Here we go again (these questions come up constantly). Your post and the responses demonstrate a misunderstanding of small-signal tubes (i.e., 12XA7's, 6SN7's, etc. found in tube preamps and tubed DAC's/CD players). In fact, 24/7 operation EXTENDS the life of small-signal tubes in most preamp and DAC circuits. Consider the following 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 [small-signal] 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).
Small-signal tubes pass little current in most preamp and DAC circuits, and thus experience virtually no filament wear. If left on 24/7, they either die within the first 250 hours or so due to "infant mortality" or they basically last forever. What kills small-signal tubes is two things: one, the thermal cycles that result from turning equipment on and off, and two, the voltage rush they see at power on. This occurs if they are in a component with solid-state rectification (i.e., solid-state power supplies), which is almost always the case. The rare component with tube rectification can be a different matter because the tube power supply functions like a variac to apply current gradually (for the same reason, many tube components have "soft start" circuits so as not to decapitate the tubes at power on). This, not 24/7 operation and heat, is the main culprit of tube death. 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 Colossus:
"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."
In summary, turn a tube power amp on and off because the output tubes pass a lot of current and will wear out if left on. With regard to most equipment using small-signal tubes, leave it on. For the record, I ran three tube preamps (CAT, Hovland and Jadis) 24/7 for a combined eight years, a DAC 24/7 for five years, and my current CD player, which has six-tubes in its output stage, 24/7 for one year - I have never lost a single tube or had any tube problems. I finally swapped out the tube in my DAC after many years just for the hell of it and it tested like a new tube. People lose small-signal tubes because they turn their equipment on and off.
Regarding your point about leaving digital gear generally turned on 24/7, absolutely leave it on - on/off turns a tolerable technology into a painful technology.
