I had a /13 SJT 10/20 that had a 5879 preamp tube in it. I went through 10 before I found one that was not microphonic as hell in that amp. Great sounding tube but a pain. Dr. Z builds a better amp than ol' Fred Farkel (IMHO) and is a much classier guy so he's probably figured out what circuit that tube needs to shine. Sounds interesting.