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Fieldcoil Speaker Replacement Help


 
6/9/2005 10:32 AM
TL
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Fieldcoil Speaker Replacement Help
I need to replace the fieldcoild speaker in a couple of amps but need a little help getting started. Any assistance would be greatly appreciated.  
Thanks, TL
 
6/9/2005 12:49 PM
Chuck
TL,  
To replace field coil speakers you need to know if/what the field coil was doing in the amp. Sometimes the field coil is used as a power supply choke, sometimes a field coil speaker looks as though there is another transformer riding piggyback and it too could be a choke or even the output transformer.  
 
Replacing a field coil speaker is basically replacing the speaker with any permanent magnet speaker of the correct size/ohm load, and then replacing and retrofitting any any parts the field coil was doing the function of in the amp. There are some variations so I can't give instructions like "Always do this with a field coil speaker". But if you have a little amp experience it shouldn't be too dificult to get a handle on. And if you have questions that are more specific once you start, you can always ask then.  
 
Here's one answer in advance. In an amp where the field coil speaker was acting as a power supply choke, you can sometimes remove the field coil and core from the speaker and continue to use it as the choke. And in the case of the OPT attached to the speaker you can still do it that way but you must reroute the leads that ran to the field coil so that they now run to the new choke. You need to be sure that the only leads connected to the new permanent magnet speaker when you are done are the OPT output leads.  
 
Chuck
 
 
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6/9/2005 11:03 PM
TL
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There are four wires coming from the the amp. Two go to a transformer mounted on the speaker whiuch I assume I need to mount on the new speaker and two wires go to the magnet and from the magnet to the speaker. What do I need to do with these wires?  
TL
 
6/10/2005 5:30 AM
Wild Bill
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TL, you need to give more info before anyone can answer your question.  
 
We need to know the impedance of the old speaker if you're going to use the old transformer. Some of those old field coil speakers had an oddball impedance. The transformer doesn't have to be mounted on the new speaker. 99% of the time it's mounted on the chassis but there may not be room. The wires to this transformer carry high (ouch!) voltage.  
 
If you have a multimeter you can measure the DC resistance of the speaker coil. This will NOT be the impedance but often it's enough of a hint we can take a guess. A 4 ohm speaker might measure 2-3 ohms, an 8 ohm speaker maybe 5-6 ohms and a 16 ohm speaker more than 8 but less than 16. This is because the DC resistance of the wire in the speaker coil is only part of the impedance.  
 
You can also measure the impedance ratio of the transformer but that's kinda "technical" - you can go to a site like R G Keen's Geofex and poke around for a FAQ on how to do this.  
 
The field coil can be wired in one of two ways and again you're going to have to find out a bit more info. A field coil can be wired in as either a choke or just a load hung from the high voltage rail. If it's a choke it will have a DC resistance of less than a few hundred ohms and if a load it will be many thousands of ohms. If you find this out you can post back and get more advice.  
 
The simplest solution is to take the coil off the old speaker (assuming the old speaker is shot and you don't want it anymore!) and leave it connected to its old wires.  
 
Sorry to answer your questions with more questions but like most things in life it's "just not that simple"! However, after you've learned how to handle this problem you'll be well on the way to becoming a techie yourself! :)  
 
---Wild Bill
 
6/10/2005 10:11 AM
TL
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Fieldcoil Speaker Recone
Would it just be easier to have the speaker reconed and if so who does them?  
Thanks, TL
 
6/10/2005 11:44 AM
Wild Bill
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Field coil speakers were all the rage until about 60 years or so ago when they learned how to make strong enough permanent magnets.  
 
That being the case I would assume it would be kinda hard for a re-coner to get the proper replacement cone for one of those old models.  
 
I'm having enough trouble finding a local re-coner for modern speakers!  
 
I'm not saying you won't find one but I'm sure glad it's you looking and not me!  
 
---Wild Bill
 
6/13/2005 6:05 PM
John Culp
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"Field coil speakers were all the rage until about 60 years or so ago when they learned how to make strong enough permanent magnets."  
 
Hammond was putting 'em in organs till at least 1959.  
 
Those speakers are still easy to find (from Hammonds that have been scrapped), but I'll bet it'd be a bitch to get one reconed!  
 
Pretty unlikely that a Hammond field coil speaker would swap directly for the ones in question in this thread, too.
 
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