Tube Life

I’m using a phono pre-amp with a pair of telefunken 12AX7 and one 12AT7. I purchased these tubes several years ago and they were rated at about 50% life. Are tube noise and tube rush the only reasons to change out tubes? Everything is silent and yet I can’t help but think that several years of use should indicate some sort of wear. Any experience or suggestions will be appreciated.
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When Thor Audio was around I talked with Paul Marks, its founder. His view was leave it on 24/7 and just change the tubes each year. So that would be about 9000 hours of use. Make a guess at how much you have used them and that's a very ballpark estimate of whether to get new tubes. Sound is another good guide.
Preamp tube usually go out with a wimper. And not a bang.
So, unlike amp power tubes.. you can just use preamp tubes until they annoy you.
And I leave my VAC Standard tube preamp on 24/7.
I bought it used a year and a half ago, left it on 24/7 so far.. still no problem. (with used old stock tubes)
Do it the right way,,When in doubt,buy another set and have them on hand..If anything changes ( in sound ) dump the old set for the new...Always try to stay one step ahead of trouble.......
I have a friend using 6sn7tubes in the amp input and they are 15 years old. Working well, no noise or problems. Use until noise is an issue. Life ends with noise for me. Jallen
Telefunkens are known for their durability so they may be chuggin' along just fine. Or there may be a decrease in sound quality that you haven't noticed because it occurred gradually in imperceptible increments.

If you plan on keeping the phono pre you will eventually need new tubes so you might as well get them while you've got time to look for a deal. When you get the new ones plug them in and see if they sound better. When the new ones sound better, it's time to retire the old ones for emergency duty only.
if your stereo sounds 'sleepy' time for new tubes - which will be more defined and vibrant I have found...
The rate at which tubes wear out and how they sound as they do varies with the circuit they're installed in. The general tips above (and any that follow) are useful but if the OP wants the most applicable guidance he should post the preamp he's using these tubes in.
I appreciate all your help and your suggestions. I was slow about posting the actual unit due to the unlikely chance that anyone would have one or have heard of it. It’s a handmade unit “Sheer Audio MM-77” that I purchased off ebay from Vintage Audio Labs.
Thorman's advice is good--with an extra "new" set, you can switch out tubes to determine if the sound has started to go bad. Weak tubes sound dead and lifeless compared to newer tubes. The only problem with this approach is that you have to pay for an extra set that you may never need.

There are some line and phonostages that run tubes so gently that they may not go bad in your lifetime. This would particularly be the case with the Telefunken 12AX7 and 12AT7 (reputation for long life). I have the other Telefunken 12AX7 equivalent (ECC803S) and I am hoping that that is the case (REALLY expensive variant).

On the other hand, there are designs that run tubes pretty hard. I recall that a dealer who kept Counterpoint linestages on for 12 hours a day had to change tubes every 6 months.
Having an extra set of tubes lets you gauge the sonic deterioration of your main tubes. Such is good practice. Also, tubes do fairly often catastrophically fail (one channel goes silent... that kind of thing) --- and, by having an extra set, you can use those tubes to assess which of your tubes died (that is, if, for example you have 4 pre-amp tubes per channel and a channel died, you can readily swap out systematically the tubes to discern which tube died).
my experience has been as tubes begin to "poop out" you lose a little something sonically. I have noticed this after tubes become and noisy and before as well. I have noticed on both my line preamp and phone preamp.
I see no reason whatever to leave a preamp on all the time, unless the unit is solid state. if you have expensive rare tubes (e.g., the TFKs you guys are discussing), why let them go to waste when you are not even getting the fun out of them? It is true that there is some stress on a tube at turn-on, but that can be ameliorated by a "soft-start" circuit and by only applying plate voltage after the filaments have warmed up (which takes only about 10-20 seconds for 9-pin miniature triodes like 12AX7). Both of these goals are easily achieved by any thoughtful designer. Most tube units will be in "full song" within 20 minutes or so after turn-on. (So will I.) When a tube ages, it loses "gain" and input sensitivity. Thus the report that the sound gets flabby or dull or lifeless makes perfect sense and is a good guide to tube replacement.

In contrast, SS units are not at their best unless or until operating temperatures have stabilized, which can take hours. Since SS devices are at even greater risk than tubes at turn-on, and since their lifespan is much longer than that of a tube. it does make sense to leave SS gear on at all times.
Lewn, I aqree totally with you. I don't see any reason to leave tube gear on all the time. Most sound very good within a pretty short time after being turned on, while solid state gear can take hours to come up to full song.

Concerns about the thermal shock at turn on are over exaggerated. When cold, the filament will conduct more readily so that there is a quick in-rush of current and rapid heating at turn on. But, with tube filaments, this does not happen with the kind of speed and severity of something like a light bulb.

In any case, all my tube gear use tube rectification, so the signal tubes are protected from instant on conditions--it takes a while before the rectifier is putting out full current so it acts like a soft-start circuit.
Depends on the voltage how high.DUDE!!
Right, a tube rectifier helps to protect the downstream tubes from simultaneous application of filament and plate voltages. The plate voltages will come up slowly. Also the simple use of a thermistor in the filament transformer primaries will protect both the transformer and the tubes from rapid application of filament current. Most modern gear, even including tube gear, uses SS rectification and so does not afford the natural protection of a tube rectifier. However, it is a simple matter to employ a relay device that effects a time delay between application of filament voltage (first) and plate voltage (second).

I would not want to have to go hunting for ECC803S TFKs on an annual basis.
Having an extra reference set to compare to is the best way to know for sure how another set is performing.

Or else you might just trust your ears enough to detect a problem that matters when it sneeks in.
I have TFK 12ax7s in a lot of places and 50% could mean 10 more years depending on the circuit. Trust your ears and the poster above who said older tubes become sleepy I think is right with regard to phono stage tubes.
VAC recommends leaving their Signature-type preamps on 24/7 unless you're going to be away from the system for an extended period. That's what they do. I was told to expect to change tubes every 5 years using this regimen. Clearly, they expect a lot of tube life in their circuit. YMMV.
Definitely diggin' it!
Thank you all for your time and experience. Apparently, the ears have it. I was a mite suspicious before posting this and I swapped a set of electro-harmonix in for the telefunkens. While the telefunkens beat the electro-harmonixs originally it was no longer the case with age. What to do with the telefunkens that are now a bit slow. Alas.....