Looking at the board it looks like channel 1&2 bypass the tone stack!
With both 1&2 mixers bypassed you will find 3&4 relatively flat, although I see 3 has a volume pot bypass cap.
Those models have different arrangements for the input stage’s cathodes, yeah.I think I’m going to have him set it up to where two of the channels are 1986 specs and the other two 1987.
Why not use half of that extra preamp tube to turn one channel into a 2204? That totally leave the 1987 circuit alone. You just add the cascaded gain of the 2204 in one channel. I'm thinking of ways to do that to one of my own amps. But it doesn't have the luxury of eight input jacks to play with. Just the regular four. I'll repurpose channel 2's low gain jack to be the cascaded input.
When I started to write that reply, your recent reply wasn't visible yet. I got distracted by other business and hit the POST REPLY button after you'd posted yours. You don't have to be jerky about it.
Awesome work!
Now, could I ask you for a favor?
I make reproductions of the front and rear panels for many older Marshall models. And the super PA is one type I don't have information on.
I'd like to get the dimensional information on your amp's front panel that will let me start designing replicas of it.
The simplest way to do it would be to lay an inch scale ruler alongside the front panel and malke photos, so I can see the total length of the front panel and the hole locations with a reasonable degree of accuracy.
The dimensions are almost certainly all going to be fractional inches, not metric. Marshall always used Imperial fractional measurements in the 60s and 70s and I'm not sure if they ever changed over to metric for that.