I made the following two posts in a thread last week:
Post #1:
"The general answer to your question is that all high-end equipment should be left on 24/7, with a couple of exceptions and a caveat.
There are two reasons to keep gear constantly powered up: (i) gear sounds a lot better when left on 24/7, especially very resolving ultra-high-end gear, and especially, especially solid-state power amps and digital gear, which take anywhere from 12 hours to a week (Naim amps) to reach thermal stability after being turned on; and (ii) when gear is turned on and off, the parts inside heat up and then cool down (and therefore expand and contract), these thermal cycles being harmful over time to the structural integrity of the parts -- it's basic physics (the same phenomenon, along with heavy trucks and salt, helps shorten the life of highways in the Northeast and Upper Midwest). Leaving gear on will shorten cap life a bit (they are constantly being beaten by the A/C waveform), but caps are relatively cheap and easy to replace, and powered up prolongs the life of everything else.
Two exceptions: (i) Class A-biased solid-state amps, because they burn way too much electricity and turn most listening rooms into saunas if left on 24/7; and (ii) tube amps, because the output tubes pass a lot of current and therefore see real wear if left on constantly. As for gear that uses small signal tubes such as tube preamps and tubed CD players, they should, contrary to common wisdom, also be left on 24/7. They pass very little current if left on 24/7, but die prematurely when turned on and off because of thermal cycles and because most tube gear uses solid-state rectification, which basically kicks the tube in the nuts every time the unit is powered up (that is why some gear, e.g., CAT preamps, have a "soft-start" feature that applies current slowly and progressively to the tube filaments upon turn on). If a piece of gear uses tube rectification (most does not), it's a tougher call - tubes in the power supply can see too much juice to be left on 24/7 and thus may die prematurely, and tube rectification powers up the filaments in the tubes in the circuit slowly (like a soft-start mechanism), so the voltage rush problem with power up is not present.
Caveat: whatever type of gear you have, if your house or apartment is prone to electrical surges or electrical storms, you have to completely unplug your gear to protect it. For this reason, I now use my tube amp from July to September, and my solid-state amp at all other times (as it needs to be powered up 24/7).
It's not green to leave things turned on 24/7, but like auto racing, this sport is not very green."
Post #2:
"Returning to my above post, I want to reemphasize the point about the importance of leaving small-signal tube gear (e.g., tube preamps and tubed CD players) powered up 24/7, and not because it sounds better, but to EXTEND TUBE LIFE.
Quoting from the "TIPS & ADVICE" section of the owner's manual to my VAC Renaissance tube amp:
"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).
The Colossus computers used in World War II to decipher enemy radio transmissions used thousands of small-signal tubes. The Wikipedia entry for "vacuum tube" has this to say about operation of the computer:
"The Colossus computer's designer, Dr Tommy Flowers, had a theory that most of the unreliability was caused during power down and (mainly) power up. Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the wartime mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colossus Mk2 and the addition of another 500 or so tubes. Another 9 Colossus Mk2s were built, and all 10 machines ran with a surprising degree of reliability. The 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year—nearly all of it for the tube heaters."
The Wikipedia entry for the Colossus emphasizes this point:
"Colossus used state-of-the-art vacuum tubes (thermionic valves), thyratrons and photomultipliers to optically read a paper tape and then applied a programmable logical function to every character, counting how often this function returned "true". Although machines with many valves were known to have high failure rates, it was recognised that valve failures occurred most frequently with the current surge at power on, so the Colossus machines, once turned on, were never powered down unless they malfunctioned."
Expanding on this point, Kevin Hayes of VAC states that with gear using small-signal tubes, if left on 24/7, defective tubes will generally die within the first 200-250 hours due to "infant mortality", while normal tubes will last years and years.
I never powered down my three tube preamps (Jadis, CAT and Hovland) or my tubed DAC. In a combined thirteen years of continuous operation, I never lost a tube - not one - and only retubed if I sold the piece, which I did only to make the piece easier to sell.
Except where the power supply also contains tubes, tube preamps should be left powered up 24/7, with the volume turned all the way down and/or mute engaged when not in use. Digital gear with tubes in the circuit should simply be left turned on (as should digital gear without tubes)."
