freestompboxes.org

I am going to try to build the preamp section of the Mk I Mesa Boogie. I have found several schematics with some variations. I think I understand how all 4 stages of the 2 12ax7s are switched in for the high gain channel. The query I have is in some schematics Input 2 is shown with a 220k resistor hard-wired to ground. In another, the resistor is wired to connect only when Input 2 is not used, shown in the attached picture. Any ideas on which one is correct please?

My guess would be that the 220k is NOT hardwired. It could be mesa did both at one time or another however. Mesa did switch it up a lot. You might try asking in the music electronics forum. There will be some techs there who have been inside one of these. I have never even played a MK1.

Agree both are likely right, Mesa changed the circuits all the time in those days.
It's so trivial anyway, why not try both and see what you prefer?

Mesa was also known. [ or merely said to have, possibly by themselves ] to release schematics with intentional mistakes

Thanks for the suggestions. I can mount the resistor on the potentiometer so it should be easier to try out.