[Radioroslyn]

There's one other pair of wires on the opt the yellow and green ones. These form a negative back circuit (sometimes called a loop) in the audio to boost the low frequencies at low volume levels. This one is kind of an odd way to do it as most won't have a separate winding on the opt for it. It's more common to see a tap in the opt 2dy or cap & resistor from the plate of the output tube back to the preamp stages. Or to a tap in the volume control.

A lot going on in that little puppy...

GL