Tube Life: 6SN7, 6DJ8, 12AT7 et al.

A tube designed to last 10,000 hours with a plate votage of 375 volts will not last 10,000 hours if subjected to a voltage of 450 volts in that application. A tube designed to last 10,000 hours subjected to a plate voltage of 450 volts will last far longer if only subjected to 375 volts in the application it would be used in. Follow? Test measurements will only tell you if the tube is good or not (aside from microphonics), and allow you to compare with a new tube spec'd from the same manufacturer. The circuit it is used in will factor heavily on the projected tube life.

I have read in threads about tube life that it is unwise to purchase tubes sold as "used, test good" because tubes will test as new for a good part of their lives and then will deteriorate more rapidly as they move down the good scale. I can't verify this from my own experience maybe someone else can.

If you talk to the tube guys who really know their stuff, they will tell you that tons of tube testers out there are inaccurate, uncalibrated or wrongly-calibrated, and the honesty and expertise of many tube sellers is questionable.

This nonsense about used tubes measuring as new needs clarification. Besides the factors listed above, it is true that tubes can test fairly high for a good part of their life, then suddenly start to rapidly decline. Lesson? It is foolhardy to trust many tube resellers who proclaim themselves experts and make claims on how close to new their tubes are. Caveat emptor applies here. Unfortunately, you have many foolish purchasers who think they have gotten what they paid for, leave positive feedback, and then the prospective buyer believes he is dealing with a reputable seller.

Anacrusis, IME tubes degrade gradually along a fairly linear curve. So much depends on the circuit and the individual tube though that beyond that any generalizations can be wildly inaccurate. IOW, if you are looking at a particular tube product, see if you can ask the owners how well it holds up. There are forums where this can be fairly easy, for example with our stuff you can inquire here or at http://www.audioasylum.com/forums/otl/bbs.html

Independent forums are likely to be less biased.

Usually you will get more accurate data this way, and its an easier tack then the one you are taking, IMO.

Do tubes wear out faster if they are driven harder? For example will listening at higher volumes wear them out faster?