Listening for tube degradation

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.

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.