I've never found a "regular" 12AX7, either current or NOS, that could compare sonically with (either) the RCA or Sylvania 5751 triple mica black plates. For more on these tubes, as well as 6DJ8/6922 and 12AU7, I strongly suggest you read the (classic) Joe's Tube Lore here: http://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html
