I don't know much about the history of electronics, but in the 5F6-A (released in 1958, i think) schematic, they're using a solid-state diode for the purposes of having a bias voltage: So they clearly had solid state diodes back then, and knew how to use them. So why didn't they use them in place of the rectifier tube as well? Why did it take fender a few more years (maybe roughly 1963?) until they started using diodes in things like the Twin? Why did it take until 1965-ish for Jim Marshall to come along and decide that solid-state diodes were a suitable replacement for the rectifier tubes in this style of circuit (I think he first did it in the JTM 100)? Were the 2 to 4 extra solid-state diodes necessary to replace the tube significantly more expensive back then than including a rectifier tube, extra octal socket, and transformers with dedicated rectifier heater filaments, as well as costs of drilling an extra socket hole?