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I am relatively new to tubes, having had a CJ tube preamp for about a year and loving it. My question pertains to life expectancy and test numbers for tubes like 6922s and 7308s which are rated for 10,000 hours. If used old stock tubes are purchased that in fact test new, should they be expected to last anywhere near 10,000 hours? Is the decline in performance linear and only noticable when they approach the minimum good test numbers? After about 5,000 hours of use, will the test numbers reflect that they are half gone, or will the numbers and performance hover near new until the last 1,000 hrs or so and then decline rapidly? Thanks.
Follow Ups:
A lot also has to do with how hard a tube is driven in a particular application. I have a pair of Tungsol 6sn7s that tests very weak (low transconductance) but sounds fine in my amp. I talked to someone at the manufacturer, and they said that the tube is used in a very unstressful manner and will even sound fine when it tests bad, something I confirmed.On the other hand, I know of some linestages that eat up tubes like crazy and so tubes had to be replaced every 6-8 months or so.
I would keep at least one whole new set of tubes on hand. Once in a while switch out the tubes with the reserve set. If the sound comes to life dramatically, then at least one of the tubes in the old set has become weak. You can then decide whether to do some switching to determine which have gone bad, or you can replace the whole set and not have to worry.
Right until the very end, a tube could still test as new or even better than new. Realize tube testers were originally intended for repairmen to quickly discover faulty tubes in a 2-dozen tube Television set during a house call; testers were never intended for critical Hi-fi Audio evaluation as we would like to apply today.
I picked up a 1946 Emerson bakelite table radio (with the "American Five" octal GT tube complement) on Ebay last week which had what appeared to be four original tubes....at least they looked ancient and were labeled "Emerson." The 35Z5 half-wave rectifier was a Ken-Rad of unknown vintage.The radio tunes all of the usual local AM stations with no drift, and the audio is strong and clear.
In general tube sound good though 80% of their life. They do soften a bit as they get beyond the 80%, but still will sound OK. The best thing to do is not be parinoid and just enjoy the amp. That and purchase one or two replcement sets just to be sure. You will always be able to sell the tubes later if your interest change.
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Long Live Dr.Gizmo
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