[morzh]

Well...

1. They put two windings of the same number of turns, both in the plate and in the cathode of every output tube. This makes the tube a partial cathode follower (катодньiй повторитель).
This cuts the output impedance 4 times.
Here's why: because the two windings are in parallel AC-wise (they contribute to the same part of the magnetic flux), they double the current. Meaning they only need 1/2 the voltage.
1/2 the voltage means 1/2 Number of turns.
1/2 number of turns means 1/4 of the impedance.

2. The windings are wound bi-filar , and it is for both tubes (they actually call it tri-filar due to the third one for Screen Grids). This eliminate leakage inductance and stray capacitance. They turned a stray (leakage) inductance of about 10mH into 0.5uH.
Also the third winding that moves the SG together with Cathode "cheats" the tube into thinking that the SG vs Cathode is not moving.

3. The windings in P and K introduce 100% feedback, hence the gain of "1."

4. Shared magnetic coupling between the two tubes results in the tubes never "stop conducting", so the "notch" (crossover, "ступенька") do not occur.
To be more specific, the tubes DO stop conducting, but because they share the magnetic flux, which is in this transformer never stop / falls to zero, so when one tube is fully open, the flux simply "jumps" to the other tube, so it is never interrupted, as it would be in a regular push-pull transformer.
Also the fact that all windings share the magnetic flux (because the plate winding from end-to-end (P-P is one winding,, after all)) by being wound bi-filar, this what helps this crossover elimination.



https://www.youtube.com/watch?v=W58Rqx_4u2w

This is Gordon Gow explain his Unity Coupled Transformer.

And, again, the 100% feedback smooths what otherwise would be a hard tube shutdown into a soft transition.