the fat lady design comes from the phillips ev4001-6 range early 1960s. shes a balanced 3 stage variable gain amp. the control stage is a pair ef83. a complex and hefty 7 tube chassis. all point to point wired for extra juiceyness. i call them fat ladys. zephyr and the blue fat lady twins . sugarlicks fat lady . OO1 fat ladys. the ekadekatron twin fat ladys and twin eqs.