1985 Mesa Boogie Mark IIC+

The Mesa Boogie Mark II amp was first released in 1978 and quickly became popular with pro players. It is also famous for it’s innovative design features including true relay based channel switching, buffered effects loops, and Simul-Class modes which allowed the amps to operate at different wattage levels and have the choice between Class A or Class AB topology. These amps influenced pretty much all major manufacturers of tube amps from the time they were released. They are now considered classics in their own right, fetching very high prices for good examples. Famous users include Keith Richards, Metallica, John Petrucci, and Trey Anastasio.

This one came in with a cap job and general maintenance having already been performed but needed a new set of power tubes and also a new reverb tank.

The amp was given a partial restoration to complete the previous one from years earlier. The competency of the older restoration (Cap Job with F+T/IC brand Electrolytics) was actually good - but there was some preventative maintenance left undone. It would've been nice if the power dropping resistors and screen grid resistors had been replaced with upgraded and new units of the same value. As those parts tested in spec and showed no signs of heat-damage it was decided to leave them rather than disassemble the entire amp and deal with the unnecessary added cost.

The original Ruby branded reverb tank was shot (internal short in output transducer) and was replaced with a MOD brand 9AB2A1B reverb tank. This is a long tank with 3 springs and a medium decay time. The tank had to be machined out a bit to fit the unusual setup and screw hole sizes of these 80's era Mesa builds. It is extremely tight in that cab with the chassis, tubes, tank, and fan all jammed in there.

The bias on these is a non-adjustable fixed bias - which in order to use tubes that are not “Mesa” branded, either the bias circuit has to be modded with a trim pot, or the power tubes need to be in a specific range of current draw. I went with sourcing the right power tubes for this repair as the customer did not want it modified.

The amp was partially re-tubed with the following compliment - The preamp tubes were kept as they tested fine. V1-V5 = Mesa 12AX7A, V6/V9 = JJ KT77 Matched Pair, V7/V8 = JJ 6L6GC Matched Pair. I picked out this lineup specifically for this amp to give the best balance of tone, reliability, and low noise. I think the KT77’s sound great in this amp rather than the regular EL34’s. They add a clarity and low end boost that compliments the tone of this circuit. The Fixed Bias was measured to be at Factory Spec. All pots were cleaned and hardware tightened.