This is 3 valve filters wired in parallel, 2 bandpass and one that is switchable from low-pass to high-pass. The Q is adjustable from subtle to oscillation.
The 3 filters can all be controlled using 1 master control knob. There are other adjustments to make the master control knob have a different effect on each filter. So when the master knob is turned the filters sweep in different directions and by different amounts.
The result is very rich complicated filtering. This is further enhanced by imperfections of the valve design. There is lots of soft distortion and cross-talk between the 3 filters. Because the cross-talk is in both directions, it results in chaotic behaviour. The chaos is nice and subtle when the Q is low, and quite extreme when all 3 filters are oscillating.