As in to get a cleaner sound from it?
OK, so what I did to get a cleaner sound was to remove the clipping diodes D7 and D8 and the feedback resistor R21 from the 4558 chip. Do I need to bridge the holes left by the diodes? Cos I did bridge the feedback loop. No there's some weird background noise(mouthy envelope like) when I let notes ring out.
There is some inherent grit in the Rustbucket but it's more prominent with the edge switch on (2nd half of the video).
This is what it should sound like. If this is not the result you are getting there might be something off in your build - but I wouldn't go removing parts. Instead, I would leave R27 and the diodes in and experiment with higher values of R23 to see if it is more to your liking.
I did get it to sound almost clean by removing said parts, the idea was to turn the 4558 into a buffer. I played through an actual amp and the envelope background noise was almost non existent(before I played with a headphone amp, which I suppose compresses the hell out of the sound). I suppose that leaving R23 in the negative feedback loop causes the issues. I've checked schematics for TL072 based buffers and the negative feedback loop in connected without any components in it. However a 4558 buffer has a cap and a resistor in it.