Can you substitute a 12bh7 for a 12ax7?

you can substitute them, meaning that it will function in a circuit that uses common dual triodes, but if your using it in some audio/instrument/microphone application it probably wont perform the way you want it to. It IS a medium gain dual triode and wont overdrive in a guitar amp the way a 12ax7 does (BTW thats why your amp came with 12ax7''s and not 12bh7's in it) . Also this is a general statement.