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

[52 Bill]

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.

[waspclothes]

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?

[52 Bill]

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.

[waspclothes]

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?

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.

[52 Bill]

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.

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.

[waspclothes]

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.

[waspclothes]

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:


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?

[Leo_Gnardo]

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.

[The Dude]

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.

[Enzo]

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.

[J M Fahey]

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.

[waspclothes]

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.

[The Dude]

You might try selling "as is" on eBay. You'd probably be surprised at what someone will give you for it.

[Silvertone Jockey]

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

[Silvertone Jockey]

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