Listening for tube degradation

Hey, just wondering what specifically there is to listen for in regards to tube degradation. I don't have access to test equipment so I have to rely on my ear to determine if certain tubes are on their last legs. I've been able to notice a definite "scratching" or muddied sound whenever one or more of the tubes are in need of replacement. However, I find myself replacing ALL of the tubes since I can never pinpoint which exact tube or tubes needs replacing. Also, are there any visual cues to look for (ie.output tubes glowing brighter than others)? I have a 15W integrated tube amp that you can look at on my system page.
I also never had the equipment (or the knowledge) but learned a way after I made Heath and Dynakits, take cover off the amp and let it warm up,do not touch any part of the amp (be sure the selector switch is set to Lp or extra/cd whatever you are using, volume up a bit) and tap the tubes with a pencil. This may not tell you how much life is left in a tube as a Tester would, but if any tube makes any noise comming from the speaker when you tap it, static, spitting, burping, harmonics etc, it is done for and should be replaced.
Len W
What I have noticed is that the sound does get a little muddy (or fuzzy) and you get less gain. This latter one is how I ended up confirming it - I had to turn the volume up higher for the same loudness. Since my preamp has a numerical volume display, this was easy to notice.

I have since bought a restored tube tester. It turns out all the tubes in my amp were worn out indeed. I am very happy being able to monitor tube performance other than with my ears because I want to keep the sound at peak performance and don't want to risk having a tube die on me and breaking something... Tubes can fail catastrophically if you use them too long. I have witnessed this first hand before.

Len W- not to be contrary, but what you are hearing is microphonics. Some tubes are microphonic when new, some are not. I don't know if this can also occur due to aging. It may be sign that the tube needs to be replaced, but it is NOT a reliable indication of aging. BTW, some microphonic tubes can benefit greatly from a tube damper.
Performance that deteriorates slowly, in tubes or anything else, is difficult for a user to recognize. Think about your eyes. They seem to be OK until the optometrist says you need new glasses. One nice thing about ss amps is that, unless they emit a puff of blue smoke, they are probably OK. With tubes I found that I was constantly worrying about them, listening for problems, which detracted from the musical experience.
I agree with Eldartford, most often tube performance deteriorates slowly. Some can last a year while others may survive 20 years. Too many variables about circuit application, user care, etc. so time alone cannot be the measure.

I believe anyone with more than say 2-3 tubes in their system should try to find a tube tester and learn to use it. I'm not knowledgeable about electronics but I found a B&K tester at a swap meet and bought the tube charts and calibration instructions from them. That was a few years ago but the $ and effort paid for itself many times over.
Treat your tubed components just like you do your car. Simply put them on a maintenance schedule and routinely replace the tubes accordingly. I replace power tubes at about 2000 hours. This takes care of incurring a serious loss of sonics, or major shorts, due to age. I replace my small tubes, depending on use, about every 4000 hours.

Some may consider this early, but at least I don't worry about subtle degredation in the sound quality. Why invest all that money in audio stuff and then cheap out on the tubes by seeing how long they will last

Oh, power tubes lose bias as they age and you will find that you need to increase bias (ma's) as the tube ages. If you let them age enuf you will be unable to get the up to recommended bias settings.

Small tubes can get noisy and or dullish, i.e. loss of detail with age! I replace them all at the same time if it is a noise issue related to age to retain good channel balance.

Newbie, how often do you listen, because 2000 hrs on power tubes at 2 hours every day equals about 3 years? I don't have any friends that put that many hours on their system.
I 'work', when I actually do, out of my home. I turn my 'casual' system on at about 7AM and turn it off at about 4PM every weekday. So, on this system, I change my power tubes in my amp at least once a year.

I have a second 'system' (different electronics - same speakers) I use for 'serious' listening sessions, those when I'm not multi-tasking - usually at night or on weekends. In that system I've maxed out power tube performance and average about 1500 hours on the power tubes. I get to change these about every three years.

I guess I'm more than a bit wierd but I just love music and don't watch TV in my spare time. FWIW.
It always seems that I find tube failure being preceded by increased noise, such as when a tube in my phono stage started kicking off big thumps through my speakers and subwoofer. It is often difficult to hear when tubes have aged because they slowly deteriorate over time, whereas when you change them you get all of the performance back at once. Often you will just get a dulled performance over time.
I've considered moving towards a SS amp as a result of this or investing in a tube tester. I get paranoid as well because I'm never sure if I'm getting optimum performance. I love the sound of tubes but I'm not sure if it's worth the maintenance effort. The best thing to do is to probably keep a stock pile of tubes on hand just in case.

Any recommended tube testers?
The easiest, and most reliable approach is to always have at least one new set of replacement tubes in reserve. Every year or so, replace the tubes with the new set and see if the sound has changed markedly for the better; if so, it is time to replace the tubes. Do this separately for small signal and power tubes because small signal tubes usually last MUCH longer.

Tube testers are generally NOT reliable for this. Most testers are good for spotting gross problems but not subtle deterioration. Most testers are not terribly reliable because they need to be calibrated every so often and who really sends them away for that kind of maintenance?

One of the best testers out there is one that is in current production and is quite expensive. It is the Amplitrex tester. It is pretty much idiot proof (a screen tells you exactly what to do step by step), accurate, free of need for recalibration and can test for a wide range of factors. You can even hook it up to a computer to trace curves. One nice feature is that it will give real, meaningful readouts (e.g., transconductance) and not some proprietary "value," provide the tubes specifications to compare with the test results, as well as provide gross evaluations ("excellent," "good," "weak" and "bad."). I have this tester and like it a lot. The only big weakness is the limited number of tubes that are programmed into the tester (you can do your own programming if you have the proper data or you can have the factory program a CD ROM to update your machine).

The problem with any tester is that, depending on the application, a tube may test extremely bad and still be perfectly fine for the particular use in the amplifier. I have a number of extremely weak 6sn7 tubes that sound WAY better than any current production tube in my amplifier. I doubt this would be the case if the tube were pushed much, but, in this amplifier the tube's capabilities are taxed very much. Even the Amplitrex is best used for testing to see if a tube is generally good, and the extent to which the vendor stretched the truth as to its "age," and not for making any kind of objective evaluation of the sound of the tube. My amplitrex has already proven itself by showing that a "new" rectifier should not be used (no voltage drop in EITHER direction, i.e., shorted out).
Hitman and Larry:

As I stated earlier, I'm not an expert in electronics but I do believe an inexpensive tube tester can be a great value to most hobbyists with any tube gear. B&K and Hickock both made a number of good testers that can be found in the used market. I got a B&K 707 which, with the added Instruction Manual (including simple calibration instructions) and tube charts, cost me about $25.

The key is mutual conductance testing. I don't simply rely on "Replace" readings for tests but I record the Gm reading directly on new tubes (with a Sharpie), then note changes after a period of use. In general, I've been confident with readings within 15% of the new test.

For anyone playing their system on a regular schedule like Newbee, estimating hours of play can be relatively easy to indicate time for replacement. My listening is much more erratic so estimating hours of play for me would be a real WAG.

If you have a simple circuit with only one or two tubes it may be easier to simply replace periodically or have a dealer test them for you. Otherwise, for a sense of involvement with your system, find an inexpensive tester and learn to do it yourself.
I bought a restored and calibrated Superior TV12. While I agree that a "bad" tube can sound good (I've had this happen several times), the part that worries me are leaks and shorts - and for that, a tube tester can test very well and reliably. But a tube with a low transconductance score means that it has spent a good part of its life and leaks won't be far behind. So the tester can be a good predictor as well. As for sound quality, I use my ears for that.

Now, do you really need a tube tester? Well, new production tubes are not as bad as some people like to say so if you have them powered up all the time, just replace them all once a year. But if you like playing with NOS and used tubes, or have many tubes in your system (I have 15 tubes in mine), then it is best to keep track of leaks so you don't end up with anything damaged.

great thread that I don't want to lose track of, hence my posting, thanks all!
This is a very informative & great thread as Karelfd mentioned above. I also have to agree with Swampwalker that just because a tube exhibits microphonics does not indicate the tube is worn out. I have purchased brand new tubes that are microphonic in some circuits but are not microphonic in others. The use of tube dampers can greatly reduce or eliminate microphonics in most cases. I think I'm a little paranoid over purchasing gear that uses too many tubes and for that reason prefer a more simpler design.

If you don't have a tube tester the only other thing you can do is watch out for a sudden deteriation in sound or a drop in gain. I have had tube preamps where the gain suddenly drops and the tubes cease to function period with no harm and they sounded very good up to that point. I would think that one would want to exercise more caution here in regards to tube life with tube power amps since the sudden death of a worn out tube could cause circuit damage or in worst case scenario a small fire. So as mentioned above it is always prudent to have a spare set of tubes on hand. I don't know why most manufacturers of tube power amps don't include a electronic metering device that lets you know the amount of hours you have put on a new set of tubes.
i have noticed an increase in treble energy just before a tube dies. i have also experienced a crackling sound that can very load just before an amplifier output tube dies. it always pops a fuse, rather than damging my vtl deluxe 120. once i experienced a failure of 2 12ax7 tubes on my mac 275.
there was no warning sound prior to the tube failure. here again, a fuse was blown and i was able to determine which tubes needeed replacement.
Hi Pryso,

I don't mean to suggest that a tube tester, particularly one that can be obtained cheaply, is not a worthy investment. Certainly for my friend, finding out that a rectifier he had not ever used was shorted was very useful information.

My only caveat has to do with relying on data, even useful data such as transconductance, to try to ascertain how the tube will sound. I have current production Electro Harmonix 6sn7s that test spectacularly high in terms of transconductance, that sound like crap compared to near dead Tungsol roundplates.

I agree that combining measurements from a tester with experience IS very helpful. I merely intended to reduce the expectation that a tester will be a quick and easy way to select the right tubes for an amp and an easy way to estimate when a tube should be replaced.