Marshall JCM800 2×12 combo amp repair

Marshall Input JacksI took on a new and simple project today. A friend’s amp started Marshall Circuit Boardgoing on the fritz, and he was ready to give it to the doctor. I’m no doctor by education’s standards, but I love to take what is hurt and broken and make it work again. I derive a pile of joy from the challenge of diagnosis and even more enjoyment when the job is done.

The problem that was described to me was that the amp wasn’t acting normal. The beautiful sound was yucky, and it lacked the power that it once had. Yippeeee! Not in that the amp is busted, but the symptoms describe a simple problem. The output tubes are going out or are on the way out.

I took the amp home and turned it on. I let the heaters warm up, and I watched them as they began to glow. Almost immediately, heat began rising out. I touched my dry finger to the envelope of the output tubes; one was hot, and one was not. Boom. There’s the problem. One of the Electro-Harmonix 6550EH tube’s heaters was burned out.

Output tube Electro-Harmonix 6550EHThe only other problem that I found was that the amp’s low input didn’t work. Upon further inspection, someone soldered the ground wire to the wrong tab on the jack. The hot wire was correct, but the ground wire was not connected to the barrel ring. The jacks are very wobbly so I am going to replace those.

I called up my customer and let him know what was up. He would need two matched  output tubes and two input jacks. Most of the labor will be in the replacement of the jacks. Checking the bias should be fairly simple task, and replacing the tubes will only take a minute.

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