Suggest Russian Military 6H23EB, Electro-Harmonix 6922, or NOS Tungsram E88CC
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The tube of choice for this preamp is Amperex Bugleboy. Any Amperex 6DJ8/6922 will sound good. Mullards also sound good. Be sure you get NOS tested and matched as this pre does not like any microphonics from the installed tubes. None of the current production tubes will sound as good. Believe me I have tried them all.
Okay. Fact: AI sounds great but eats tubes. You have a choice best sounding vs. longest lasting. I've bought over 100 NOS tubes in the past 2-3 years. Best sounding in order (combinations of 7308 6922 & 6DJ8) Amperex, Telefunken, Seimens, Tungsram-some lasted less than 1 hour. I could hear a degradation even in Tungsram over months and they were the strongest. Longest lasting Sovtek 6H23, Telefunken PCC88(sounds surprisingly good, very good really & bought them for almost nothing)-have yet to burn any of these out yet. I've not rolled much recently and have left in now for 1 year at least E288CC. They sound great and appear to be indestructible. They cost alot. Note they're so tall the top of the case can only be reinstalled with spacers. Too expensive really. My advice, if all you want to do is enjoy music rather than enjoy tube testing, call Kevin Deal at Upscale Audio and get his best 6H23 for $25. If you turn off the unit between sessions like me they may outlive you.
Amperex 7dj8, Siemens e288cc like mentioned above, and the late model single getter, no shield 7308 Siemens. These all lasted 2 years in my previous m3a. The 7dj8 Amperex really sounded killer, as did some Toshiba 7dj8's (sounded similar to Mullard cv2493). I still have a few of each if interested (the NOS sleeves of 5).
The key to the 7dj8/PCC88 is to have a pair in use with a pair of regular Russian 6h23 or the Siemens single getter 7308 tubes. In the M3a the tubes are run in-series, so by having a pair of 7dj8 in the line stage, and 7308 in the phono, the extra voltage going to the 7dj8 lowers the filament voltage seen on the 6922/7308 tubes (so they last a LOT longer). Really does work. I had my M3 then M3a for quite a few years, and they lasted a long time in this confiquration.