How do i know when to change the tubes in my Conrad Johnson PV10 preamp?

[Kaboom]

Hey!
how do i know its time to renew the valves? does the sound just suddenly go wrong, or is it a progressive deterioration which is much harder to notice?
what should i be looking for?
i know there is a "number of hours" gauge, but i got the amp second hand and my listening patterns are very erratic so its impossibe to keep track of the hours.
any help?
cheers!

[oaqm]

America's Goddess of Thermionic Emissions, has this to say about that:

FAQ #16l. Other than outright failure of a tube, how do I know when it is time to re-tube?

Generally speaking, for the small tubes, if you notice an unacceptable increase in background noise ("hiss") then the tube who is responsible for making the gain in the circuit probably needs to be replaced. The tube(s) making the gain will usually be shorter than the output tube. Common types we use for gain in most of our circuits will be 12AT7, 6201, 12AU7, 5814, 12AX7, 5751, or 6072. The output buffer tube in most of our line-level circuits will be either the 7044 (or 5687, same difference), 6414, or 12BH7. These tubes usually don't cause too much trouble and either work or don't work. Turn the lights off and see if you see the little tubes glowing. Look for one that looks like it has cocaine in it.

For the power tubes in our amplifiers, after a few years if you notice a small revolt going on where several of the output tubes are misbehaving or getting hard to bias, you might consider doing a full re-tube. Keep the old ones that did not join the revolution as emergency spares.

http://www.manleylabs.com/containerpages/FAQ.html

Well worth reading through.

[Kaboom]

Excellent advice, thank you very much!
aparently the only hiss so far is coming from my sub-standard computer DAC, and its only noticeable when no music is playing and the volume is cranked way past 12 o'clock. considering i never, EVER go past 11 when listening, this doesn't bother me at all.
thanks again!