U.S. Patent No. 1,874,159 - 08/30/1932 - Burnice D. Bedford
Comments Concerning the Patent Gerald Stanley 03/03/2003: "Many present switch-mode amplifiers are billed as new, but if that is true, Burnice Bedford (filed in 1930) must be their contemporary. Using only three tubes, Bedford made the first push-pull switch-mode amp."