1990 Mesa Boogie 295

This is a pretty radical (90’s reference) stereo power amp that comes in rack mount form, and is great for that 80’s/90’s arena rock sound - or for this customer, a cool way to do the “wet” in a wet-dry-wet rig. It came in with a lot of blown out and physically broken power tubes, the preamp tubes were shot, and the power cord was in need of fixing too. 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 mini 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.

A matched quad of Tungsol 6L6GC STR (medium current draw), a matched quad of JJ EL34 II (low current draw), and a matched quad of Sovtek 12AX7LPS were put in to remedy the symptoms of the amp and biased up to %70 Class A/AB Plate Dissipation with a B+ of 498VDC. The interesting thing about this amp is that it uses a Constant Current Source Diode to bias the phase inverters, and it runs a pair of EL34 tubes in Class A Triode mixed in parallel with a pair of 6L6GC tubes in Class AB Pentode (or Beam Tetrode depending on how you see it) per channel resulting in 95 Watts each. Kind of a strange design but it sounds good - very clean and defined. The power cord was fixed and the amp was in need of a deep clean. I usually recommend doing cap jobs when an amp reaches 30+ years old but Mesa wisely used Sprague Atom electrolytics which seem to last longer - I told the customer to come back for a cap job in the next few years as they all tested good for now.