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Thread: How do I replace field-coil speaker with magnet speaker?

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    How do I replace field-coil speaker with magnet speaker?

    Hey guys,
    I have an old Valco amp and the speaker is farting out, I can hear the voice-coil rubbing while I move the speaker.

    I'm wondering, what is the procedure to replace those old field-coil speakers with modern magnetic speakers?

    I'm under the impression that a choke should replace the field coil and then the speaker gets hooked up with the other two wires? What's the criteria for selecting the choke?

    Can I mount the choke onto the inside of the cabinet to avoid modifying the original chassis, or is it best to mount the choke to the chassis?

    Last question would be, what sonic differences would I expect if I'm replacing an old field-coil with a modern speaker? Were the field-coil speakers more/less efficient than today's speakers?

    I can't find the exact schematic right now for this amp, will post it if I can - or trace it out if I need to once I crack the amp open. Thanks guys!

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    Supporting Member The Dude's Avatar
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    Where is the coil now? In other words, does it attach to the speaker and is removable, or is it inside where it can't be gotten to?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Supporting Member Chuck H's Avatar
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    I think wasp is implying that the field coil is acting as the magnet for the speaker... I think.

    I can't say about the choke specs. It probably matters less than you think. The new speaker will almost surely be more efficient (good thing IMHE). This alone will make the amp sound and feel more lively. Any difference the choke specs make will be peanuts by comparison. If the choke is between the plates and screens I'd just pick any OEM Fender choke for a similar power amp. If the choke is feeding the plates as well then you'll need to know your amps whole current draw to source a part.

    And you've got it right as to how it hooks up. If you can accurately locate where all the leads go then the coil in series with the power supply is the choke and the other leads go to the permanent magnet speaker.
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    Supporting Member The Dude's Avatar
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    Right. The field coil does act as the magnet for the speaker. The replacement speaker will not need it to operate. The amp will. The reason I asked about the original coil is because you can sometimes remove it and mount it elsewhere saving the hassle and cost of a new coil or high wattage resistor.
    Last edited by The Dude; 01-08-2015 at 04:17 AM.
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    Supporting Member Chuck H's Avatar
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    +1 That would pretty much remove the need to select the right part

    EDIT: I've never done it myself. I'd expect you'd need to include anything acting like a "core" to preserve the inductance. I guess you pretty much just remove the speaker from the frame, cut off the cone and don't hook up the speaker coil leads? Might need some creative framing with aluminum plates or something but it seems like a good solution.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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    Supporting Member The Dude's Avatar
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    I haven't done it for quite some time. But I recall some speakers (Jensen maybe?) where you could remove the rear bell of the speaker and easily get to the field coil. The pole piece of the speaker would be the "core" I guess. The times I did this, I didn't (maybe wrongly) worry about it (the core) and everything worked just fine. I don't think it's that critical. I mounted the field coil with large washers and screws rather like they mount toroidal transformers in some amps these days.
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    Supporting Member The Dude's Avatar
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    It just occurred to me, you may want to check into what the speaker is worth first. For whatever reason, sometimes these old buggers bring some good money- even nonworking. If that's the case, just sell it as is and buy a coil.
    Another consideration would be to recone it.
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    Old Timer Leo_Gnardo's Avatar
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    You might get that FC speaker reconed. My local recone guy was able to do an excellent job an an early Jim Lansing (later known as JBL) in a 30's Rickenbacker amp. Ask your reconer or you can PM me and I'll direct you to mine in upstate NY.

    If you do swap in a PM speaker, and substitute a regular choke, be aware that the voltage drop across a field coil is typically a lot more than a standard choke, and you'll wind up with a noticeably higher DC voltage to run the amp. To compensate you will have to increase the value of the output tube cathode resistor to keep bias current within reasonable limits. Some say they hear a difference in tone with higher voltages, brighter & more "taut". Despite the chart-readers' claims that it's rubbish, I do hear that difference. Another way to diminish the B+ is the zener diode trick. You put a high wattage (5W should be sufficient) zener between center-tap of the hi voltage winding and ground. I've never used this technique but if you're interested just mention it & I'm sure one of our members more experienced with this technique will fill you in.

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    Quote Originally Posted by waspclothes View Post
    Hey guys,
    I have an old Valco amp and the speaker is farting out, I can hear the voice-coil rubbing while I move the speaker.

    I'm wondering, what is the procedure to replace those old field-coil speakers with modern magnetic speakers?

    I'm under the impression that a choke should replace the field coil and then the speaker gets hooked up with the other two wires? What's the criteria for selecting the choke?

    Can I mount the choke onto the inside of the cabinet to avoid modifying the original chassis, or is it best to mount the choke to the chassis?

    Last question would be, what sonic differences would I expect if I'm replacing an old field-coil with a modern speaker? Were the field-coil speakers more/less efficient than today's speakers?

    I can't find the exact schematic right now for this amp, will post it if I can - or trace it out if I need to once I crack the amp open. Thanks guys!
    Post up a picture of the speaker, and also measure the resistance of the field coil. As long as the field coil isn't burned up, the speaker can likely be reconed (contact Weber Speakers) and be good as new. I just reconed two field coil speakers, a Rola and Jensen F12N. They are high efficiency speakers because they have a small gap and a strong electromagnet which makes for a large BL product - high flux density in the gap makes for a strong speaker motor.
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    The most common version of Valco amp with a field coil speaker used a 10 inch Rola. The field coil was used as the choke for the power supply and was about 1K ohms in resistance.

    I've seen people use a separate choke and I've also seen a simple 1K power resistor used to replace the field coil. Normally the output transformer is riveted to the speaker basket, so if yours is mounted this way you will also need to remove it or replace it. Reconing is probably easier in some ways.

    Tell us what model amp you have or at least what tubes it uses.
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    Hey guys, thanks for all the replies so far.

    The amp is a "Bronson Electric Sound Amplifier" that says "Detroit 26, Michigan" on the front. It uses two 6V6's as the output.

    Here are some pictures of the amp and speaker:

    img_1042.jpg
    img_1037.jpg
    img_1045.jpg

    So the output transformer is also mounted onto the basket, that's pretty wild. It might be kinda neat to bolt the transformer (and choke?) onto the basket of the new permanent magnet speaker - any caveats on doing something like that? Just tossin' that out there.
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    Quote Originally Posted by Leo_Gnardo View Post
    You might get that FC speaker reconed. My local recone guy was able to do an excellent job an an early Jim Lansing (later known as JBL) in a 30's Rickenbacker amp. Ask your reconer or you can PM me and I'll direct you to mine in upstate NY.

    If you do swap in a PM speaker, and substitute a regular choke, be aware that the voltage drop across a field coil is typically a lot more than a standard choke, and you'll wind up with a noticeably higher DC voltage to run the amp. To compensate you will have to increase the value of the output tube cathode resistor to keep bias current within reasonable limits. Some say they hear a difference in tone with higher voltages, brighter & more "taut". Despite the chart-readers' claims that it's rubbish, I do hear that difference. Another way to diminish the B+ is the zener diode trick. You put a high wattage (5W should be sufficient) zener between center-tap of the hi voltage winding and ground. I've never used this technique but if you're interested just mention it & I'm sure one of our members more experienced with this technique will fill you in.
    Thanks Leo, reconing does seem like the easiest option. I don't know enough about reconing to know if voice coil problems are something that can be fixed as well? If you can PM me your recone person's info that'd be great, but I should probably try to find one in Ontario Canada to save on shipping if anyone knows one - or how I can find one.

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    Supporting Member TomCarlos's Avatar
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    This type of speaker is new to me... interesting. I just found a video where a guy shows how to test one of these speakers using a computer power supply. Click here...

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by waspclothes View Post
    The amp is a "Bronson Electric Sound Amplifier" that says "Detroit 26, Michigan" on the front. It uses two 6V6's as the output. >>snip<< So the output transformer is also mounted onto the basket, that's pretty wild. It might be kinda neat to bolt the transformer (and choke?) onto the basket of the new permanent magnet speaker - any caveats on doing something like that? Just tossin' that out there.
    Looks familiar- I worked on one just like it 2007. OT mounted on basket was standard back in the good ol' days. Some baskets still have provision for this but not many. I wouldn't go to the trouble of drilling a speaker & take a chance/probability of having steel debris find its way into places it shouldn't be, stuck there magnetically. Either find a spot to bolt a new OT to the chassis, or mount it to wood on the base of the cab if possible, or on the side if not.

    It was a nice mellow "old" sounding amp in its original form, but the owner wanted to boost it a bit & get a brighter tone. VC was scraping as you describe on the FC speaker. So, as I described above, new PM speaker, new filter choke, higher B+ voltage, higher value Rk to compensate & he got what he wanted. Bit of bad luck for me, not only did he get what he wanted but he got it free, never paid and moved 2000 miles out of town. Grr.... so much for trust.
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    Yes, that is a 10" Rola. If you don't want to recone it, you could replace the field coil with a choke and add a new output transformer to the the chassis and just add a new PM speaker. Then sell the original speaker on eBay. There are lots of folks looking for them to restore their old Valcos.

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    Quote Originally Posted by 52 Bill View Post
    Yes, that is a 10" Rola. If you don't want to recone it, you could replace the field coil with a choke and add a new output transformer to the the chassis and just add a new PM speaker. Then sell the original speaker on eBay. There are lots of folks looking for them to restore their old Valcos.
    Thanks Bill, can I just reuse the output transformer that's attached to the old speaker if I choose to not sell the speaker on ebay?

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    Quote Originally Posted by waspclothes View Post
    Thanks Bill, can I just reuse the output transformer that's attached to the old speaker if I choose to not sell the speaker on ebay?
    Of course you can, it's your amp.

    A few months ago I fixed an Epiphone Elektar that originally had a field coil speaker. Someone had cut up the original speaker basket so that they could keep the coil intact and bolted it to the inside of the cabinet. They also had the original output transformer mounted to the chassis. So if you want, you could hack up the speaker and get the magnet structure off and use that instead of buying a new choke.

    I only offered up the eBay suggestion as a possible way to deal with your problem. The money made from the sale of the speaker could help cover the cost of the new parts.

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    Well I've decided I'd like to replace this original speaker with a modern permanent-magnet speaker.

    I had a look at the amp again today, there are 5 wires leaving from 5 terminals on the back of the amp.

    Terminal 1:
    Black - From Rectifier - to Field Coil

    Terminal 2:
    Empty

    Terminal 3:
    Orange - From other 6V6 Plate - To Output Transformer.

    Terminal 4:
    Yellow - From Field Coil - to B+
    Red - From B+ - to Output Transformer Centre Tap.

    Terminal 5:
    Blue - From 6V6 Plate - To output Transformer


    The field coil measures 1K DC resistance and is used as a choke. Do I replace this with a choke AND a 1K resistor?

    The last thing I'm confused about (and then the questions will stop!) is that the output from the Output Transformer doesn't go straight into the speaker, it goes into the field coil in series with the speaker. This winding or whatever it's doing in the field coil measures about .8ohm. What's the speaker output doing going to the field coil?



    Quote Originally Posted by 52 Bill View Post
    I only offered up the eBay suggestion as a possible way to deal with your problem. The money made from the sale of the speaker could help cover the cost of the new parts.
    I appreciate that Bill, it also lets the speaker live again and make someone else happy.

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    Quote Originally Posted by waspclothes View Post
    The field coil measures 1K DC resistance and is used as a choke. Do I replace this with a choke AND a 1K resistor?
    No, the new choke should have a similar dc resistance (about 1K), so the choke alone replaces the field coil. Or you can opt to use a large wattage 1K resistor (10 watt maybe) to replace the field coil.

    Quote Originally Posted by waspclothes View Post
    The last thing I'm confused about (and then the questions will stop!) is that the output from the Output Transformer doesn't go straight into the speaker, it goes into the field coil in series with the speaker. This winding or whatever it's doing in the field coil measures about .8ohm. What's the speaker output doing going to the field coil?
    Believe it or not, that is a humbucking coil. With the new permanent magnet speaker, you will wire the output from the transformer secondary directly to the voice coil.
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    Quote Originally Posted by 52 Bill View Post
    No, the new choke should have a similar dc resistance (about 1K), so the choke alone replaces the field coil. Or you can opt to use a large wattage 1K resistor (10 watt maybe) to replace the field coil.

    Believe it or not, that is a humbucking coil. With the new permanent magnet speaker, you will wire the output from the transformer secondary directly to the voice coil.
    Ah yes, I see that a Fender Deluxe style choke has a resistance of 1500 ohms. Thanks guys, I believe I have everything I need now. Will order some parts and post back the results when everything is completed.
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    Hey guys,
    I just wanted to update this thread. Everything is working great, thanks for all of the advice. I replaced the field-coil speaker with a weber alnico.

    I removed the OT from the original speaker basket and mounted it in the cabinet along with the new choke. I also replaced all of the capacitors in the amp while I had it open. This thing sounds great!

    Here's a pic:
    img_1266.jpg

    Thanks again for all of the advice, it really demystified field-coil speakers for me. To be honest, the field-coil arrangement seems very elegant (and light weight), why was it phased out in favour of permanent magnets?
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by waspclothes View Post
    To be honest, the field-coil arrangement seems very elegant (and light weight), why was it phased out in favour of permanent magnets?
    Wait long enough and history will repeat itself. Eminence has had a FC speaker for a couple years now. I'm not 100% familiar with it but I have heard of their system which includes a variable power supply for the magnet. You can dial up the field for max efficiency and clean bright full-toned sound, or dial it down for a funky "blues" response. Or anywhere in between, whatever makes your ears happy. Not cheap though, I think it's over a grand.

    There's also at least one hi-fi speaker builder making FC speakers for well-heeled corque-sniffeur audio fans.

    What's old is new again.

  24. #24
    Supporting Member The Dude's Avatar
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    Early magnets powerful enough to efficiently drive a speaker were extremely large, heavy, and expensive- making using them somewhat impractical. A field coil setup was much lighter, more efficient, and cheaper. As magnets got lighter, more powerful, and cheaper, the field coil was phased out. So, technological advances and like everything else in manufacturing........COST.
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    Senior Member Enzo's Avatar
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    Field coil speakers were efficient, and they could make a strong magnetic field. Permanent magnets back then were not up to it. Why did they stop? Speakers had to have the extra wiring, the coil had to be wound and installed, it had become more expensive than magnets. Some people may be making them again in small numbers for a niche market, but they will never return as the main form of speaker.

    Most of our guitar amps used the FC as a choke, but there were others that ran the coil off the B+ to ground. I used to rebuild jukebox amps, and I kept a 5k 50w resistor in my drawer to take the coils place on the bench. The big ceramic resistor had a couple clip wires soldered to it so I could insert it into the circuit.

    Most of the time, if I have to replace a field coil speaker, I just use a resistor. I seldom find a choke makes much of a difference.
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    Old Timer J M Fahey's Avatar
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    Fully agree.

    To put some numbers int it , I make speakers and transformers so I have the real costs of both technologies, as seen in that speaker:

    Field coil, you need to:
    * cut and bend out of a strip of 1/4" sweet (very low carbon) iron the square magnetic circuit frame (same for an Alnico slug "permanent magnet" speaker)
    * make a precision hole in one side of that square box for the voice coil
    * turn a piece of bar stock , also with precision, which will be the coil core and the pole piece
    * wind a quite fine wire field coil, a couple thousand turns, around a proper form/bobbin
    that alone is a deal killer: slow, expensive, you use a lot of copper wire measured in pounds (even if 1/4 pound it soon adds up at the end of the day, copper is expensive)
    * assemble and firmly mount the whole enchilada
    so you wasted a lot of time (say a man-hour) , had to work with precision, used expensive materials, that field coil assembly probably costs U$30 to make today.

    Modern ceramic :
    * 2 disks cut from 1/4" plate in a big mechanical or hydraulic press: around U$ 2 or 3
    * 1 polepiece lathe cut from sweet iron bar stock: around 1$
    * a voice coil hole cut in 1 disk, usually first roughly punched and then finished in a lathe, 50 cents.
    * a ceramic ring magnet, around U$2
    * one of the disks is resistance spot soldered to the frame in around 20 to 30 seconds; the pole piece is "hammered" into the other disk, the magnet is epoxy glued in seconds.

    So the modern ceramic magnet technology costs around 1/5 the equivalent power and size FC , and needs much cheaper "trained monkeys" to assemble.

    Just as a free sample of how it's done today:
    https://www.youtube.com/watch?v=P-pzTfc5C08
    it takes 56 man-seconds per magnetic frame assembly vs. , say, 1 man-hour for a FC one.

    It's a no brainer why F.C. is not used any more and the few available prices go through the roof.

    FWIW with a couple partners we made 30" field coil speakers in the early 80's (no large enough magnets were available) and we couldn't sell them because of the price, so only way to recover expenses was to mount them in "Earthquake" type enclosures and rent them, for a monthly fee, as literally wall (and floor, and roof) shaking cabinets to Disco houses.
    End of the story?: we never recovered what we invested.
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    Quote Originally Posted by J
    It's a no brainer why F.C. is not used any more and the few available prices go through the roof.

    FWIW with a couple partners we made 30" field coil speakers in the early 80's (no large enough magnets were available) and we couldn't sell them because of the price, so only way to recover expenses was to mount them in "Earthquake" type enclosures and rent them, for a monthly fee, as literally wall (and floor, and roof) shaking cabinets to Disco houses.
    End of the story?: we [B
    never[/B] recovered what we invested.
    Thanks for that info, I can see why PM speakers took off.

    Does anyone want the old speaker? The OT has been removed, but the FC is still there. The voice coil has a rub, but the cone is in great condition. I'm not sure if anyone has a use for it, but if you're willing to pay the shipping I'll send it out - otherwise it gets buried.

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    Supporting Member The Dude's Avatar
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    You might try selling "as is" on eBay. You'd probably be surprised at what someone will give you for it.
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    Senior Member Silvertone Jockey's Avatar
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    Quote Originally Posted by J M Fahey View Post

    FWIW with a couple partners we made 30" field coil speakers in the early 80's (no large enough magnets were available) and we couldn't sell them because of the price, so only way to recover expenses was to mount them in "Earthquake" type enclosures and rent them, for a monthly fee, as literally wall (and floor, and roof) shaking cabinets to Disco houses.
    End of the story?: we never recovered what we invested.
    I bet in today's world of competition mobile audio where people spend big bucks to pimp out a ride for db bragging rights those speakers would sell, even noting that some sort of inverter power supply would be needed for the field coils

  30. #30
    Senior Member Silvertone Jockey's Avatar
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    This is thread drift but curiosity got to me, so a search came up with at least 1 modern field coil speaker manufacturer that specializes in 'corque-sniffeur' products as Leo put it

    http://www.feastrex.com/products.html

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    Senior Member Silvertone Jockey's Avatar
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    More thread drift, high end PM speakers with north and south double voice coils

    http://www.makeitlouder.com/loudspeakers.html

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Silvertone Jockey View Post
    More thread drift, high end PM speakers with north and south double voice coils

    http://www.makeitlouder.com/loudspeakers.html
    Oh the car stereo kids will eat that shi+ up. You know,......the ones who think wattage = SPL...... the "mine's bigger than yours" type.

    And, by the way, IMO, it's only important to have an amp that's flat to 10Hz if you're trying to mate elephants.
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  33. #33
    Supporting Member The Dude's Avatar
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    Since we're rambling a bit here, how many of you remember or have worked on these?

    servodrive.jpg
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by waspclothes View Post
    Hey guys,
    I have an old Valco amp and the speaker is farting out, I can hear the voice-coil rubbing while I move the speaker.

    I'm wondering, what is the procedure to replace those old field-coil speakers with modern magnetic speakers?

    I'm under the impression that a choke should replace the field coil and then the speaker gets hooked up with the other two wires? What's the criteria for selecting the choke?

    Can I mount the choke onto the inside of the cabinet to avoid modifying the original chassis, or is it best to mount the choke to the chassis?

    Last question would be, what sonic differences would I expect if I'm replacing an old field-coil with a modern speaker? Were the field-coil speakers more/less efficient than today's speakers?

    I can't find the exact schematic right now for this amp, will post it if I can - or trace it out if I need to once I crack the amp open. Thanks guys!
    I didn't read all the replies so your question may have been answered. I guess the first question is do you want to retain originality? If so you would be best to re-cone the speaker.
    If originality is not important, buy a decent speaker of the same size and impedance and install as required. Would have to locate the output transformer in a suitable position but keep it away from the power transformer, then you will also require a filter choke which doesn't have to be to fancy as long as it can handle approximately 100mAs. I would suggest that you replace that output transformer as well as it looks a litter tired, and while you're at it replace those horrible looking paper capacitors and also the filter capacitors.
    That field coil shouldn't be called a hum-bucking coil as in many old radios there was another coil on the speaker which was wired in reverse polarity which actually did hum-buck the 60Hz hum. But that is another story!
    The field coil acts as a filter choke and at the same time magnetically energizes the speaker so that the voice coil of the speaker can vibrate (backwards and forwards) with the incoming audio signal producing the sound that your ears hear.
    Spend some money and time on this amplifier and you will end up with a nice classic that you can actually use.
    Good luck, Mickey

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    Quote Originally Posted by Mickey View Post
    I didn't read all the replies so your question may have been answered. I guess the first question is do you want to retain originality? If so you would be best to re-cone the speaker.
    If originality is not important, buy a decent speaker of the same size and impedance and install as required. Would have to locate the output transformer in a suitable position but keep it away from the power transformer, then you will also require a filter choke which doesn't have to be to fancy as long as it can handle approximately 100mAs. I would suggest that you replace that output transformer as well as it looks a litter tired, and while you're at it replace those horrible looking paper capacitors and also the filter capacitors.
    That field coil shouldn't be called a hum-bucking coil as in many old radios there was another coil on the speaker which was wired in reverse polarity which actually did hum-buck the 60Hz hum. But that is another story!
    The field coil acts as a filter choke and at the same time magnetically energizes the speaker so that the voice coil of the speaker can vibrate (backwards and forwards) with the incoming audio signal producing the sound that your ears hear.
    Spend some money and time on this amplifier and you will end up with a nice classic that you can actually use.
    Good luck, Mickey
    Hi Mickey, sorry I thought I updated this thread.

    I replaced the speaker with a Weber AlNico, I kept the original output transformer and mounted it inside the cabinet along with a new choke.

    Can capacitor was replaced and film caps were replaced with Mallory's. Sounds great!

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