[MrFixr55]

Hi DConnant,

The reason why C127 is isolated from ground is because sets of this vintage did not have the center tap of the HV (B+) power supply grounded. Instead, they grounded the cathodes of things like the output tubes and developed a negative voltage for the grid bias. They developed the grid bias or "C-" by a voltage divider network. Between choke #122 (the speaker field coil) and R 121, there is 378Ohm between Ground (B-, C+) and the B Supply center tap (C-) Note that the can (- terminal for Cap #123 is also not grounded but is connected to the center tap of the HV supply like C127 is. The + terminal is connected to Chassis ground. Measure between the + and - terminal of this cap and you may see the missing voltage.

I looked at the Philco Service Info and the Nostalgia Air schematics fdor this set and am missing the voltage documents. However, reading the RCA Receiving Tube manual, Specs for a 6B4 are the same as for the 2A3 except for Filament voltage.
Voltage between plage and cathode is 300V max. Grid bias at this voltage should be 62V. So this is close to the 370V that you see between the terminals of C127. So it sounds like you are close, especially when you fire this thing up with the tubes and speaker in place.

Hope this helps.