Bad NOS tubes...

I am somewhat with stevecham on this one, not that I can't tell the difference in sound, but it is not a sure thing that NOS will always outperform current production tubes.

I believe, like many things in audio, it is just a matter of matching. Different brands of tubes (NOS or current production) do have different sound, so the users have to match them to their systems. For example, in one of my amps, I prefer the plain EH6922 to NOS Amperex 7308.

Think about it, most current tube amp manufacturers are probably tuning the sound of their amps with current production tubes, so NOS tubes probably don’t have any advantages.

Add me to the list of those who can hear differences between tubes and who enjoy fine tuning the sound of their systems with old stock tubes.

aol, Talk to Kevin Deal at Upscale Audio. He is the importer of PrimaLuna amps and the expert on them. He also has a great selection of NOS and current production tubes that are priced from very reasonable  on up.

He can probably tell you if there is a possibility that the tubes that went bad on you were not right for your preamp or if you just got some bad tubes, as you suspect.

Before you talk with him give the stock tubes a good listen and decide what kind of change in sound you want to hear with a change of tubes. He will give you his honest opinion, even if it’s not what you want to hear, which causes people to rant about him on this forum from time to time.

Buying old stock tubes does have risk. You can minimize the risk by buying from an established seller with a good reputation. Don’t let your bad experience sour you on tubes. They can definitely improve the sound of your system, IMHO.

Here are some other good sellers:

Tubemonger

Vintage Tube Services

Jim McShane

Brent Jessee

Talk to Andy bowman at vintage tube services.  He will take care of you.

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.