6DG6GT Tube Amp Design Update 4-27-13

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This is just a quick update. I have finished drilling, painting, and installing the hardware. I installed the heater wires this afternoon. A few other random wires were also connected. One issue with reusing “iron” is the fact that almost all the wires were cut very short. I had to splice and heat shrink quite a few leads from the transformers.

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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.

My travels in repairing broken electronics (2)

Today I had an electronic gate brought to me that stopped working. Here’s the thirty minute way to figure and fix…

  1. The “customer” came in and asked me if I could fix a circuit board. I said, “I can definitely try, but I can’t make any promises.”
  2. I asked him what happened the day it quit working. He said. “I hooked the power up backwards, and the fuse blew. I put a new fuse in, and it blew, too. I imagine something else is blown.”
  3. Based on his testimony, I assume that something has caused a short across the positive and ground. I tested the circuit without power attached, and there was zero ohms between red and black.
  4. Continue reading

My travels in repairing broken electronics

Here are my first steps in the repairs this week:

  1. Search Google [or your favorite search engine] for published schematics.
  2. Start at the wall and work your way in.
  3. Check solder connections and part integrity all the way through. Broken jacks, resistors, solder joints, etc. could lead you to the source of disaster.
  4. Look for burn marks or abnormal-looking solder connections. [Solder can and occasionally will defy gravity.]
  5. Continue reading

Review of the Low Voltage 12au7 Tube Headphone Amp

http://blog.makezine.com/archive/2010/04/low_voltage_tube_headphone_amp.html

12au7 Headphone amplifier
This is the link that I found on Makezine’s website a few weeks ago. Of course, the actual link is on the page I linked to. It is definitely worth checking out. I pretty much had everything available with the exception of a 12au7 tube. I just happened to win an eBay auction with about eight 12au7 tubes. I paid $25 dollars for 14 tubes.
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