Most I have seen were turned into 300B SE's. I believe Cary has a fixed cost for this mod. Sad that the 572-3 is no more. You can't substitute an original 572 because the pin connections are different. Have you exhausted every source for these tubes? If I had these amps I would get my hands on all I could find before they are totally unobtainable. I heard they don't have a long service life.
