[Arran]

Greg;
I will have to look at the full diagram for a model 20 again, but in most tube radios the primary of the output transformer is connected to the B+ ahead of the rest of the radio since the audio section consumes the most current, and requires the most voltage. Pin "D" connects to the output filter cap in part #26, which is in a metal box, and #27 which is a filter choke. If would have been nice had they illustrated what goes to which pin of the speaker plug socket the way Mr. Fix shows. It's also NOT a smart design since the input filter cap of the power supply can still be energized with the speaker unplugged, leaving the B+ supply with almost no load, a lot of sets have a jumper on the speaker plug that disconnects the cathode of the rectifier tube from the input filter cap. That being said the model 20 uses a pair of #71As so the B+ is going to be well under 200 vdc, so it may only reach 250 vdc without a load, but Philco designed a lot of sets this way right into the mid 30s with much higher voltages involved, this may explain why so many of their sets had hard wired speakers.

https://philcoradio.com/library/download...l.%201.pdf

Regards
Arran