How do you know when to replace tubes

I say use your ears, and have backup tubes around from your god damn tube hoarding fetish...oops...got carried away there...there is NO way anybody else can suggest when to change the tubes on an amp they're not personally listening to so stick the backup tubes in as a reference when you suspect aging... I got about 8000 hours from a set of KT120s with great performance until I fired them (laid them off?) for getting soft, and until my KT150s arrive I'm enjoying the hell out of my backup 88s and 6550s. Note that all of this depends on how hard your amp drives these things, and power tube explosions aren't any more likely than SS amp failure...it happens, but rarely in well designed amps. I recently replaced my 12AX7s after rolling my various tubes around and found that my previous ones were also a bit dull...new set of Gold Lions put that right. This is the fun part of tube amps...love my Flashing Mullards...great band name...

Wolf - Seriously? 8000 hrs on power tubes?? I'm impressed.

Regarding 6H30s...did recently replace some Sovteks in my pre-amp with EH
gold pins. The Sovteks had hours on them but tested strong. Noticeable
improvement in sound quality with the EHs installed, however. I wasn't
expecting much and was pleasantly surprised given these are out of the same
factory, gold is just cosmetic, blah blah blah. Very reasonable prices on the EH
6H30 gold pins too.

Maintaining a listening log is a good way to keep track of hours on power tubes.
When they go, they can do damage.

I didn't count the hours on my 120s but it was about 2 and a half years of
use...even 5 hours a day (I have my rig on more than that usually) is over 5000
hours, and they still held their bias and sounded fine until that last couple of
weeks.

I really AM impressed, WG.

When it comes to power tubes, I'm more concerned with catastrophic tube failure than SQ. Following the recommended tube maintenance schedule is dependent on the amp and manufacturer.

I've had power amps where the tube just gracefully dies but with ARC, I'm definitely following their recommendations.