I can't comment on whether 12bz7 is a substitute for 12ax7/5751 however if it is my advice would be take advice only from those who have experience of your amp or go out and listen for yourself.
For me Sylvania 5751 are excellent, better than any 12ax7 I've tried but in another implementation they may not work at all.
Can be good fun exploring different tubes though expensive and a good measure of frustrating
For me Sylvania 5751 are excellent, better than any 12ax7 I've tried but in another implementation they may not work at all.
Can be good fun exploring different tubes though expensive and a good measure of frustrating