[Mondial]

AVC voltages seem a bit low, but it really depends on how strong the signals are in your area. Is the voltage about the same at the detector output as it is at the bottom ends of the ant and RF coils (after the filtering resistor)? Sometimes you can have a gassy tube which will put a positive voltage on its grid although the AVC voltage at the detector will still be negative.

I would try swapping the 6A7 and 78 with known good tubes and see if it makes a difference.