[Arran]

That would be on the grid of the 50A5 and the plate of the 14B6. I said 7C6 as the first audio as most Philco AC/DCs use the first generation Loctal tubes with 6.3 volt 150 ma filaments. A 14B6 is a second generation Loctal type, I'm not sure when they came out but they use a 12.6 volt 150 ma filament, like a 12S#7 type, so I think they were a post war development. It's possible that one or more of the tube may be weak, but check the plate voltage on the 14B6, and check the plate and screen grid voltages on the 50A5, if they are low then the rectifier tube may be bad, if everything is wired correctly. If they are normal according to the schematic then either the 50A5 or the 14B6 are weak, again if you didn't make any wiring mistakes like connecting a cap or a resistor to the wrong tube pins.
Regards
Arran