Almost there

[notneb]

O.K. what did I do? When I first built this: http://beavisaudio.com/bboard/projects/bbp_DOD250Overdrive_Rev_1_1.pdf I swapped red LEDs for the 4148's and loved it. The 4148s were more crisp than the LEDs (which I liked) but with the LEDs I could roll the volume off a tad on my guitar and it would clean right up. Dig in a bit and the drive came back. I rebuilt it and THINKING everything is the same, the LEDs produce a nasty buzzv distortion when the string is first hit. It smooths off but the attack sounds very jagged. Leave everything the same and swap the 4148s back in and it goes away. But with the 4148s I don't get the same cleanup with the volume knob. Very smooth but not as much control. I'm afraid when I first built it I had something in there that wasn't on the schematic, a happy accident, that I now can't reproduce.

[teemuk]

LEDs have higher forward voltage than silicon diodes and "harder" characteristic curve.

Did you try to compensate any of this? (e.g. matching forward voltages of silicon diode strings to that of a LED, inserting series resistors to alter forward current of diodes).

[deafbutpicky]

a guess: did you put the leds back to back?

[Quackzed]

thats my guess too. if you had both leds accidentally in the same orientation / not back to back you might get odd misbiased sounds, i bet the first time they were back to back and the second time they were not and sounded different/bad...

[PBE6]

If you like the way the 4148s distort but want more headroom, add a 1k pot in series with the diodes connected to ground (both the forward and backwards diodes should be connected to the same lug on the pot). The extra resistance reduces the current through the diode (given the same voltage) and increases the voltage required to turn it on. This increases headroom and will allow you to roll back your volume knob to clean it up.

Alternatively, you can put multiple diodes in series to get the proper headroom as noted by teemuk.