Dating blackface fender amps
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.The first 4x10 Bassman amplifiers started with a batch of prototypes in November and December 1954, model 5D6.
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.
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.
During early 1960, Fender began producing the 5F6-A Bassman with Jensen P10Q speakers.
The P10Q Jensen speakers are more able to manage stronger electrical input power and generate better "clean" output sounds than previous installed P10R Jensen speakers.
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.
No schematic for the 5D6 circuit has ever been found, but Ken Fox and Frank Roy have created a few from originals, and copies are freely available online.
Only 11 of these early 5D6 Bassman examples are known to have survived.
Fender ceased production of 5B6 Bassman amplifiers during the spring of 1954.