and do no harm to the amplifier they're in.
SOME tubes fail in a more spectacular manner. But tubes that arc over internally or develop a space charge short are uncommon, especially among tubes that have already lasted a long time, because those kinds of faults usually manifest early in the tube's life.
In most cases it's perfectly safe to run tubes until they stop glowing. Filament death at umpteen thousand hours. But it'll be non-functional as a tube long before that point.
Back when I used to work on projectors that use three CRTs (which are a form of vacuum tube...), once I worked on a projector that had something like 45,000 hours of usage on the CRTs. 10,000 hours is full rated life. The projectors have their own hour counters in them that can be displayed on screen. The tubes were so toasted that I had to turn off all the lights and crank the projector contrast and brightness settings all the way just to get enough emission out of the tube face to be able to read the hours meter. I have never understood how it could have had that many hours on it as it stopped delivering a usable picture many tens of thousands of operating hours ago. Why even turn it on if it can't show a picure anymore? For the same reason I don't expect to encounter amps with tubes that are so worn that they don't generate any kind of usable sound.
SOME tubes fail in a more spectacular manner. But tubes that arc over internally or develop a space charge short are uncommon, especially among tubes that have already lasted a long time, because those kinds of faults usually manifest early in the tube's life.
In most cases it's perfectly safe to run tubes until they stop glowing. Filament death at umpteen thousand hours. But it'll be non-functional as a tube long before that point.
Back when I used to work on projectors that use three CRTs (which are a form of vacuum tube...), once I worked on a projector that had something like 45,000 hours of usage on the CRTs. 10,000 hours is full rated life. The projectors have their own hour counters in them that can be displayed on screen. The tubes were so toasted that I had to turn off all the lights and crank the projector contrast and brightness settings all the way just to get enough emission out of the tube face to be able to read the hours meter. I have never understood how it could have had that many hours on it as it stopped delivering a usable picture many tens of thousands of operating hours ago. Why even turn it on if it can't show a picure anymore? For the same reason I don't expect to encounter amps with tubes that are so worn that they don't generate any kind of usable sound.