This amp is the classic rock amp used by all famous Marshall users. It was built from 1975 to 1981 and Marshall's first master Volume amp. Built into the same cabinet as the 50w model you can only determine the 100 watts power by the heat grill on the top side wich is absent on the 50w models. The amp has one single channel, no reverb or effects loop, just the distinctive Marshall sound with three ECC83 in the preamp circuit and four EL34 in the power stage.
The amp shown is #7694J and was built in October 1977. It was modified with a functionable effects loop using a fourth ECC83 tube to feed the signal in an out without effecting the sound. This effects loop can be switched from parallel to serial is controllable by two pots (send and return) on the rear side.