The 1959SLP is a recreation by Marshall of the plexi Superlead. They used a pcb and trannies by the original manufacturers (Dagnall). For unknown reasons they never quite used the exact spec of the originals.
The Superlead as a plexi ran from early '67 to mid 1969 although the circuit of the early metal panel amps was quite similar up to 1972. They are simple circuits and derive much of their colour and tone from over driving specific parts (the basic trannies, the type of caps and resistors the relatively low filtering).
The first amps were relatively soft and relatively clean, most distortion was a pleasing crunch but kick then with a good pedal and they are great. These are shared cathode on V1 a mid biased tone stack and large PI couplers and big trannies for extended low end, filtering low.
68 saw split cathode but still low filtering, much more rock orientated and can can be quite aggressive
69 had the same preamp but tighter filtering
70 to 72 same bu then tweaks for brighter tones and less NFB for more raunch.
Roe's time scale documents these or for extended read try this
http://forum.metroamp.com/viewtopic.php?f=3&t=1211
and this:
http://forum.metroamp.com/viewtopic.php?f=3&t=106
schems here:
http://forum.metroamp.com/viewtopic.php?f=25&t=9680
