[morzh]

20 Ohm is too little; while not likely to damage anything, the filtering quality will be compromised as the RC time constant would be 25 times less so the hum might actually increase.

Here's what you could do: leave the field coil connected but only disconnect the speaker's voice coil from the output xfmr and connect a PM speaker to it.
Or: same about the field coil, but disconnect the primary of the output transformer and use another output transformer together with a PM speaker (if you have one from Zenith portables or whatever similar).

Either will do. The first one won;t require you to find another output transformer.

This way you do not need a resistor as the Field coil stays in place.