10-28-2020, 11:57 AM
For the radio in question:
1-If the field coil is shunt connected across the "B" power source, positive to negative then it is not part of the filter network for that source.
2-If the field is series connected in the positive of the source with one or more filter caps on either side the resistance is important to the supply and the inductance part of the filter network.
3-If the field coil is in the negative of the positive supply with a center-tap transformer the field is part of the resistance required for developing negative bias.
The these solution could be applied in most instances...
#1 Replace the speaker with a PM type if the resultant "B" voltage is too high add a sutable shunt resistance, but it does not have to be as low as the old field coil.
#2 Replace the field with a similar value resistor, it will usually be 10 watts. Increase the value of the last filter cap by 50%. Do not increase the value of the first cap as that will often incense the B+ and put a strain on the rectifier.
#3 This one is a bit tricky as there can be filter caps that reference either side of the filter but these are primarily to insure there is no hum in the bias, some sets also obtain a reference AVC level there too. For most receivers the field in this location can be replace by a resistor. If there are bias resistor shunting the filed or the field has a center tap resistors can be adjusted to provide the same bias. Be aware all current for the "B" system returns via these resistors so appropriate wattage's must be used...
For auto radios with the 6-volt field these can be replaced outright with a PM speaker, YMMV as there may be exceptions.
It is nice to have a PM speaker solution IMHO it does require these carefully made changes I feel that it is better to re-cone or rewind the fields than to do all the labor and sometimes marginal results from extensive electrical mods. YMMV... chas
1-If the field coil is shunt connected across the "B" power source, positive to negative then it is not part of the filter network for that source.
2-If the field is series connected in the positive of the source with one or more filter caps on either side the resistance is important to the supply and the inductance part of the filter network.
3-If the field coil is in the negative of the positive supply with a center-tap transformer the field is part of the resistance required for developing negative bias.
The these solution could be applied in most instances...
#1 Replace the speaker with a PM type if the resultant "B" voltage is too high add a sutable shunt resistance, but it does not have to be as low as the old field coil.
#2 Replace the field with a similar value resistor, it will usually be 10 watts. Increase the value of the last filter cap by 50%. Do not increase the value of the first cap as that will often incense the B+ and put a strain on the rectifier.
#3 This one is a bit tricky as there can be filter caps that reference either side of the filter but these are primarily to insure there is no hum in the bias, some sets also obtain a reference AVC level there too. For most receivers the field in this location can be replace by a resistor. If there are bias resistor shunting the filed or the field has a center tap resistors can be adjusted to provide the same bias. Be aware all current for the "B" system returns via these resistors so appropriate wattage's must be used...
For auto radios with the 6-volt field these can be replaced outright with a PM speaker, YMMV as there may be exceptions.
It is nice to have a PM speaker solution IMHO it does require these carefully made changes I feel that it is better to re-cone or rewind the fields than to do all the labor and sometimes marginal results from extensive electrical mods. YMMV... chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
