What does this diode do ? (EKO Multitone Repeat Percussion)

[Vivek]

What does this diode on the Collector do ? (EKO Multitone Repeat Percussion)

[antonis]

Sorry but I can't discern nothing more than a UJT..

[r080]

It looks like it is bringing the voltage down a diode drop for biasing, for some reason.

I found another schematic for the EKO repeat percussion on the other forum that had the diode, but made no sense at all. I also found schematics for the Vox repeat percussion, some with the diode, some with a resistor.

[Rob Strand]

The tremollo is done by the PNP so the diode is probably unrelated.

Perhaps they are trying to linearize the amplifier (the npn transistor).  Do a spice sim of the amp only.

Can's see the resistor value on the far right, it may have an impact on the final behaviour.

[anotherjim]

The modulator appears to be a reverse-beta NPN acting as a VCR on the signal feed to the NPN output amp.
As the output amp bias is fed back from the collector, the diode might be to stop a strong signal from glitching it? Some organs, the RP only operated on one voice tab and could be before the voices tone filters, a fistful of notes could be very strong if that's the case.

[duck_arse]

is there a better, readable copy of the schematic available?

[Vivek]

EKO Multitone Repeat Percussion Schematic

[Rob Strand]

The tremolo is done by the PNP so the diode is probably unrelated.

On your new pic I can see there's no PNP.  The transistor I was referring to is the reverse  connected NPN.

Perhaps they are trying to linearize the amplifier (the npn transistor).  Do a spice sim of the amp only.

I had a look at this and to me it looks like it doesn't not help linearize the output.

So that leaves only speculative guesses,

- Some sort of bias tweak where the designer considered more symmetrical clipping.
   In this case I don't see the benefit as ultimately the diode reduces swing.
   By the time you factor in resistor tolerances and transistor Vbe and hFE tolerances
   the tweak would only make sense on the prototype.

- Some sort of tweak to the tone.   The designer thought it sounded better, probably when clipped.
   And for the reasons on the previous point, possibly not a reliably tweak in production.
   If under hard clipping it sounds better then that would make sense but you would need to test it.