1966 – TheJTM45 changed to JTM50 on panels: Block logos now changed to white script with gold coloured plating: Valves changed to EL34s and rectifier phased out: Transformer uprated. 100 Watt production was now standardised with a single 100 Watt transformer and EL34 output valves. The 1961 and 1962 were upgraded in line with the heads and cabinets slimmed down. All cabinets now had corner locked joints. As a method of quality control all cabinets were signed inside.
1967 – Chassis were all upgraded from aluminium to steel construction. The first 200 Watt Majors appeared, with just treble, bass and volume controls, “active” tone circuitry and KT 88 valves. 10 and 18 Watt combos introduced in 1×12, 2×10, 2×12 tremolo or non tremolo versions. Fret cloth on cabinets changed to basket weave in late 67.
1968 – The Major was modified to match the normal panel layout and active electronics dropped. Gold logos changed to white.
1969 – New gold metal panels were introduced across all ranges. Serial numbers now followed a dating system with the letter A appearing after the number. By 1970 cabinets had all changed to check fret cloth. From this point onwards, Marshall amplifier history is much more easily accessible from the serial number and the inspection tag attached to every amp chassis.