I also have used 5751s in place of 12AX7s for years. The triple mica black plates from the 50s are the best. Don't buy the gray plates or the later two mica versions.
My personal preference is RCA, then Sylvania, then GE. All are excellent IMHO. GE is best bang-for-the buck.
Full disclosure: I currently have Sylvanias for sale on eBay and have a pair of GEs that I am getting ready to post.
My personal preference is RCA, then Sylvania, then GE. All are excellent IMHO. GE is best bang-for-the buck.
Full disclosure: I currently have Sylvanias for sale on eBay and have a pair of GEs that I am getting ready to post.