In this video, we'll take a look at Kentucky Headhunters guitarist Greg Martin's early 1980s Mesa Mark IIB 60W combo. This one has several factory options such as the Graphic EQ board and Reverb. He's complaining it's too noisy. We'll try to determine why and along the way, we will discuss why even vintage Mesa amps are so hard to work on and why many techs don't like taking them in for repair. This one appears to have also been modded at the factory (perhaps later?) from the typical Mark IIB schematic with a post effects level control which operates like an overall Master Volume for the preamp. In the end, we will substitute a 12AU7 for the 12AX7 in the V1 position and demonstrate how that has little effect on the amount of overdrive these amps can produce. Hard rock lead tone is one thing they do VERY well.
Since publishing this video, a viewer has alerted me to the fact that this example has a popular factory mod done by Mike Bendenelli "Mike B" at Mesa. It includes the post effects level control on the back panel and a movement of the effects send and return from V2A cathode follower to another point. The change allows V2A to act as a traditional gain stage, thereby increasing the overall gain of the amp and putting it closer to a Mark IIC+ in terms of overdrive.
#Mesa-Boogie #repair #troubleshooting
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