Besides being a popular and important amplifier in its own right, the Bassman also became the foundation on which Marshall and other companies built their high-gain tube amplifiers.
5B6 Bassman amplifiers had two 6SC7 or 6SL7GT pre-amp tubes, two 5881 power tubes and a single 5U4G rectifier tube.
Designed by Freddie Tavares, longtime R&D man at Fender, Instead of the single 15" speaker, four 10" Jensen Alnico P10R speakers were used.
The circuit had two innovations: a fixed bias for the power tubes, which increased power in comparison to the earlier cathode bias design, and a cathodyne phase inverter, using half of the 12AX7 tube and allowing a third gain stage on the other half.
During November 1954, Fender introduced the newly designed 5D6 Bassman amplifier offering four ten inch speakers and was designed utilizing two rectifier tubes.
The 5D6 was a major departure from the earlier 5B6 Fender Bassman model.
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.
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Initially intended to amplify bass guitars, the 5B6 Bassman was used by musicians for other instrument amplification, including the electric guitar, harmonica, and pedal steel guitars.
Fender began making other models with tweed covering, a similar open backed cabinet with a rectangular grill cloth and a narrow (just over an inch wide) tweed covered panel at the top and bottom.
Produced from 1954 until 1960, these models are called the "narrow panel" tweed amps .
Only 11 of these early 5D6 Bassman examples are known to have survived.