Dating a marshall amp
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 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.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 LTD came equipped with the original 5AR4 rectifier tube, and four Jensen P10R reissue alnico speakers, which was period correct for the original amp.
Similar preamp changes were also incorporated in the 5F8 Twin Amp at about the same time, but not on other large size Fender amps.
During 1958, Fender introduced the model 5F6-A Bassman model.
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.