In July 1957, Fender introduced the model 5F6 Bassman. This model also had four Jensen P10R speakers, but the power supply was redesigned around a single 83 mercury vapor rectifier tube, and a new preamp circuit was introduced that included a three knob tone stack, with separate controls for Treble, Mid and Bass. The power amp included a "long tailed pair" phase inverter, an innovation that noticeably increased the "headroom" or clean power output capability of the amplifier. Similar preamp changes were also incorporated in the 5F8 Twin Amp at about the same time, but not on other large size Fender amps.