After having the Marshall JCM mod for a year or so and mainly playing around with C-3 on the circuit board (the capacitor on C-3 determines the amount of bass that is send into the gain stage) i remembered how crunchy the modern distortion mod sounded and wanted a similar tone but i liked the snap & slight buzziness of the JCM Mod.
Reading up on LED's as clipping diodes and testing a few out i ended up using a red square LED. I tried 3mm, 5mm of various colors, even blue LED's and the best sound to my ears was using a red square LED. This LED has a lower forward voltage than the regular run of the mill 3mm LED's so it would clip a bit earlier, after trying out an LED in D5 & D4 and various diodes i ended up using the JCM mod & the Modern Mod together.
So along with the Marshall JCM mod (click here for my previous post with the list of changes) i ended up using the following clipping stage;
-D-4 = 1N4148 Connected in Series to another 1N4148
-D-5 = Square RED LED
This type of clipping with two diode in series & one diode on its own is called asymmetrical clipping, its where the wave form is clipped harder on one side and has more of a tube like sound. The new clipping has changed the tone a lot, it still has the JCM mod character but sounds more crunchy with a little less distortion, slight volume boost & a touch more bottom end.
If you want to know more about different types of clipping click here:
The pedal sounds great even though i wish it had a little more bottom end, I'm thinking of looking into the tone stack parts of the circuit and using the Duncan tone-stack calculator to get some more lows, however, its now more suited to my needs than the standard JCM Mod and i like it.
Below is a Pic of the clipping diodes with the Square LED.
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