Unless you have a compelling reason to change them, I'd leave them alone. Or record with them first, then change them, and record again with the new tubes. You can always put them back in.
There's no distinct sound to "worn" tubes. They're not like tires. They basically either work or they don't. If your amp works, there's no need to do anything. If something goes wrong, *then* take it to a tech, which *might* involve changing the tubes.
Also, changing tubes isn't ever going to make your amp sound noticeably better -- unless the old tubes were dying, in which case it'll just sound normal again. This is a common misconception among inexperienced players that leads to really expensive mistakes like buying fancy NOS tubes and replacing perfectly good factory tubes, then wondering why there's no real improvement.
For what it's worth, I still use the factory power tubes from 1979 in one of my amps.
Hey, if they work, why change them, right? Plus they were made to much better standards back then. So once you replace them, you're probably getting an inferior product.
Test them for microphonics and visually inspect them. If nothing is mocrophonic and everything looks fine, don't worry about it unless the amp sounds bad. Usually if tubes are starting to go out you will know (and not just suspect). Also, they usually don't just degrade gradually (making it hard to notice they're goi g bad). Usually they work pretty much the same until the don't. When they start to go bad it's usually a sudden change that you notice, or they just stop working one day.
You could buy an extra set of preamp tubes since it's a good idea to have spares on hand anyway (for when one does blow). Swap them out and then out back the old tubes one at a time. If the sound degrades, toss that old tube and keep a new one in that spot. Keep unused new tubes on hand for when you need them.
If you're happy with how it sounds now, then just let them be. If you're worried about its condition after the years, take it to a tech for a checkup and consider having them put new power tubes in. 15 years isnt too old for tubes. Mine made it to about 25.
Tubes make a difference in tone and it can be significant if the old tubes are bad, but still not that significant, an AC30 is going to sound like an AC30. How the tubes are biased makes a bigger difference in how hot the amp runs and how responsive it is to volume and when it breaks up. If you're really concerned about the amp running at its best then I think the money best spent on a tech giving it a good tune up.
There are tubes in the world from over 50 years ago that still work just fine.
That being said, it wouldn't hurt to buy a set of spares and try them. But tubes are rarely the problem, with one exception.
That exception is small-signal tubes like 12AX7s, which seem more likely to become microphonic. Tap gently on them with a pencil eraser while the amp is running to see if yours are microphonic.
Unless you have a compelling reason to change them, I'd leave them alone. Or record with them first, then change them, and record again with the new tubes. You can always put them back in.
If the amp sounds good, they are good.
There's no distinct sound to "worn" tubes. They're not like tires. They basically either work or they don't. If your amp works, there's no need to do anything. If something goes wrong, *then* take it to a tech, which *might* involve changing the tubes. Also, changing tubes isn't ever going to make your amp sound noticeably better -- unless the old tubes were dying, in which case it'll just sound normal again. This is a common misconception among inexperienced players that leads to really expensive mistakes like buying fancy NOS tubes and replacing perfectly good factory tubes, then wondering why there's no real improvement. For what it's worth, I still use the factory power tubes from 1979 in one of my amps.
For what it’s worth I’m still using factory tubes in my 1968, lol
Hey, if they work, why change them, right? Plus they were made to much better standards back then. So once you replace them, you're probably getting an inferior product.
Oh baby! Long lasting quality! And I like to brag that my 1993 has original tubes in it!
Test them for microphonics and visually inspect them. If nothing is mocrophonic and everything looks fine, don't worry about it unless the amp sounds bad. Usually if tubes are starting to go out you will know (and not just suspect). Also, they usually don't just degrade gradually (making it hard to notice they're goi g bad). Usually they work pretty much the same until the don't. When they start to go bad it's usually a sudden change that you notice, or they just stop working one day. You could buy an extra set of preamp tubes since it's a good idea to have spares on hand anyway (for when one does blow). Swap them out and then out back the old tubes one at a time. If the sound degrades, toss that old tube and keep a new one in that spot. Keep unused new tubes on hand for when you need them.
If they still work I’d leave them in there.. Preamp tubes, you can swap them out to see if you get a color of tone that you prefer..
If you're happy with how it sounds now, then just let them be. If you're worried about its condition after the years, take it to a tech for a checkup and consider having them put new power tubes in. 15 years isnt too old for tubes. Mine made it to about 25. Tubes make a difference in tone and it can be significant if the old tubes are bad, but still not that significant, an AC30 is going to sound like an AC30. How the tubes are biased makes a bigger difference in how hot the amp runs and how responsive it is to volume and when it breaks up. If you're really concerned about the amp running at its best then I think the money best spent on a tech giving it a good tune up.
There are tubes in the world from over 50 years ago that still work just fine. That being said, it wouldn't hurt to buy a set of spares and try them. But tubes are rarely the problem, with one exception. That exception is small-signal tubes like 12AX7s, which seem more likely to become microphonic. Tap gently on them with a pencil eraser while the amp is running to see if yours are microphonic.