This video explores the B&W Model 350 2Q4 Audio PSN that will eventually be installed in a homebrew Phasing SSB Transmitter. This PSN is over 70 years old --so keep that in mind.
see: http://www.n6qwradiogenius.us/Phasing_SSB.html
I have simulated the circuit in LT Spice and the actual circuit seems to work OK midrange in providing a 90 Degree shift (Lissajous Figure) but beyond 2kHz folds. So the good news ---it works but the not so good news needs a bit of help at the higher frequencies. This is just a starting place.
Pete N6QW
