CHANGES TO THE 7 SERIES PREAMP SCHEMATICS
One of the key changes is the reverb mixer resistor - R60.
Clearly the schematic as we have it was redrawn at some point in or after November 1966, as the changes it records are already incorporated in the circuit drawing. All we can say therefore is that by 15th November the reverb mixer resistor had been changed, formally, from 470K to 220K.
However, some amps have 100K, which may represent the very first value, the scenario being that 100K was found to be too low, and 470K too high.
Why these changes? Primariy an attempt to stop the amps feeding back at high volume when the reverb was engaged.
The question of how much authority the 7series preamp schematic, as we have it, carried for those actually building the amps remains to be seen. Probably not much. Clearly its greatest value will have been to repairmen. One imagines that Triumph had a multiplicity of formal and informal reference sheets, all annotated, indicating how the boards were to be wired up, what should go where, and so on.
100K
Resistor colour code = brown, black, yellow = 1 0 0000 ohms = 100K
220K
Resistor colour code = red, red, yellow = 2 2 0000 ohms = 220K
470K
Resistor colour code = yellow, purple, yellow = 4 7 0000 ohms = 470K