[Synchrodyne]

Across a broad range of valve-era products from the setmaker to the hi-fi level, it seems that taking the main feedback loop from the OPT secondary winding, rather than from a separate (tertiary) winding, was the modal choice. Possibly the transformer design difficulties along that vector were perceived to be more easily managed than those arising from the separate winding vector. Or perhaps the transformer manufacturing industry had effectively made that decision. I recall that one argument mustered in favour of taking the feedback from the secondary was that it was the actual output to the speaker that was being sampled, not a once-removed facsimile. But I have not seen any quantification of the in-practice difference.

Given that, then the use of the OPT secondary as the source of cathode feedback was a oui-sequitur, so as to speak. As already said, in the Quad case, the need for a push-pull feedback source for the cathodes pretty much dictated that a separate winding be used. Maybe a biphase secondary could have been used, with the speaker fed from one side and the cathode feedback from both, but that would have been lopsided, with different currents in each leg. Quad also used global feedback from the secondary, so the cathode feedback was another loop, internal to the global loop.


Cheers,