During the 1990's Norman Koren designed a power amplifier using 6550 tubes and toroidal transformers. After correspondence with Menno van der Veen I have all of the information about the toroidal transformer that is used in Norman's Amplifier. Menno is the designer of this transformer and also has written a book that explains the theory about his transformers. The original PAT 4006 CFB transformer was special designed for Norman but for production Menno renamed the transformer to PAT 2100 CFB/H which is available at Plitron as well as Menno's book Modern High-end Valve Amplifiers based on toroidal output transformers. Please send my any technical thoughts.