The 7247 is the same tube as a 12dw7 and wouldn't make for a very satisfying 12ax7 substitute. It's a dual triode (like other 12XX7 tubes) but one half is like a 12ax7 and the other is like a 12au7. A 12ax7 as a gain of 100 for both triodes. A 12au7 only has a gain of 20. So... A 12ax7 would be 100/100 and a 12dw7 (7247) would be 20/100.
It's not especially unsafe (there are minor exceptions) to use any 12XX7 type tube in another 12XX7 socket. The circuit will most likely not be ideal for the different tube type though.