Quote Originally Posted by Filterlab View Post
6SL7
6SL7GT
ECC35
6H9C
5691
6SU7
6SU7GTY
6N9S

All interchangeable with little issue I understand. There’s probably more monikers for the same convention.

Correct me if I’m wrong.

Yep, seen all these. I'm just a little wary nowadays as I like the idea of getting optimal usage out of each valve, the 5691 escapade has made me slightly paranoid.