# Field Coil to Modern Speaker Conversion



## Tone Chaser (Mar 2, 2014)

I did my search several years ago, and have forgotten what I read due to age and unfamiliarity.

I don’t know how to determine what ohm speaker is required; to many wires to choose from.

I thought about finding a good used field coil speaker, or recone; but being able to try modern speakers could open up a world of possibilities. I have been way to busy with things that life has been throwing at me, and the amp has been in storage. I am losing my storage, and it has to come home, or be sold off.

The amp is my old, 1946 Sound Craft. I had it made safe and quiet. It is kind of an original 5e3 circuit. It has been 3 years since I looked at it. It was fun when it was a noisy, nasty, possibility dangerous amp to play. 

I had the schematics posted several year back.

Any guidance is appreciated.


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## nonreverb (Sep 19, 2006)

The conversion is fairly straight forward as long as you underdtasnd what's going on in a field coil amp. A question first. Do you have the original speaker? if not, you can probably quess that it's going to be either an 8 or 16 ohm speaker. If you still have the schematics, they should tell you the speaker impedance.....Now onto the more important piece of information. Pretty much all those old amps used the speaker electro magnet as a choke in the power supply section of the amp. What needs to be done is either install a 1Kohm resistor (average value) where the wires were that went to the coil. This will bring down the B+ voltage at or near normal operating values. It will need to be around 10 watts, maybe more. The other option, if you want to spend more money, is install an actual choke coil.


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## mhammer (Nov 30, 2007)

Richard has good advice from the technical perspective. I'll simply supplement it at the conceptual level by noting that the basic difference between "normal" and field coil speakers is that normal ones have a permanent magnet of some category (ceramic, neodymium, alnico), while field coil speakers have to _create_ a magnet when you turn them on; the term "field coil" referring to the additional coil used to create the magnetic field.

Apologies if this aims too low, but there are a LOT of folks unfamiliar with the technology that preceded their birth...or that of their parents.


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