Thanx...that's what I did...and it works perfectly. Now I can experiment with any combos I want...I might even make another similar switch for the other diode (D4), now that I know the first one works. I might even do this with all of my distortion/overdrive pedals.
This is how it starts
A good trick is to buy sockets. You can get rows of socket pins on ebay for a couple bucks, clip off one with your wire cutters, and solder that into the board in place of one end of the diode. So you need 4 to replace two diodes, but then you can swap one diode after another.
1N914/1N4148, 1N60, 1N4001 and 2N7000 MOSFETs wired as diodes all sit +/-50 of 650 mV, and the 1N34a usually sits around 300-350 mV. LEDs will sit up around 1.6-1.8 V usually. What I find usually works best is either:
A) Mixing types at about the same voltage drop, so a 1N60 and a 1N914, or a 1N4001 and a 2N7000, etc. The 1N4001 doesn't sound as much like the 1N914 as most Si diodes do, it's slower acting and has similarities to Ge diodes. Combining a "fast" or "bright" sounding diode with a slow one results in one side of the wave being clipped differently from the other, and this can add pleasing complexities to the distortion.
B) Mixing voltage drop-offs so that one side is about double the other. So a 1N34a and a 1N914, or a 2N7000 and an LED. It'll seem to have additional harmonic content that way. I find the effect isn't very pronounced until you get to having about a 2:1 ratio between the two opposing diodes.
I've landed on a small handful of options that I like and try first when modding a pedal.