Tried out a new tube in my Mark IV,the 6AU8A, the tube socket is designed for a 6GH8A and I simply made my own adapter from a socket saver, the tube did surprisingly well,it's very close to specs,with slightly higher power handling capability. I'm sure the curcuit could be tweaked a little more for even better performance. They price about the same as the 6GH8A.[url=][/url]