Dating a marshall amp
Fender ceased production of 5B6 Bassman amplifiers during the spring of 1954.
During November 1954, Fender introduced the newly designed 5D6 Bassman amplifier offering four ten inch speakers and was designed utilizing two rectifier tubes.
Later on, production was moved to Ensenada, Baja California, and the model name was altered to "59 Bassman LTD".
The LTD came equipped with the original 5AR4 rectifier tube, and four Jensen P10R reissue alnico speakers, which was period correct for the original amp.
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.
5B6 Bassman amplifiers had two 6SC7 or 6SL7GT pre-amp tubes, two 5881 power tubes and a single 5U4G rectifier tube.
Fender introduced the model 5D6 "DK" in November 1954 followed by the 5E6 Bassman Amp during early 1955.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.This final 1950s Tweed Bassman model product line included a change from the 83 mercury vapor rectifier tube 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.