i bought a used at-100 head, i have a new matching cab. its a great amp from what i heard, and i really love the tone. its works fine, but i know the tubes must be at least a year old. and they are ****ty chinease. also my tone breaks up at louder volumes and i know thats exactly what tubes DONT do. i know its a good amp, so can old bad tubes cause tone to break up at louder volume?
It's supposed to break up at higher volumes, that's what tubes do. 1 year isn't very old and btw almost all current tubes are made in china or russia.
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Your tubes should still be fine. Manufacturing processes are much better these days so a tube can last quite a while. If you're after a change of tone however, new tubes can generally be the ticket. Head over to eurotubes and give them an email and see what they recommend.
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jsut to clear things up they are like no name brand, really really bad quality.
when you say break up, you mean distortion right, thats normal, the higher your amps cranked the more gain you`ll have

but if you think there a problem, send your amp into a music store and have them look at it, they can tell you in 2 seconds if it needs tubes