Repairing a Mysterious Guitar Amp

About a year, year and half ago my friend Gary, who is the drummer at my church asked if I wanted this amplifier. I guess one of his friends left it at his house years ago and didn’t want it back, so he asked me if I wanted it. Thanks Gary for the amp!

When he gave it to me he said he didn’t think it worked and he thought that the previous guy had just used it as a speaker cabinet. So when I got it I opened it up to have a look at it and saw that there was no speaker, so it must not have been very loud, but this amp looks like something that is cool enough to get working again.

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Now the biggest hurdle with this amp is that it appears to be just something that somebody made. The faceplate says showman amp, but this is definitely not a Fender Showman. On closer inspection the faceplate looks hand drawn, and a lot of the holes are kind of sloppy and look like they were drilled by hand. So since this amp is kind of an odd ball, I obviously don’t have a schematic and am kind of on my own when it comes to repairing it.

It looks like a pretty standard dual 6l6 power amp, with bias modulating tremolo. If anybody thinks they know if this amp is a clone of anything, let me know. I just worked off of schematics for a Fender Bandmaster, and a Vibro-verb, which were pretty close.

amptop


The first thing I did was to install a 3 prong power cord on the amp, if you interested in that there is a separate article here.

3-Prong Power Cord

Next I took a look at the tubes to see if those were still good, it had these Toshiba power tubes in it, but those were pretty burned, and they turned out to be no good. I don’t have a tube tester so I had to use a multi-meter to check the tubes.

This isn’t the best but you can check the heater pins on the preamp tubes, which are pins 4,5, and 9 on 12ax7 style tubes for continuity. If there is continuity between those pins and the tube doesn’t looks like it still has vacuum in it then it is possibly good.

However when I fired up the amp later I found that most of them were super micro-phonic, so I replaced those.

Next it was time to flip the amp over and check some of the components. I started with the large filter capacitors and sure enough they were dead, so I replaced those. Next I tested the old can capacitor and found that 3 of the 4 capacitors were no good, So I added a terminal to the amp and replaced those 4 capacitors with individual components.

Next I looked around the amp and saw that this one potentiometer that just spins and spins and is clearly broken, so I replaced that. And I tracked down a couple of wires that had never been soldered, so I fixed those.

At this point the amp was working but there was a helicopter noise coming from the tremolo circuit. It was controlled by the speed and intensity knobs so I knew it had to be in the circuit. I suspected the only electrolytic capacitors left in the amp, so I pulled the one in the tremolo circuit and replaced it.

IMG_2534

Since this amp isn’t anything special and has no value for being original, I went ahead and replaced all of the other electrolytic capacitors at this point as well to be safe.

This helped the noise a lot but there was still the thumping in the tremolo circuit when the intensity was all the way up. I figured that maybe there was simply to much gain from the tremolo effect and I tried swapping out the 12ax7 tube that had been in there with a 12at7. This solved the problem of the tremolo thumping and also had the added benefit of taming the total output volume of the vibrato channel. This means that the normal channel gives you more clean volume, while the vibrato channel dirties up a little sooner with lower volume. I am happy with the result and the added difference between the channels add some nice versatility to the amp.


Now that I know the amp is working I am going to focus on the cosmetics.

img_2533.jpgI pulled the faceplate off and by hand bent it to get rid of the warp and make it flat again.

And then I cleaned it carefully with some windex and a paper towel. There was some spots that were pretty nasty and had a lot of grime on them so my scrubbing ended up taking some of the paint off. Once it was clean I sprayed some clear coat lacquer over the faceplate to protect the remaining paint from coming off.

Next I could put the plate back on and get to work on the knobs. The amp came with these pointer knobs that I really did not like. What was worse was that the knobs would run into each other if you turned them the right (or wrong) way. I was thinking that I could go full Fender blackface emulation with this amp since it had that look to it already, but the knobs I had were too big and looked silly since there were numbers painted on the faceplate on this amp, unlike an actual Fender.

In the end I found I had enough of these guitar pedal knobs to put on the amp and I was happy with the way that they looked.

All That was left to do was to check the bias on this amp. There is a bias potentiometer hidden on the inside of the amp so this was pretty straight forward. I just checked the plate voltages of my power tubes and set the knob accordingly.

For a free amp I really can’t complain. In fact even if it wasn’t free I wouldn’t complain. It sounds like classic Fender clean tone, and I am sure that the massive vintage transformers help it to really sing.

I am calling this amp done for now but I need to decide if I am going to put it back into the cabinet that it came in and get a speaker for it, or if I am going to build an enclosure to turn this amp into a separate head.

Certainly turning it into a head would be a more interesting project but we will see. Let me know which one of those options sounds better to you.

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