Guys, Here are the facts as I see them:
(1) The OP bought 12AU7s for a preamplifier that presumably uses 12AU7s, so there is no question that he bought the right tube type. In terms of operating parameters, all 12AU7s are alike. (The OP should report to us if he changed tube types, but he did not say that he did.)
(2) A tube that looks good on a tester can only be relied upon to have been good at that moment. Tubes can fail unexpectedly and prematurely, whether NOS or current manufacture, but the hooker here is that 4 out of 6 tubes appear to have gone bad in such a short time. That defies the law of averages for tube lifespan, when tubes are used within the recommended limiting parameters.
(3) For this reason, we suggested that the OP might want to check out the voltages in his preamplifier; one cause of such a catastrophe would be abuse of the tubes, for sure. They DID last for more than a month before showing signs of early death.
(4) Against this hypothesis, the OP also tells us that his set of OEM tubes, presumably also 12AU7s, are all working fine after many more hours of use in the very same preamplifier. Moreover, he re-installed the OEM tubes and they are sounding fine.
(5) So, if we want to exonerate the vendor, one more hypothesis remains tenable: Perhaps something went bad in the preamplifier subsequent to the installation of the new tubes, something that killed 4/6 of them prematurely. If this is so, then the OEM tubes ought to go bad too, after a while. To test this hypothesis, I advise the OP to continue listening with the OEM tube set for a month or two longer; see how that works. Some or all of them will fail as well, if the preamplifier has developed a power supply problem. (This could involve either the filament supply or the B+ supply.) OR, take the unit now to a good technician for a check-up.
Peace. Out.
(1) The OP bought 12AU7s for a preamplifier that presumably uses 12AU7s, so there is no question that he bought the right tube type. In terms of operating parameters, all 12AU7s are alike. (The OP should report to us if he changed tube types, but he did not say that he did.)
(2) A tube that looks good on a tester can only be relied upon to have been good at that moment. Tubes can fail unexpectedly and prematurely, whether NOS or current manufacture, but the hooker here is that 4 out of 6 tubes appear to have gone bad in such a short time. That defies the law of averages for tube lifespan, when tubes are used within the recommended limiting parameters.
(3) For this reason, we suggested that the OP might want to check out the voltages in his preamplifier; one cause of such a catastrophe would be abuse of the tubes, for sure. They DID last for more than a month before showing signs of early death.
(4) Against this hypothesis, the OP also tells us that his set of OEM tubes, presumably also 12AU7s, are all working fine after many more hours of use in the very same preamplifier. Moreover, he re-installed the OEM tubes and they are sounding fine.
(5) So, if we want to exonerate the vendor, one more hypothesis remains tenable: Perhaps something went bad in the preamplifier subsequent to the installation of the new tubes, something that killed 4/6 of them prematurely. If this is so, then the OEM tubes ought to go bad too, after a while. To test this hypothesis, I advise the OP to continue listening with the OEM tube set for a month or two longer; see how that works. Some or all of them will fail as well, if the preamplifier has developed a power supply problem. (This could involve either the filament supply or the B+ supply.) OR, take the unit now to a good technician for a check-up.
Peace. Out.