DIY Tube Overdrive

One post that really intrigued me and got me thinking about designing my own overdrive effects pedal was Dave Mac's version of the Matsumin Valve Caster. The Valve Caster is a simple circuit built around a 12AU7 tube running at 9 volts. The 9 volts feed the tube's heater and also provide the tube's plate (anode) voltage.

At 9 volts, the tube operates way below its intended operating voltage. Most designs use plate voltages of between 100 and 300 volts. I was skeptical that at 9 volts, the tube would sound and perform the way I had in mind. I decided to use a higher plate voltage. I also decided to use a higher mu (amplification factor) tube.

I finally picked the 12AT7 tube for my design. In terms of mu it lies between the 12AU7 and the 12AX7. The 12AX7 is a very popular tube used in the pre-amp stages of many great guitar amps. The main reason I chose the 12AT7 over the 12AX7 was its higher idle current - something I considered an advantage in a low voltage design.

So the heart of my overdrive circuit would be a 12AT7 tube running at more than 9 volts. However, I wanted my pedal to be able to run off the standard 9 volts of power supply most pedals use. As a result, I needed an internal DC/DC voltage converter in my design. I also decided to run a pre-amp stage in front of the tube with adjustable output in order to vary the level of overdrive the tube would generate. A simple EQ would be nice too I thought, to independently shape the lows and highs going into the tube stage.

The following diagram shows the main stages of what I had in mind for my circuit.

Overall block diagram - the guitar tone going into a simple EQ, then into a booster (with adjustable output) and finally into a tube amplification stage producing (hopefully) a nice over-driven guitar tone.

The challenge now was to turn this conceptual design into a working electronic circuit! Quite a challenge considering that my knowledge of electronics and amplifier design circuits was (and still is) very limited. I needed to do some serious research!