Hallicrafters S120 hum at no vol and hum with volume

[Electronic M]

I have seen 2-3 AA5 sets with field coil electrodynamic speakers, not common, but some did it.

What matters with the output grid is that it be negative with respect to the cathode. One thing you can do is connect a large cap (.22-10uF) between the output grid and signal ground to bypass any hum from proceeding stages to ground, then put the meter common lead on the grid and the hot lead on the cathode...That should give you a positive voltage indicating the cathode is positive WRT the grid and that the grid is negative WRT cathode.