Eldartford is correct. It is a myth that leaving small-signal tube gear on 24/7 shortens tube life. I reprint here my response to a thread posing a similar question:
"Many people believe that tube gear using small signal tubes such as tube preamps and tube DAC's are best left on 24/7, this practice in fact lengthening tube life (and providing better sound). Quoting from the 'TIPS & ADVICE' section of the owner's manual to my VAC Rennaisance 140/140 Mk. III tube amps:
'How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current.' (Emphasis original).
As for solid-state equipment, the most prevalent view (and I think the right view) is that running it 24/7 extends component life, and this, for the same reason that it is best to leave small signal tube gear on 24/7-- the heating up and cooling down that occurs as a result of turning a component on and off causes the internal componentry to expand and contract, compromising their physical integrity over time (it's basic physics). It is a settled point that leaving solid-state gear powered up improves the sound (many solid-state DAC's and solid-state power amps take two to four days to fully stablize). If you look through the threads with respect to solid-state equipment (and there must be 100 of them), there seems to be some consensus about these points.
As for tube gear and the threads, there are many, many people who report experiences that mirror mine (one thread in particular describes how an A-Gon member ran the infamous "tube-eating" Audible Illusions 3A preamp 24/7 for five years with the same set of tubes). I have had three tube preamps, a Jadis, a CAT and a Hovland, all of which I left on 24/7 (my CAT for three years straight), and I never lost a tube in any of them. Furthermore, they all sounded better with the "old tubes" than "fresher" tubes I substituted for comparison. I have an old Cal Audio Labs Sigma II DAC in my second system that I basically ran 24/7 for seven years and the original tube still sounds great. As for my tube amps, I tend to turn them on one or two nights a week during the week for after-work listening, and then on Saturday morning straight through to Sunday night to ensure top weekend performance and to keep the on/off (thermal) cycles to a minimum, this, as per the manufacturer's recommendation. Getting back to small signal tube gear (like tube preamps, tube DAC's, tube tuners, etc.), one of my dealers explains that, provided the component is left on 24/7, small signal tubes basically either die of "infant mortality" within the first 250 hours or so, or they last forever. The issue is probably moot anyway unless a person is using really expensive new-old-stock tubes, as it costs very little to retube a preamp or a DAC (rarely more than $100, and usually much less).
Of course, if one objects to leaving equipment on 24/7 for environmental reasons, that's another issue (those who do should probably buy mid-fi gear, as they are absolutely not getting the performance they paid for if they are running top-notch equipment cold)."
As a poster above mentioned, there are a few preamp designs that really stress signal tubes, but they are few. Most of the time, the best advice is to leave them on.
