Watch it with the12AT7. It's not really an audio tube, and can become microphonic, plus plate and screen voltages that are ok for a AX7 may not be ok for a AT7. Plus it's a fairly noisy tube and not really recommended for Audio Gain stages. Go ahead and play with it, but I think you'll be going back to 12ax7's. One tube you CAN USE is a 5751. It's a 12ax7 with gain of 70, just like the 12at7. As it's a 12ax7 derivative the heaters are the same.