how long do these last?

[daz]

heres the deal....i have maybe a dozen 12ax7's from the 80's, those square getter chinese. They are my fav 12ax7 and being that they haven't been made for years i worry about the tone degrading. At my age that dozen should easily outlast me as far as actually dying on me. But tone is a whole other area and i haven't had a new one of these for many years. So i have no idea if these are sounding thier best or are worn out with no new ones to compare. But of the dozen there are no doubt some i've used far longer than others and yet i still hear no difference between them. Could be they just stay tonally intact nearly forever, OR that they are all degraded already. So i have been wondering whether i should spring for some NOS ones and pay thru the nose or if these are really that stout that they can last decades w/o degrading tonally. Thoughts ? (besides calling me anal....already know that)

[km6xz]

Well, their tone does not change of course because they have no tone but they might change slightly from sitting a lot time by the vacuum altered by outgassing from the interior materials. They should have very little change in any measurable characteristic. What they sound like will more depend on what aging your amp has done or if you changed amps, then you will notice a change in tone.

[daz]

Well, their tone does not change of course because they have no tone but they might change slightly from sitting a lot time by the vacuum altered by outgassing from the interior materials. They should have very little change in any measurable characteristic. What they sound like will more depend on what aging your amp has done or if you changed amps, then you will notice a change in tone.

Ok, so preamp tubes last forever unless they actually stop working. Otherwise they will never change. OoooooKayyyy...

Anyone else want to comment on that? I ain't saying it's not true, and i'm gonna be realy happy if it is, but it does go against literally 100% of what I've heard/read forever. So forgive me if i find it hard a bit hard to accept that as the final answer.

[Justin Thomas]

I've heard that if left unused on a shelf in a box, tubes are about the LONGEST lasting components. How long does space junk last in a vacuum? Aside from pins rusting, etc. We still buy NOS tubes all the time and nobody says they don't work. Tone is a whole different story. But function? I bet you'll never run out if you don't abuse them.

Justin

[daz]

But function? I bet you'll never run out if you don't abuse them.

Justin

Well, yeah, thats what i said. But thats not my concern. My concern is that like anything, things that degrade over long periods are not noticeable to you unless they degrade to a great degree. Therefore a set of NOS chinese like i use might make me realize how much punch and sweetness they've lost. Then again maybe not, but thats what i'm asking....how long they last tonally before they lose that and it would be worth buying some NOS.

[km6xz]

So you concluded that I said there is never change forever? What about the comment that there is change over a long time in the condition of the vacuum but they should change very little in any measurable characteristic?
What proof do you have otherwise? Since it goes against everything you have ever read or heard, maybe it says more about what you prefer to hear than the source. Why, then do NOS tubes that are 60 years old command such high prices?

Small receiving tubes, if run within their design specifications last a very long time in operation but guitar amps are not designed within spec mechanically or electrically so players get the impression tubes are fragile and need replacing often. They do not unless abused.
Take a well designed test instrument or consumer hi-fi, a voltage gain, summing, oscillator, or follower circuit should work in normal operation 30-50 years. They generally do unless there is a manufacturing defect.
Take a look in an old piece of electronic test or lab equipment where the designers stayed within design spec for the tubes and you will see most of those are originals. Back in storage I have s Tektronix 570 vacuum tube curve tracer. It has about 30 tubes in it and all but the power oscillator tube are original. It was built and was used almost every day since 1957. That is 56 years and still meeting spec. If that is in an operating unit, how long would those tubes be expected to operate if just sitting in their box on the shelf?

Tubes in guitar amps are used with little concern over reliability. For example combos give the tube elements the environment closest to a 3 axis multi-mode shaker table so elements loosen and they become micro-phonic. A tube can be put into a vibration resistant socket, suspended by rubber or other isolation, then surrounding the glass with sound wave damping material, which would mean tubes would last much longer before becoming microphonic. Amp makers had incentive to increase the cost of production just to greatly increase tube life. Makers of military, lab and any sort of industrial products did concern themselves with service life of tubes and their products however.

So tell us about this 100% of your information tells you they do not last a long time. By the way, no one said "forever" except you. Were your sources testing labs? Materials or component engineers? Ok, must have been at least degreed electronics engineers, right?

[Jazz P Bass]

Would it not be prudent to set up a test circuit & test each tube.
You could use the first gain amplifier stage & a scope to actually see how the tubes react, gain wise & frequency wise.
If you really want to quantify the different tubes, do you not need data?
As to the tubes ageing, short of gross deformities within the tube, about the the only item that can 'wear out' is the cathode coating.
And that would show up as less gain.

[defaced]

Well, yeah, thats what i said. But thats not my concern. My concern is that like anything, things that degrade over long periods are not noticeable to you unless they degrade to a great degree. Therefore a set of NOS chinese like i use might make me realize how much punch and sweetness they've lost. Then again maybe not, but thats what i'm asking....how long they last tonally before they lose that and it would be worth buying some NOS.

Unfortunately you're asking for a quantified answer regarding an unquantifiable characteristic. Until someone figures out how to measure "tone", it's not really possible to determine a lifespan of said "tone".

[Steve Conner]

If they're just sitting on the shelf, they should last forever. Tubes are made from ultra-pure materials, heated red hot in a vacuum to remove the last traces of stuff that might contaminate them. There isn't really anything that can go stale.

If you're using them, I'd expect a life of about 10,000 hours if you're lucky. This was the specified lifetime for most small tubes. Good NOS ones might be able to exceed it, a bad batch of Chinese ones might not. You can check the plate voltages in circuit with a meter. I think anything that affects the tone ought to affect the DC bias point too. Well, except for microphonics, but you can test those by tapping the tube.

I once had a PI tube that slowly went bad. I didn't realise how bad it was until I swapped it into my hi-fi amp while just messing around at random. It made the channel sound weak and distorted compared to the other one with a known good tube in it.

[daz]

I once had a PI tube that slowly went bad. I didn't realise how bad it was until I swapped it into my hi-fi amp while just messing around at random. It made the channel sound weak and distorted compared to the other one with a known good tube in it.

Thank you ! Theres proof that it IS true that preamp tune can degrade tonally, which is what i have always heard till this thread. But i guess thats as close as i will get to finding out whether mine are worth replacing. I suppose not since some of the dozen or so i have have been used a lot and some probably not near as much and they all sound pretty close. I just began getting a bit paranoid knowing that eventually they'll be impossible to find. There are other 12AX7's i like, and some seems to sound better when i A/B them. But if i use them a while, eventually i always seem to notice something just isn't the same and pop the Chinese back in only to find that they are still the best to my ear. Just something about them i can't quite put my finger on, but if i had to guess i suppose they are just so perfectly neutral that theres nothing that sticks out about them that can get tiring like i find with other ax7's.

[Tom Phillips]

… My concern is that like anything, things that degrade over long periods are not noticeable to you unless they degrade to a great degree. Therefore a set of NOS chinese like i use might make me realize how much punch and sweetness they've lost. Then again maybe not, but thats what i'm asking....how long they last tonally before they lose that and it would be worth buying some NOS.

I agree with km6xz that outgassing of the internals will be the biggest factor to degrade vacuum tubes that are in storage. That and bad glass to metal seals at the pins. A more significant thing to consider is that the Chinese 12AX7s that you like and that you already are storing are most likely not aging any differently than expensive NOS Chinese tubes that someone else has been storing. Therefore, it would not make any sense to buy NOS from someone else to replace your NOS. Right?

I have another personal concern about buying NOS anything. The concern is that, as the remaining NOS supplies shrink, the NOS supply that is left is potentially more picked over. Others may have already sorted out the best of the litter. Therefore, if you buy NOS now, you may be getting the leftovers which may not be as good as you expect.

Regards,
Tom

[Diablo]

I can see it now, someone is going to offer a service to pump down the vacuum on these old tubes and make 'em good as new.
Maybe they'll throw in a free cryogenic treatment....

[boots]

In my experience, preamp tubes last a long time, at least compared to output tubes. But they can definitely go bad. I am a big fan of used NOS tubes (is that an oxymoron?). You can still find used vintage tubes with lots of life left in them for decent prices if you keep your eyes open. Once in a while, I come across a tube that just doesn't have as much pop as the rest of the bunch - that would probably show up as low emission on a tube tester. But by far most of the used pulls I find sound incredible. Even the weak tubes can still sound good, just without as much gain.

You might be very pleased with how good some of those vintage pulls sound. They might not be new, or NOS, but they are worth the search.

[Chuck H]

I usually have enough tubes in my inventory (Inventory is a big word. I usually have about a dozen "extra" 12ax7 tubes, and a few of the other common preamp tubes. 12at7, ay7, au7, etc.) A few will be low microphonic and in top working order. These are marked and will be used as the first gain stage in amps I build. The others are fine, but might be a bit too microphonic to use as a first gain stage in a combo amp. I test all my tubes for tone and gain by listening to them in my personal amp. Now...

Just to satisfy Stan, I don't actually test for tone. There is no such test. I simply listen to them in my personal amp and decide if they sound good. That could simply mean that with the operating parameters in one particular amp that a given tube sounds better than another. I think Stan would have us believe that tubes are simple amplification devices that are no more prone to differences than transistors given the correct operating conditions. I don't believe that. But it's a valid argument none the less. Because the differences are truly minimal outside of the first amplification stage. Because a guitar amp may amplify the input tens of thousands of times, tiny differences, including those that only show themselves under clipping conditions, are sometimes clearly audible with tubes used in the first amplification stage. JM2C. Ideally, outside of microphonics, any tube can be made to perform like another within it's product designation by simply matching the operating parameters to the tube. I say that's hogwash. But I'm probably going to take a beating for it.

Preamp tubes can last for many years with regular use. Microphonics not withstanding. It's spotty. Many tubes in my amps (and amps I've sold) started life as the first preamp stage. Then became a little too microphonic and found application in a later stage. Where most still live happily today. The first preamp tube can be a tender issue. Especially in combo amps, as noted. Rubber mounting and damping rings DO help, as noted. I use both. You should too. But my point is...

Just because a preamp tube doesn't sound good in the first gain stage or is too microphonic doesn't mean it's bad. It can still live a long, happy life as a later stage, an oscillator or a phase inverter.

And finally... The best way to know if a tube is worn out, other than outright failure, is to replace it with a known good tube and evaluate the difference for yourself. I do have a few "known good" tubes. Not everyone does. But it's not hard to get one. Most tube vendors offer extra services like selecting for low noise and low microphonics. That tells you nothing about how a particular tube will sound in your particular amp. Other than it won't be noisy or microphonic. But IMHE the difference won't be much, or even important. So a known good tube can be purchased for an extra charge. If you have a dozen tubes to evaluate, I think it's a justified expense so that you have a bar. And later, probably other tubes that meet or exceed that standard. Then you have a base for comparison.

Also IMHE... The Chinese tubes sound very good. They're microphony and balance is for shit. But overall, one in five Shuguang tubes will be low in microphony and all of them have good tone and gain. They seem to last about two thirds as long as Russian tubes and half as long as old school NOS tubes. But this is just my limited experience. Overall, considering the good tone and cheap price, I think the Shuguang tubes are an excellent value. And if Stan is selling them "selected" for specific stringencies, I would be happy to buy them from him.

EDIT: An addition... Russian preamp tubes fare no better than Chinese tubes as Microphony goes unless selected. And IMHE Russian tubes can be more prone to hum and noise for a given gain level. The Shuguang tubes best the Russian tubes in this aspect. I actually can't recall ever having trouble with noise or hum with a Chinese tube. Neither the Russian OR Chinese tubes sound as good or last as long as most popular NOS (read $$$). The possible exception being any GE made tubes and their many rebrands. Those sound terrible.

[km6xz]

I am not asking for anyone to take my word for it, but expect those interested in discovering what is real or not to do their own tests. A valid test isolates the characteristics of a tube from all the other factors that only prove how complicated the interaction is between all the parts of a system.
You can do double blind listening tests if you distrust instrument measurements so much. Listening tests can be valid if they point to a consistency of subjective judgement when the listener is not aware of which tube is being listened to, or when or if it is switched.
These are tests I did many times with professional listeners and some were embarrassed how subjective their impressions are when they were not able to consistently call a change. The came up in the studio years ago when hoo-doo magic dust claims were being made for studio gear and producers were starting to demand some particular flavor of fairy dust for a project, that likely was going to bust their contracted budget for a project. When one client says he can't work without a $5000 3 meter long speaker wire, I was not about to spend that sort of money for one producer who had been hoodwinked by a salesman.
So, listening tests were done and if they were beating the statistical random odds, we could talk further. As it turned out, there were only a few times when a piece of hardware was added based on listening tests alone, but never any of the technically questionable items with wild claims.
Set up your own tests in such a way to keep you out of the loop, preferably with disinterested parties. Re-amp so no variation in playing would swamp the results. Set up a switcher that truly randomizes the active selection. Set it to very fast when normalizing the gain since out ears and brain are really poor at remembering sounds, although we are very good at remembering our impressions of sounds. Once the 1/2 second switching is used to equalize level, slow it down to 1-2 seconds. Have the listener select with a button which is better. If these are recorded on a spare track along with the switching data, a true random test can be done. If it is random, some selection changes will be to the same unit as before, some will be the second unit.
The material should be moderate level, within the most linear portion of the curves for that tube in order to hear tube differences. Driving harder and you hear differences in the onset of non-linearity that has more to do with the circuit and gain of a tube than any sound character added to the signal simply by being amplified by the tube stage.

Give the test several times, and with several people. If there is a better "sounding" tube you will see a definite selection bias in the results. I don't think you will be so confident stating that I am spouting "hogwash" after the tests. If you are proven right, your next step should be to quantify the differences in measurements. If you can't measure the effects that account for the "better sounding" unit with available parameter testing, invent your own that shows it.