[Mondial]

I think that the detector plate voltage measurement is inaccurate on the Philco schematic.

Keep in mind that the Philco tester was 1000 ohms per volt. So if they were measuring on the 100 V range and measured 40-60 V, the plate circuit was loaded by the meter's 100K resistance to ground, which formed a voltage divider with the high value plate resistor. So it was not the tube conducting current but the meter loading causing the low reading.

As you said, the detector tube should be biased to cutoff, so the true plate operating point should be very near supply voltage.