The 5E6-A Bassman model was introduced later that year and included some evolutionary improvements.
In July 1957, Fender introduced the model 5F6 Bassman.
Produced from 1954 until 1960, these models are called the "narrow panel" tweed amps .
Fender introduced the model 5D6 "DK" in November 1954 followed by the 5E6 Bassman Amp during early 1955.
The 5D6 was a major departure from the earlier 5B6 Fender Bassman model.
Designed by Freddie Tavares, longtime R&D man at Fender, Instead of the single 15" speaker, four 10" Jensen Alnico P10R speakers were used.
This final 1950s Tweed Bassman model product line included a change from the 5Y3 to the GZ34 rectifier tube, as well as a modification within the Presence control circuit.
During early 1960, Fender began producing the 5F6-A Bassman with Jensen P10Q speakers.
The 6G6 model was covered in rough Blonde colored Tolex material with Oxblood colored grill cloth.
It had a single GZ34 rectifier, two 5881/6L6GC power tubes and four 12AX7 preamp tubes.
In late 1960, Fender introduced a completely redesigned model 6G6 Bassman Amp, using the "piggy-back" design, in which the amplifier chassis is housed in a small cabinet, attached by metal clips to a larger separate speaker enclosure.
The early models were called "Brownface" because of the dark brown color used on the control panel.
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.