This is very nice and simple mod that will transform your L100 into real beast. It's called Kon's Zener Overdrive and was made by Kon Zissis. For L100 Zener Overdrive consists of one capacitor and one diode only (plus a switch and a potentiometer for controlling the amount of overdrive).
Unscrew and tilt the power amp on the side. There are two 6BQ5 valves (marked as V9 and V10 on the schematic).
Solder two wires to pins number 2 on both of them. You may need to use flashlight to see pin numbers. For really simple overdrive this is what you need to do:
Suggested values are: 100k audio potentiometer, 3.3V Zener Diode, 2.2uF capacitor and any SPST switch. It is very simple Zener Overdrive circuit. You can have more overdrive with two diodes put in parallel in opposite directions. You can try different values (especially the diode) to get different overdrive.
Because I modified my L122 for vibrato, the audio signal going through my preamp is slightly weaker than previously. Therefore to be able to get some distortion I had to add another diode in parallel. I also added two 0.1uF capacitors (and two additional switches) for different tone of the overdrive.
This is my final circuit:
C2 reduces treble response of the overdrive which gives really sweet and dark sound (especially when using Leslie) and can be disconnected by a switch (SPDT3).
C3 reduces distortion in low frequencies (bass) and can be bypassed by using a switch (SPDT2).
Because I donít use Volume Soft/Normal at all, I decided to convert it to a switch that would compensate the volume loss when the signal is distorted (as this is passive overdrive).
I replaced C401 capacitor with two 270k resistors in parallel. This way I have nice soft volume drop that helps me to keep the same volume level for clean and distorted signal. Of course only one switch (overdrive or volume) must be in ON position at a time.