[mikethedruid]

The field coil acts as a filter choke in the power supply circuit. One CAN substitute a resistor of suitable wattage and the same DC resistance as the field coil, however the power supply will not be as well filtered and one will get hum. Of course, if you change the speaker, you will need to remove the output transformer from the old speaker to use with the new one, or use a new output transformer with suitable impedance matching characteristics. If the field coil on your old speaker is OK, and the voice coil is OK, even if the speaker cone is badly damaged, it is better to re-cone the speaker than to substitute a new one. Unless the cone is badly damaged, they can often be patched using black RTV glue and pieces of paper towel.