The 5D6 was a major departure from the earlier 5B6 Fender Bassman model.
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 lowest serial number known to still exist is 0013 (Frank Roy), but 0035 (Albert Talley), 0075 (Jim Cornett), 0077 (Perry Tate), 0089 (Mark Grandfield), 0701, 0745 (Walter Horton), 0769 (Hayes Kolb), 0780 (sold on e Bay Nov 2006), and 0783 are among those still known to exist.
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.