Can a 12AX7 be replace with a 12HB7 or even a 12AU7??
I know you can do a 12AU7 in place of a 12AX7, but I don't know about the other one.
not a 12bh7. yes to a 12au7.
Not sure what an 12HB7 is but you can use a 12AU7, it just won't have as much gain.