Mesa Boogie, beloved by metalheads, terrified by techs. Truly the most contortionist style soldering I’ve had to do in a while. Here’s the story.

Mesa Boogie MkIV from beneath

     Client brings his Mesa Boogie MkIV Combo in, suffering from a blown fuse and what looked like a cooked vacuum tube. He threw that tube out, so I wasn’t able to check but he said the text that should have been white, was brown so it got pretty hot. Popped a set of Electroharmonix 6l6eh tubes in to the open spots and checked biases. These tubes bias very hot on a Mesa, but they’ll last through testing. One tube of the pair was idling at 62ma, but the one that cooked was idling at 136ma?! (FYI 6L6GC spec is max 80-90ma)

     Checking voltages, the over biased tube was only getting -32v bias, vs the other side’s -45,-46v. working my way back, I find the culprit. One of the 221k resistors that feed the bias was drifting under voltage. That was a fun one to find. No power, both sat at 221k. With power on, one side was dropping 60v to 52, the other 60v to 38v. Ruh, roh! We got’s a problem! Replaced both with 1/2w 221k resistors and now both tubes idle properly. Both sides getting their -46v-ish bias voltage.

221K resistors for Mesa bias circuit

     Looking at the pictures, you get what I mean by contortionist soldering, there really isn’t very much room to do repairs on this board without pulling the entire circuit out. What would take 10 minutes to replace on one of my amps, took a full hour of sweating and bending and hoping that everything goes into place quickly so I won’t lift traces from the board.

     Now I’m waiting for a pair of the correct Mesa tubes to make completely sure that everything works the way it’s supposed to. Then it’ll be back to rocking loud and proud!

     Circuit boards may work fine, but I’ll stick to turret board construction. Arguably stronger, but mainly easier to service down the road.

     As always, if you’ve got an amp making funny sounds, or none at all, let me take a shot at it. You won’t get a tech with fairer pricing nor quicker turnaround. If I tell you it’s fixed, you can take that right to the stage. You can reach me through Facebook, email: RauldelP@ra2amps.com, or call me at 305-345-3948. Thanks for looking!

-Raul