Finally putting the guitar equipment out of the extra bedroom they've been stored in for about two months and back into my fixed and now much nicer basement/soundroom/office-thing. However, a few minutes in, something blew.

It's been cooling off for about four and a half hours now. Unfortunately, by the time I can get out to them, any places around here that sell valves would've been closed, so I'll be getting the new tubes tomorrow. But in the meantime, I saved all five from when one of the stock tubes blew as spares, so I was thinking about putting one of them in until I can get a new set.

But as one of them is dead, and these still have more life left if they aren't the one(s) that blew, as well as my liking the sound these gave me more, I only want to replace as many as I have to. I'm pretty sure it was a preamp tube since the preamp as cranked and the master was at 2 at the time, and since it kept giving sound with it dead, albeit much quieter. How can I tell which tubes are dead and which have some life?

Baltimore Orioles: 2014 AL Eastern Division Champions, 2017: 75-87
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Take a spare, and replace the others one by one, until you find the one that blew.
^ I'm an electronics tech and that suggestion works. If you really wanted to troubleshoot it, there are some steps you could work through, but you'll achieve the same result in a little more time.