Are you looking for schematic? The early Mark I had the Bassman/Marshall style presence that added high harmonic content in the power amp feedback loop, where the re-issue had the black Face side of things with a hi roll off presence at the end of the preamp before the phase inverter/poweramp next to the Master volume, all the way up is straight blackface, turned down is darker. Interesting side note, Carlos Santana's #009? had no cascaded gain stage like the later Vol. 1 and Vol. 2, it had volume treble, middle, bass, presence. Preamp boost was by disconnecting the Middle control lug that goes to ground for the boost, basically bypassing the the tone stack controls, the treble was somewhat still active. Later a 100k resistor was put across a push/pull pot, when pulled the 100k resistor was the amount of resistance between the tone controls to ground, allowing more signal to pass, pretty much bypassing the tone controls, when pushed in, the 10k Middle pot limited the gain and the tone stack worked normally. Today we see Mesa on on amps implements a 100k audio taper (LOG) pot so that from 0-12 O'clock its a normal Middle control, 12 O'clock to 5 is a variable boost control like on the Mark V 25, 35 and possibly other models. I been doing this before they started, when I learned about how their Preamp boost worked early on but the downside sometimes is that when its really boosted, the low notes get bloaty, farty, muddy, so one has to to carefully dial in and play lighter on low notes or turn the guitars volume down a bit to avoid the farts. Probably not what you wanted to hear, but most people are not aware of this. At lower settings the Mark I can get some killer Blues/Rock tones, it's basically a Twin with an extra Gain stage. Even without the extra gain stage, using the Preamp boost or swapping out the 10k Middle pot with a 100k Audio pot allows you to get some good early Santana like lead tones, even on a Fender amp with this mod. If you want schematics, we can help you with that too.
