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A home brew amplifier

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A home brew amplifier Empty A home brew amplifier

Post by westcoastjohn Tue Jan 10, 2012 11:02 pm

Not a true home brew, more of a radical modification. I found this guitar amp laying outside under a tarp. It turned out to be a Johnson Clarion built in Winnipeg, Canada, about 1960. The chassis was steel and covered with a blanket of rust. But it had both transformers and a tube or two. I drew the schematic. It used all octal tubes, and was wired on circuit boards handmade from scraps of gold flecked formica, kitchen counter material. So it started life as a home brew of sorts, although Mr Johnson built a lot of amps over a period of about 20 years. For some reason, he put the volume control right on the input ahead of the first stage. This amplifys noise along with the signal. It is better to run a short lead to the first stage and amplify the signal before introducing noisy components.

I tested the trannys and the output transformer had an open winding, shot, but I had a good one for a push-pull (two power tubes) amp design. Stripped the chassis, sanded off the rust and gave it a paint job and a new faceplate of aluminum flashing material. I drew up a new schematic using 2 12AX7 preamp tubes. It wasn't hard to insert the smaller sockets with some aluminum adapters. Being twin triodes, they provide a double whammy of amplification. So I added an overdrive stage with a switch. I inserted a gain control between each stage. With 3 stages of amplification, there was still one stage left for a phase inverter. I found a Fender schematic that uses one triode for a phase inverter. 2 6V6's provided the power and a 5Y3 went into the rectifier socket. I copied all the details, cap and R values from various schematics, and the thing played great!

That is what I like about vacuum tube circuits. You can see and feel every component. If it isn't quite right, you can tweak it.
Finally I stripped the old cabinet and put in a good speaker. The amp was reborn. Mr Johnson would have said it had too much gain, I'm sure.

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A home brew amplifier Empty Re: A home brew amplifier

Post by jukeboxman Mon Aug 27, 2012 11:38 pm

where are the photos?

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