If you could post the schematic that would help. I have no experiance with the 6ag5 power tube but what the hell give it a shot. Whats the worst that could happen ? your out $5 . Are you planning on taking the parts out of the reel to reel and puting them in somthing else ore just gutting the reel to reel and modding it? Either way it should be a fun build. you should be fine with the 5e3circuit but anyone else is welcome to chime in.