Dating vintage fender amps by serial number
5B6 Bassman amplifiers had two 6SC7 or 6SL7GT pre-amp tubes, two 5881 power tubes and a single 5U4G rectifier tube.It was designed to generate 26 watts at an 8 ohm impedance load, and offered a cathode-based bias.
The P10R Jensen speakers were shipped within all Fender Bassmen from late 1954 until early 1960.The output was 50 watts at 8 ohm into a single 12 inch speaker, with a "Tone Ring" baffle in the speaker cabinet.In early 1961, model 6G6-A was introduced with a solid state rectifier replacing the GZ34, and two 12 inch speakers with a conventional baffle in a slightly larger cabinet (wired in parallel) with a 4 ohm output.Many professional music industry analysts have heralded the 1950s Fender 4×10 Bassman amps as the greatest guitar amp ever.The first 1954 Fender Tweed 5D6 4×10 circuit generated further Tweed Bassman amplifier development through 1960.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.
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.
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.
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.