Disclaimer: Tube amps contain high voltages that can cause injury or death. Do not attempt this project unless you fully understand the risks that come with it, and you have a working knowledge of how tubes work. Thank you, and be safe!
Anyway, on to the article:
Recently I've been working on an all tube guitar amplifier that combines some features of the Fender 5F1, 5F2, and the Fender Twin. It's 5 watts, and uses a 12ax7 in the preamp, a 6V6GT in the power amp, and a 5y3 rectifier tube. I designed the turret board in Autodesk Inventor, and I built it by hand, mostly using parts from Tube Depot. I designed the chassis myself as well (also in Inventor), and while I didn't bend it myself, I did drill all the holes by hand.
It's controls are Volume, Tone, Presence (ala Fender Twin), and a switchable negative feedback circuit, which lets you select between a 22k feedback resistor (More negative feedback, less distortion) and a 57k feedback resistor (Less negative feedback, and more gain). You can definitely hear the difference in tone between these two resistors. It's more pronounced at higher volumes though. This goes for the presence control as well. It seems to do more when the volume is set to 10, because it's a part of the negative feedback circuit. Using a 0.1uf capacitor, it acts like a tone control for the feedback.
The amp's output transformer is from an old 6V6 Hi-Fi amp from the 70s sometime. It's one of two identical ones that I salvaged from the amp. Even though the amp I built is single ended (meaning that it uses only one power tube as opposed to 2 or 4) and the output transformer is for a push pull amp, the sound doesn't seem to be affected. I just used the full primary winding between the 6V6 and B+, and taped off the center tap. This output transformer is rated for 30 watts.
The first time I tried to power up the amp, it seemed to work. There were some unwanted noises and hiss, but it sounded OK for a first try. Then the power transformer blew up... Great! Oh well, it was over 40 years old, so that was to be expected. A few days and a piece of aluminum later, I put in a newer (and much smaller, which is good for weight) power transformer. It has a lower secondary voltage than the last one by about 130vac, but it doesn't seem to matter too much. With this new transformer, the amp sounds great, and has been working for a few weeks now. The B+ voltage off the transformer is 563vac. It has great clean sounds for jazz, and an overdriven sound that reminds me of classic rock from the 60s and 70s.
Here are some pictures that I took while building the amp:
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| All the parts before I started building. |
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| The top of the chassis. |
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| The back of the chassis. From left to right, the holes are for the 5y3, the 6V6, the output jack, and the 12ax7. |
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| The output transformer. I got two of these from an old Hi-Fi amp. |
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| Front panel controls and home-made turret board installed. |
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| The turret board rests on stand-offs so it won't short with the chassis. |
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| The copper faceplate was left out of the final amp because the clear finish I put on didn't stick well. |
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| The power transformer. A Triode Electronics model. My guess is that it dates back to the 50s. It ended up failing. |
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| Wiring up the rectifier tube. |
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| Wiring the presence control and the tone control. |
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| Wiring the 12ax7 socket. |
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| Almost done, all we need now is a new power transformer. Luckily, I had one. |
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| The amp, powered up and ready to play. In this picture you can see the replacement power transformer. |
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| The rectifier tube. This one is RCA branded, and says it was "made in the U.S.S.R". |
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| The 6V6 power tube, |
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and the 12AX7.
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