[Arran]

I don't recommend replacing the field coil in such a large set with a resistor network, the simple reason is current draw. A filter choke/field coil resists changes in voltage and blocks AC ripple but allows DC to pass, so it acts like a resistor to AC but more like a length of wire to DC. It is also much more efficient at eliminating ripple then a resistor, a 10 Henry choke is ten time more efficient at eliminating ripple then a 400 ohm resistor even if the DC resistance is the same. The problem with using a resistor is that the voltage drop is the same whether AC or DC passes across it, and the voltage drop increases with current. So if you want to replace the field coil with a PM speaker use an iron filter choke or a solid state substitute, it's a top end radio not a cheap AA5, on a 38-690 it's worth the effort.
Regards
Arran