I play metal with a lot of distortion, and I play almost every day for at least a half hour. My amp has two 12AX7 preamp tubes and one KT88 power tube. How often should I change them out?
when you start to lose tone or a tube blows. it depends, it can be 5 months or even more than a year
Your power tube should last somewhere around a year and the preamp tubes will last multiple years in most cases.
I play about that frequently as well, and I changed out my JJ 6L6's and 12AX7's after about a year. The tone hadn't degraded a ton, but I had developed what I believed was some tube rattle (due to combo amp) and some scratchy-ness due to me being an idiot and not cleaning my sockets when I put the first JJ set in (to replace the stock chinese/sovtek sets).

Remember that many amps require re-biasing when you replace the power tubes.
Quote by The Hunger
I play metal with a lot of distortion, and I play almost every day for at least a half hour. My amp has two 12AX7 preamp tubes and one KT88 power tube. How often should I change them out?

off of curiosity what amp is that?

it really varies by what tubes came in there stock. what its biased at (if powertube and in your case biased from the factory). how long you allow it to warm up before heavy use. how hard you push the powertubes.

i have the factory tubes in my valveking for 2.5-3 years now, playing it for 3-4 hours a day for the first year, (less after buying more amps, and not much at all in the last six months), and turning it up for practice and shows. i am amazed they have lasted this long.
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youre just being a jerk man.

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I swapped my tubes in my XXX, power amp tubes, and they last now for about a year and a half. However I've have 3 preamp tubes go micro phonic on me, two of them being JJ tubes lol.
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Tubes shouldn't be changed unless they
A.) Die
B.) (in the case of preamp tubes) become microphonic
C.) You want a different tone than the tubes are giving.
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Quote by Anonden
You CAN play anything with anything....but some guitars sound right for some things, and not for others. Single coils sound retarded for metal, though those who are apeshit about harpsichord probably beg to differ.
I bought it as a kit from ax84.com, a website that offers a few tube amp kits. The tubes I have came with the kit. I've been playing them for a little over a year, first through the speaker from my old transistor amp and then through a Hartke 4x10 cabinet. They seem to be giving me a lot more treble than they used to of late, but that could just be me.
Can you define 'trouble'

Tube age is a bit of a misnomer. Some are kinda crappy before they even get installed inan amp. They could sound great for 5 days or they could last 10 years.

I badly biased amp will eat through them quicker. Plus it is always good to have spares on hand.