Listening for tube degradation

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.

Arthur

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.