[Jamie]

I think I solved the problem. If you look at resistor #55 on the schematic, it connects to E-cap #67 and then also, to resistor #65. It was not connected to either. As soon as I connected it, the hum vanished. I could see no evidence that it was ever connected. I know Philco frequently made changes to schematics and parts, so I was a little concerned that maybe this was a late change, but voltages are good and hum is gone, so I think the problem is solved.

Edited to add - It was definetley speaker hum. No doubt about it. I also switched the hum bucker back. Hum was very pronounced. It wasn't so bad that you couldn't listen to the radio, but it would eventually drive you crazy. The hum was constant, no change with volume control or tuning. If you numbered the volume control on a normal radio, from 1-10... with 10 being loudest and 1 being the lowest volume you can hear from a 10 foot distance, the hum was a level 3.