There is a very smooth valve distortion if you turn the pre up (marked gain), light up the clip light, and turn the post (marked volume) down. There is also a master volume.
Having the gain at 9 o'clock and the volume at 3 o'clock gives a nice clean sound. Having the gain at 3 o'clock and the volume at 9 o'clock gives a good tube distortion. Having both at twelve o'clock is nice and warm.