[morzh]

Using a PM speaker means that you must come up with a way to replace that 660 ohm field coil. Sure, a resistor can do the job...but it will also create lots of heat. The 38-5 uses eight tubes and that means a sizeable amount of current flowing through the speaker field (or resistor) which translates to plenty of heat dissipation if a resistor is used.

Ron

A little.... disagreement. The electrical truth is, the 660 Ohm choke (and this is a DC impedance) or field coil will dissipate exact same amount of heat as the 660 Ohm resistor that would replace it.
Sure the choke filters better as it is inductive and the resistance is a necessary evil.

Another thing is if one uses higher value resistor, that would filter better but dissipate more heat plus drop more voltage thus screwing B+ voltage value.

So, if I were to use a resistor, I would simply use the same DC resistance of 660 Ohm, but increase the output capacitor (after the resistor, not the one before) several times.