The shared cathode was an original part of the JTM circuit which was copied from the fender bassman. Split started showing up when, I guess, Marshall caught onto the fact that guitar players wanted these amps and so I assume they started dialling in versions of the JTM100 for guitars players (Leads) and the split cathode was one of the changes made that probably started showing up in late 67 early 68. I’m not really sure. Some of the other guys here who do or have done repairs know more about the specific dates of things like this. The tonal difference is simple. You remove the bright channels cathode from the normal channels and give the bright its own set of cathode values dialled in to be a bit brighter. Bypassing the normal channel cathode with the usual 330u or 250u would boost all frequencies in the guitars range plus freq’s far lower. .68 boosts frequencies starting more in the midrange and up. So the effect is that the bright channel was then a little brighter since all frequencies are boosted accept low freq’s. Pretty much all Lead amps past a certain date had them but as far as I know no Bass amps used them.
– Dan Schepise