My First Modded Guitar Effect - A Tube Overdrive
Last updated 3/30/04
After a few unsuccesful attempts at designing my own distortion pedal, I finally came up with something that is worth sharing, a tube overdrive. I initially wanted to use some transformers that I had lying around, so I was going to design a pedal that put 50-100 volts on the plates of a 12AX7 preamp tube. But, then I changed my mind, and kind of did a hybrid based on numerous schematics I looked at for preamp tube based distortion pedals and came up with something that intrigued me and sounded pretty good to my ears. I decided to call it the PJM Tube Overdrive, after my initials.
I looked at schematics for the Chandler Tube Driver, Real McTube II, Ruby Tuby, Matchless Hot Box, Aron Nelson's Shaka Tube and preamp module schematics for my Seymour Duncan Convertible to get some ideas. Since I already built the Shaka Tube and was happy with it and familiar with it, I thought it might be fun to try and make a sort of hot-rodded version of it. The power supply scheme is basically the same as the Shaka Tube, as well as the basic format. But, instead of the opamp at the front end of the circuit, I wanted to use an FET. I ended up using a J201 which gives me an asymmetrical waveform going into the tube section. The tube section is a rather basic cascaded triode circuit with bypass caps on the cathodes to increase the gain a little bit. I liked the sound of what I had so far, but the output was very low, lower than the bypassed guitar signal. I was perplexed for a little while on how to increase the output level, but then I stumbled onto the Ruby Tuby schematic which has a LM386 audio amplifer IC chip on the output of the circuit. So that gave me an idea on how to boost the output. I tried the amplifier that has a gain of fifty right off of the LM386 data sheet, and with a few slight modifications it solved my problem. I used a trimpot at the input of the LM386 to control the amount of signal going to it so that it could have some flexibility with low mu preamp tubes, such as a 12AU7 or a 12AT7, and keep the output level reasonably high. I haven't tried this yet, but it would probably sound cool with a 12AU7.
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Here is a view of the inside. I built this one on perfboard. It is laid out a little more efficiently than my Bulldog (Shaka Tube) is. |
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Here it is a view of the tube socket mounting. I had this left over from my Firefly tube amp build, so I used it for this project. |
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Here is a view of the exterior. It didn't take that long to drill all of those holes... |
Overall, I am pretty happy with the results. This circuit seems to be fairly versatile. It can go from mild overdrive to pretty heavy sounding, cleans up pretty well when you roll back the volume knob of your guitar, and the tone control has a pretty big effect on the sound. I also added a contour control of sorts at the input, which is essentially a 100K variable resistor in series with the input. This affects the high frequency content of the incoming signal. It's more of a subtle control, but it does have a noticeable effect.