The NU black glass and earlier gray glass 6SN7GT sound different. And the round plate 6F8G sounds different than either of the 6SN7GTs. I wonder which of these the 12J5 is most similar too....
I do have the black glass 6SN7GT, the round plate 6F8G, and now the 12J5. Both the black glass 6SN7GT and the 12J5 have ladder style plates, but they are not identical in design. I did think they sounded pretty similar though, tonally. I'll have to get the 6F8G in the mix! I haven't listened to it in a while.