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Field Coil Speakers

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5/30/1999 11:39 PM

Field Coil Speakers

I have an old amp with a field coil speaker (no magnet, but the power supply choke is a winding on the speaker to produce magnetic field). I like the sound of the amp a lot -- very early Freddie King, but I'm a bit curious what the amp would sound like through a P10R or something similar (like a Weber). Any thoughts on that? The technical issue would be how to match the impedance to the amp. Is there any half-way easy way to determine the impedance of a speaker?

THanks in advance for any suggestions,
R L

5/31/1999 3:00 AM

You check the impedance the same way you check it on any speaker...use an ohmmeter across the voice coil.(Remember, it's got *two* coils, the field coil and the voice coil) A 4 ohm or 3.2 ohm speaker will be like 2-3 ohms, 8 ohm usually between 5-7 ohms, etc.

If this is being used as a PS choke, to replace it, you replace the field coil with a resistor of the same DC resistance (you'll have to figure the wattage...just measure the voltage drop across the field coil & use Ohms Law...it'll probably be pretty big, limke 10 or 20W).If you get more hum in the output, you'll have to increase the value of the filter capacitors to get rid of it...occasionally this is necessary, as field-coils are better at hum rejection than resistors are.It'll get *HOT*, so make sure it's not near anything burnable or meltable.

If you want to use an external speaker, this is much easier..leave the field coil hooked up, disconnect the voice coil and run the leads to the external speaker.

I'm assuming the field coil really is a choke in the power supply, and not a bleeder across it. If that's what it is, you might be able to ditch adding the resistor if taking the load
off the power supply doesn't make the B+ voltage jump too much.

Many amps this old don't have fuses...install an inline one in there if it doesn't. Could save you a lot of grief if anything goes wrong. One amp fire can ruin your whole day.

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