 The upgraded 5F8 Twin-amp of 1957 with its complement of four military grade 5881's was briefly rectified with a tall mercury vapor 83 tube and special tube socket.Due to its volatile nature if broken, it was quickly superseded with a GZ34 rectifier but not duly noted on the tube chart until the second production.The final 5F8-A Twin-Amp version released in 1958 and produced into early 1960, had become the crowning achievement of Fender fifties amplification.