I wonder how many turns the PR80 OT must have.

Good question. Lots of formulas on the internet, but little practical information on output transformer design for transistor amps.

I found some info in an old Telefunken handbook. For a design using AC106 output transistors (a predecessor to the AC128), and a 5 ohm load, an EI42 transformer core is used. The primary has 168 windings of 0.32mm wire center tapped, the secondary 76 windings of 0.55mm wire.

The previous is for an OT with the speaker on secondary. But the PR80 has the speaker connected to taps on the primary, and the secondary is just for feedback. So I assume the secondary can be made from thin wire, there is hardly any current flowing, and then you can make the primary wire thicker. I suggest 180 windings ct with 0.5mm wire for the primary. The DC resistance of the PR80 primary winding is 2-2.5-ct-2.5-2 , assuming the same wire for the whole primary this tells us where the taps should be: 40-50-ct-50-40.

For the secondary, the impedance ratio is 140:35, or 4:1, so the turns ratio is 2:1. So I suggest 180/2= 90 windings of 0.1mm wire.

To be clear, I have not tried this, just a guess as to what might work.