Here is my modified version of the ToneBender MKII Professional, the MindBender. I decided to use NPN Germanium transistors for mine so that I'll be able to use a negative ground arrangement, which allows use of modern power supplies. You can use 2N388A, SK3861, AC127, and a few other part numbers. I made a few more changes to the circuit to enhance it's overall performance:
- First off, I added a 1M pulldown resistor to the input of the circuit, which will prevent "pops" when using true bypass switching.
- The second change that I made was to replace the 100K resistor on the collector of Q2 with a 47K resistor. This helps tighten up the sound a bit, making it less "farty", and it helps to increase sustain.
- Third, I changed the 470-ohm resistor that's in series with the output capacitor with a 1K2 resistor. This helps to boost the output of the pedal so that I don't have to crank up the volume control so much.
- Fourth, I changed the 1KB "attack" pot to 2KB. This change doesn't really have any effect on the tone of the pedal, but it does help to prevent Q3 from becoming saturated.
- The Fifth change was to replace the 8K2 resistor on the emitter of Q3 with a 20K trimpot so that the collector voltage of Q3 can be adjusted to exactly 4.5V-5V.
- The sixth and final change that I made to the circuit is the addition of a 1N4001 reverse polarity protection diode. This isn't necessary but it helps to protect the circuit components in case the power supply is accidentally reversed or if the battery is touched to the battery snap backwards.