My dad's building (or rebuilding) a tube amp based on KT-88s and an ultralinear output transformer. Back in the 1970s he built a stereo version of it, but it weighed close to 80 pounds. He plays accordion with an electronics tone generator (midi setup nowadays), so he used one channel for his bass, and the other channel for the right hand reeds and synth.
Anyways, he pulled it out of storage, cut down the chassis and started rebuilding it as a single channel output; he's 75 now, and an 80 pound amp is a bit much for him. The transformer is something he got out of an old PA system from the late 60s. It has a 200W core, and has a myriad of taps and isolated windings on the secondary for various load impedances, including 25 and 70V line. The transformer appears it might have been a custom design made by Hammond; It works flat from 20Hz to way out past 150Khz!!
He had an interesting way to obtain a bias supply, a small, low current 120V / 12V transformer with the 12V winding connected to the 6.3V filament winding on the large power transformer. This yields about 60V AC on the 120V winding, perfect for a full wave rectified bias supply, rather than using a large power resistor off the B+ windings with half wave rectifiction.
