PS... go to the local electronics shop and buy four or five RANDOM diodes. Zener diodes, silicons, germaniums... EVEN LED's! Anything!
Then try those diodes in the holes... they will all do basically the same thing but have different characters to the sound... often the more mismatched the diodes are, the more tube-like the sound. LEDs actually sound quite good in there!
Using a diode identical to the other two will give the most extreme effect... in fact, very high up on the neck around the 12th fret and above, you will almost start to barely hear an almost audible octave up effect! Just like a ghost of one...
Identical diode also makes the notes up there have more 'weight' to them... it seems like they punch harder but a little later... you'd have to experience it yourself to know what i'm talking about.