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. |
