i've tried a few other tubes without much success. one i remember was nos mullards. they weren't a good match as the ai preamps are very sensitive to microphonics. i would stick with sovtec's.
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I replaced the tubes on my Mod3 two tears ago. NOS Seimens super duper low noise, super duper matched, blah, blah, blah tubes at less than $70 apiece. So much better than the supposed select Ruseky crap. I don't care how many boxes of tubes you go throught to find "low noise, matched" Soveteks. These are what they are. For no more money you can get better NOS stuff that will allow you to tune your system (to a degree) via the preamp. The Seimens did EVERYTHING better than the Sovtecs. Why to you see manufacturers using tubes like Sovteks? Simple. Available supply. A manufacturer needs to maintain CONSISTENCY in the product -- so, they need tubes that are readily available. Go with NOS all the way, man. Kevin Deal seems to know his stuff. Also, Andy Bowman at Vintage Tubes Services is great. Tubes are all that Andy does. Good luck. Jim.
Stay with 6922, do not try 12AT7. This kind of statement sadly gets mis-understood and takes on a life of it's own. You probably won't hurt your preamp by doing this, but by doing the reverse (a 6 volt tube in a 12 volt hole) you can damage your gear. Stay with numbers that the manufacturer states is OK. In the case of the Mod 3 and L-1 that would be 6DJ8/ECC88, 6922/E88CC, 7308/E188CC and CCa. Some folks use 7DJ8 too. On this Russian 6922 thing...Sovtek is an export name. I have eight different types of Russian 6922 and of those there are two that sound excellent. Of those two, one of them we can get only about a 40% yield that are good enough for the Mod 3A. The other one is better. And lasts. In fact, one guy who lurks here that bought his Mod 3A from us prefers the special Russian 6922/6H23 tube to our much talked about Tungsram.