If they still sound good and perform well, I'd leave them in until they completely crap out. Why fix it, if it ain't broken?
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You should buy yourself a backup set of new tubes. The inexpensive EH brand would do just for situations like this one. Rotate the new EH tubes into the preamp and listen. If these generic/boring sounding tubes sound better than your current tubes, then you will know it's time to replace. Tubes as you have stated, do gradually wear out at such a slow rate that it is quite possible for it to happen without you noticing it.
My first thought wsa why not shut it off except when listening, but then I thought maybe leaving them on is helping them survive. Everything I cna remember reading says to shut tube gear down when not using, but leave it on if it will be unused for only a short time. I remember - vaguely - something about the heating and cooling cycles not being good for tubes, maybe the expansions and contractions of the parts damages the coatings).
If you are using single ended inputs, you only need to change the 4 tubes on the left. The 2 on the right of the main group (not the headphone ones) are used as buffers for the balanced inputs. In a recent newsletter, Cary had a picture of a modified SLP-05 they had at RMAF with 4 Black Treasures and 2 more normal tubes. People like the Black Treasures but they do not fit the holes in the top plate - you need to enlarge the holes. New Psvane tubes (CV181-T) are based on the old Treasures and are starting to become available from Grant Fidelity, at half the price of the Pope's. I thought about the Pope's but Upscale says they only sell them to people you got their pre from them. But, it sounds like your Pope's may last forever. Good luck!
6SN7's are extremely resilient tubes that were built for applications like televisions - it is very common to get 100,000 hours or more out of them - I wouldn't worry about them if I were you.
The nearly universal belief nowadays is that tubes must be turned off when not in use to preserve tube life. It's in fact more complicated than that. Output tubes used in power amps have to be turned off when not in use because they pass a large amount of current. Small-signal tubes like those used in tubed preamps and DAC's, however, actually last longer in most circuits if they are left on 24/7, as the voltage rush that occurs when tubes are powered up stresses them, and the thermal cycles they experience, whereby they got hot when powered up, and then cool down when powered down, likewise stresses and weakens tubes after numerous cycles. The key question is how close a tube is being run relative to its maximum voltage - most preamp circuits run small-signal tubes at relatively low voltages, meaning relatively little heat - in such cases, it's best to keep them on.
If the preamp uses a tube or tubes in the power supply, however, it's more complicated, as such tubes can be stressed like output tubes - it depends on the circuit.
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 yearnearly 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."
You may not THINK there is a degradation of sound but are you sure you are not maybe acclimating? it can be a subtle process and 45K hours is a LOT of hours, depending of course on the circuit and the quality of the tube itself. The only way to know for sure is to test the tubes if you can. Aside from testing trying a good set of new tubes will tell you what you need to know. I agree with Brownsfan, the Sophias are really excellent sounding tubes and worth having around if you can swing the 600.00 it will cost you. They stack up well sonically with ANY 6SN7 type I've heard.
If you throw a frog in hot water; it will try to escape immediately. Raise the temp, a degree at a time, over an extended period; it will complacently remain, and boil to death(in agreement with Tubegroover). Get some NOS tubes, try them, and I'll bet there is a marked improvement. If not: you haven't lost anything! They will be put into service eventually. I've a stock of TungSol round plates, Sylvania tall bottle Ws, Sylvania and Ken-Rad 6SN7GTs for that day. Forget about newly manufactured tubes. Even your old Popes will sound better.
Rodman - have you tried Black Treasures? They are new but were meant to sound like NOS tubes. The new Psvane may not be as "NOS"y but they are meant to be a major step above EH and other commodity tubes.
I really hope that some new quality tubes (e.g. Psvane) continue to show up. If we are relying on NOS tubes in 20 years some of the magic will be lost.
Rodman99999 while I would certainly LOVE an alternative to purchasing anything from China and always make a concerted effort to do my part in that manner the real problem is that good NOS tubes are getting harder to find. Many of the 6SN7s are used, not always a bad thing if they have been tested and you know what you are getting. One area I suspect the NOS tubes may be better overall is longevity over the tubes currently being manufactured. The metals, coatings and quality control were almost certainly better than anything we get today because they were made to military standards that were critical. Your Tung Sols, lucky you, are probably among the best along with several of the Sylvanias which I keep as back-up, the Ken-Rads well, to each his own. Have you actually listened to them?
As the NOS tubes become more scarce the price is going to continue escalating. 15-20 years ago you could purchase these tubes for a song, not today. I purchased a pair of the Sophias to try as an alternative 9 months ago and was quite taken back by their performance after reading good things about them. They really are that good. The one thing that remains to be seen is how long they will last along with the Pavanne and Black Treasures. They haven't been around long enough to determine. I generally agree with you concerning Chinese tubes, at least in my experience.
I would NOT leave the linestage on all of the time. That will shorten the life of the tubes as well as other components (heat shortens the life of capacitors and resistors).
Your Cary has tube rectification which provides a soft start turn on of the tubes. There is no benefit, in terms of preserving tube life, to leaving the gear on all of the time. There is also a small risk of fire from leaving gear on unattended.
Small signal tubes, like the 6sn7 can last quite a long time, provided that they are run well below specified maximum parameters. It appears, from your experience, that that is the case with your Cary linestage. Still, with valuable tubes, it makes little sense to use them up heating your room--they are NOT making new tubes that are comparable to some of the better 6sn7s of yesterday.
Dearing is correct about the care of small-signal tubes - provided they are not run in a circuit at near maximum voltage, which exposes them to a lot of heat, they will generally last longer than they would if you turn the unit on and off.
That said, Larryi mentions that the Cary has tube rectification. As Dearing notes, tubes in a power supply can be run hard and this complicates the question of whether to run the Cary 24/7. In addition, Larryi says that the Cary features soft-start, which, assuming proper functioning, eliminates the damaging voltage rush that occurs at power up.
The best way to determine whether 24/7 operation is best in this unit is to begin by retubing it - that way, you are starting with tubes that have not been damaged by previous on/off operation. Then just try it. Kevin Hayes of VAC, who is generally an advocate of 24/7 operation of small-signal tubes, explains that when run 24/7, small-signal tubes either die within the first 250 hours or so from "infant mortality" or they last basically forever. 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).
Mr T- My Cary monoblocks use 6SN7s as phase splitters & drivers. I tried the top ten 6SN7GT/VT231's(all bottom gettered, but for the Sylvania 'W' tall bottles, from the 40's), mentioned on the Chimera list(http://www.dehavillandhifi.com/6sn7_vt.htm) in them, the first year of ownership(2002). I found the cleanest, most liquid, least tubey(to me=unnaturally warm) and most extended(both directions), to be the four that I mentioned as having in reserve. The Sylvania and Ken-Rad VT231/6SN7GTs only seem to give up a bit of sound stage width & depth, to the TS's and 'W's. That's using mixed pairs, in the mentioned positions. I tried every possible combination, in that year.