[Mondial]

Check the two 10 ohm filament shunt resistors (flex type). If one of these is open it will cause a hum. 

These two resistors set the electrical center of the 2.5V filament winding which is referenced to ground. They perform the same function as a center tap of the filament winding. This is important because the 47 output tube has a directly heated filament, so to cancel hum the two filament pins must be balanced to ground.