Mesa Boogie Mk I input resistors

Tube or solid-state, this section goes to eleven!
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Yazoo55
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Post by Yazoo55 »

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?
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bmxguitarsbmx
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Post by bmxguitarsbmx »

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.

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Optical
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Post by Optical »

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?

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Post by okgb »

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

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Yazoo55
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Post by Yazoo55 »

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

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