A brief history of the JTM45

[Cygnus X1]

Clipped from LP Forum, who clipped from somewhere else:

The first JTM45 was pretty much a Tweed Bassman, no tremolo circuit and 5881 tubes. Stolen from Dr. Tube's site:

"JTM45, 45W head
This was the very first Marshall amp. It was built from 1962 up to 1966. It didn't actually have a model number at first, but later it was called a JTM45.

The circuit was a 1959 Fender Tweed Bassman (circuit 5F6-A) clone. It is said that the first prototype and early production amps were fitted with two 5881 beam power tetrodes, three ECC83's and a GZ34 rectifier tube, but soon KT66 tubes were used instead of the 5881's.

The Fender Bassman used a 12AY7 as the input tube but the JTM45 was fitted with a 12AX7/ECC83 which has a higher gain. The mix resistors (which mix together both channels after the first triode sections) were originally 270kΩ on the early models. Sometime (late) in 1966 these mix resistors were raised in value to 470kΩ.

These amps had 2 channels with each a volume control and two inputs (making a total of four inputs). Next came the low, mid, high & presence controls. There was also a power switch and a standby switch with a power indicator.

The first JTM45 amps had no model number printed on them. Later (probably in 1963) aluminium panels were used having JTM 45 and MK II printed on them (implying the first series of amps were the MK I's)."