Hey y'all!

I have a VOX AC4TV (1 x 12AX7 (pre) / 1 x EL84 (power) )
I bought it about 2.5 years ago.

I know valves inside amps are disposable. So how can I check if they need replacement ?

Does the amp work fine?

You don't need replacements.

However, you may want to play with different preamp tubes for a different sound. I'm really liking a long plate JJ in V2, and plan on getting a NOS 5751 as a phase inverter and throwing something really special in V1
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It's not quite accurate, though. Tubes can start to go bad, microphonic, or just generally sound crappy without ever letting you know with a loud bang. The amp might still work just fine, too.

Really the best way to check is to get a new set of tubes and swap them out. You should have a set of spares anyway, so if it turns out the old ones were fine, you've got backups. Preamp tubes especially can just start to sound dull after a few years and it often happens slowly enough that people don't notice anything is wrong until the tube dies completely.
Simplest way to know when to change them that I go by is simply: when you notice that your sound is different than it used to be, and you don't like it anymore.

Otherwise, you'll know. Because your amp won't turn on because your power tubes will have died.

Further clarification, tone changes usually indicate dying preamp tubes, volume drops and fluctuations usually indicate dying power tubes.
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if they are microponic (which is how most of my preamp tubes die) tap the tubes with a pencil eraser noise and hear loud noise when you tap.

color is no way unless you see redplating (which is obvious you will know when you feel it)

i think i have 10-12 tube amps (hybrid musicman and ampeg included) i have only lost like maybe three preamp tubes and one set of 6L6's in the last few years).

the only ones i am ever concerned about are kt88's. they are expensive suckers. even from JJ.
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