This is a pretty standard op amp type distortion. I tweaked the diodes a bit to allow a tiny bit more saturation current to soften the peaks of the sine wave properly. The potentior adjusts the overall gain. At the lower settings there is virtually no distortion on the signal. At > 50% the tops of the sine wave start to soften amd distort. I once read that in theory the 'warmest' distortion should have very soft peaks on the top of the sine wave. I am not sure if this design works but I built a very similar one a few years ago based on a FET based op amp and it worked very well.
