:) great pedal grate tutorial thanks ! Now it is time to expariment. I am useing germ diodes now but will try leds next. Also I want to bump up the gain . My guess is that the 1M resistor is the component I want to change maby sub in a 820 K or so ?
cool. i've been out of town and so could not answer until now.
the amplifier in this circuit is the probably most common setup of an op amp and it's called "divided negative feedback." you can see one explanation of it on all about circuits, volume 3, chapter 8. here's the link: divided feedback (http://www.allaboutcircuits.com/vol_3/chpt_8/5.html). or you can look at this wikipedia entry for op amps: Basic non-inverting amplifier circuit (http://en.wikipedia.org/wiki/Operational_amplifier#Basic_non-inverting_amplifier_circuit).
the bottom line is this: increasing the 1M resistor (R5) or decreasing the 4.7K resistor (R4) will give you more gain.
there are limits, of course. you can't get anymore gain than the 9V battery will allow. the largest amplitude that you can get in this circuit is almost 4.5V. if you try to go beyond that, you will get clipping. that is, the audio signal will have its peaks chopped at 9V and its troughs cut off at 0V.
and this circuit reaches those limits within the range of the gain pot as it is already set up. that's why the circuit has overdrive/distortion. the characteristic thing about the DOD Overdrive 250 and its brothers and sisters is that it has op amp distortion.
so what you will actually achieve with those resistor changes is more like making the op amp distortion happen earlier in the rotation of the pot. at the highest gain setting, you will be just a bit closer to a square wave output.
so, have you tried any resistor changes? what did you find?