#2
The life of tubes depend on many variables, so it's hard to say exactly when you really have to change them.
But if you don't play a lot, 9 months seem a bit too soon for the power amp tubes to go, let alone the preamp tube (which should last years).

But if you notice a change in sound it may be time to change the 6V6's, yes.
Just make sure that you get a matched pair.
Squier "VMC" Stratocaster
PRS SE Singlecut
tc electronic polytune
CMAT MODS Signa Drive
Blakemore Effects Deus Ex Machina
DIY gaussmarkov Dr. Boogey
EHX Small Clone
Mooer ShimVerb
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Ampeg GVT52-112
#3
totally variable. preamp tubes should go multiple years. however, most stock tubes are trash. a 15-30 dollar v1 preamp tube, just one, cna make an audible difference in tone in a good way. will teh crowd notice? perhaps not. but as a player it can be a subtle difference that makes you happier. lots of subtle differences add up...

power tubes should last the average guy atleast 1 year. if you are van halen or a pro touring musician perhaps not. but i would say the average bro, easy 1 year. then again, your stock ones probably suck.

NOS tubes can last many many years. i had the stock tubes go in my egnater after about 1 year. it blew. ive been running my replacements for about another year fine. i dont play that hard / much, master low in my bedroom. i am comforatable riding it out for another year or so.
Carvin CT624
Walden G630ce Acoustic
Carvin V3M, Avatar 2x12 WGS Reaper, vet 30
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#4
There's no way to predict when tubes will go. And that includes expensive NOS tubes. I've seen tubes go within two weeks of replacement, and then THEIR replacement ran well over three years until I replaced it just because.

I mostly buy tubes from Bob at Eurotubes or Doug's tubes. I've got some gear that takes a LOT of tubes (one rack setup alone has a Mesa Triaxis that needs five - I think - 12 AX7 tubes, a Carvin Quad-X that contains nine (!) 12 AX7 tubes and a Carvin power amp that needs three or four 12 AX7s plus four EL34s). I figure that if the tubes are running past a year, they're doing better than expected.

Do a bit of research on tubes and you'll find that there are far fewer tube manufacturers (the guys that actually have a factory that build the tubes) than there are tube brands. Some of these "branders" buy tubes from one factory, and will then switch to another and to another, and you won't know until you spend the time to compare the tubes themselves. Another result of this is that you'll find a fairly wide range of prices (depending on the branders) for the same tube from the same manufacturer.

Both Eurotubes and Doug's Tubes have been reliable in the matter of "matched" tubes. I've picked up a set of "matched" tubes from another retailer on occasion and their idea of matching seems to be limited to making sure they're the same kind of tube. A very tiny fraction of a percentage of guitarists will ever get a chance to actually test their tubes (the testing machine is over a grand), and as a result, the number of people who buy "NOS" tubes that have no idea at all what they've just coughed up big bucks for is rather large. But they will always talk themselves into believing that their amp now sounds way much better.