I have read that the Tung Sol 12ax7 is a finicky tube that will die fairly quickly in the cathode follower position which I found out first hand before reading that. I was curious about its durability in the phase inverter section. I really like that tube in the phase inverter slot in my amp along with one in V1. Hard to explain but the difference in my amp was obvious. I order my tubes from Doug's Tubes and he didn't think it would be a problem but he's never heard of anyone using that tube in that slot when I talked to him.