I have a PYE 52D. The output section is single ended 6AQ5 and the output transformer has an extra winding on top. It is asymmetric, such that the plate load section is ~340 ohms and the top section to tap is 8 ohms. It appears to create an RLC filter in my mind, but benefiting only the previous stages. Its been mentioned to me that this is to alleviate conditions contributing to motor-boating on the basis that the magnetically linked coils will provide some feedback. I am too many years separated from ‘theory’ to describe what this approach accomplishes.
I though I’d post here to see if anyone is familiar with the design, and could comment.