Field-coil speakers

[Rhodesplyr]

I was wondering if anyone has tried out 1950s field-coil speakers like the Jensen F12N for use with low-powered guitar amps.

I'm not going for the magic of obsolete technology; I'm just curious as to whether they actually have higher sensitivity than the PM models that followed. As an organ tech, I have access to them for much less than the nutty prices some are paying for them on eBay, and building a stand-alone power supply is easy enough.

I have four Leslies, and three of them use the field-coil Jensen F15LL, which was Don Leslie's personal favorite of all the woofers he tried in his cabinets. I've tried running guitar through them via the Leslie amps, but as fifteens, they don't have much high end, not to mention that the Leslie amps have some low-pass filtering built in. (You can plug the woofers directly into the amps, bypassing the crossover.) However, in the bass region--with 300V across the coil--a 20W 21H amp with a field-coil woofer will run circles around a 122 with a PM speaker.

[J M Fahey]

Great info !!!
I am also sure that, at least in the beginning, they were better than the "novelties".
If you have any free time , please post pictures, DCR readings, DC current it eats, some physical measurements, whatever you can.
If you recone one, coil physical measurements of the old one, some pictures.
Have you tried it , side by side to some "modern" speaker?
Very interesting stuff.
PS: they *had* to be good, even before the Rock age, they were meant to be used in Churches, some of which are scaringly big and seat a *lot* of people !!

[Rhodesplyr]

With limited wattage from power amps and few Alnico V speakers in production, they had to resort to field-coil speakers to get sufficient SPL for large spaces. All Leslies until 1958 use them. In general, they are similar to the Jensen P15LL except for the magnet structure. The coil DC resistance is nominally 2,650 Ohms cold, rising to 2,800-3,000 as it warms up. The lower powered Leslies (21H, 44W) put 276V/104mA across the coils, which were wired between the screen supply and the output tube cathodes. The higher-powered Leslies put 305V across the coils for 115mA--35W of power just for the speaker coil.

Hammond tone cabinets and spinets like the M-2 and early M-3 used the F12N or similar and put 100V/142mA across a 700 Ohm DCR coil--14.3 Watts.

You can Google for images of the F15LL. The coil/magnet structure is very heavy. They were built before dB figures, Thiele parameters, etc... were published, so the actual specs on them are a bit vague. I'd estimate the F15LL at around 101dB, maybe more.

Removing field coil speakers from Leslies is strictly forbidden. Thou art cursed to the lowest pits of perdition if thou doest so.

[Albert Kreuzer]

Removing field coil speakers from Leslies is strictly forbidden. Thou art cursed to the lowest pits of perdition if thou doest so.

Word.

[J M Fahey]

Thanks for the info.
My fingers are itching to make one.
I'll talk with my lathe man.

[loudthud]

There was an article years ago in Speaker Builder magazine about a guy working on horn speaker design in 1930s Germany. His field coil speaker had a flux density of about 5 Tesla. The JBL D series used a large AlNiCo magnet and spec'd the flux density at 1.1 Tesla or 11000 Gauss. It would seem that a field coil speaker could have a very high efficiency if the power required to power the magnet is ignored.

[Rhodesplyr]

It would seem that a field coil speaker could have a very high efficiency if the power required to power the magnet is ignored.

Exactly. People go down the path of thinking that if they get rid of that big drain on the amp's power supply, they can rebuild the amp to put out more Watts, which they can. But then, when they get it all put back together, it's not as loud as it was before (unless they use high sensitivity PM speakers like some of JBL's). They forget that speaker sensitivity trumps Watts output.

But with the arrival of Alnico PM speakers and higher powered amps, speakers with field coils and the larger power supplies necessary to energize them were no longer the most cost-effective option.

[Steve Conner]

Field-coil speakers are back and they're boutique priced. FluxTone Speakers reduce speaker volume without tone destruction

I bet the original Jensens sound great. The only reason people don't use them is probably, the average boutique or hobbyist amp builder doesn't know what to do with the two extra wires coming out of the speaker.

Tubes are no longer the most cost-effective option. They're hugely wasteful of money and energy compared to transistors. If we consider building tube amps at all, why not consider using field coils too.

I know a guy in Denmark who built his own field coil speaker from scratch. It was gigantic, with something like a 24" cone, and a magnet assembly that looked like the McGuffin from a Bond movie. Unfortunately (or maybe fortunately? ) I've never heard it working.

[GibsonLover]

This is great, timely information. I have an amp with a dead F15N. I've heard about the higher sensitivity, but now I gotta hear it. I just hope they can still be serviced(?) A Googling I will go...

[Rhodesplyr]

Is that a dead F15N or a dead F12N?

Most speaker places I've ever talked to can recone a Jensen field coil speaker as easily as any other Jensen speaker. If, however, the field coil is shorted or open, you may be SOL.

And, as if the powers that be heard my request, I just got home with a working 1956 Jensen F12N that I saved from the trash can. Another local tech was getting rid of a trashed Hammond M-3. He called me and told me I could have whatever parts I wanted. Now, I just need to jerry-rig a 100V power supply.

[jrfrond]

I don't want to break up the warm and fuzzy feeling in this party, but the one thing with FC speakers that ALWAYS concerns me is the B+ pipeline that novices can potentially get their hands on, or better yet, remember the old 4p XLR umbilical on B15's? Hmmm...... :O

[J M Fahey]

Well, now you only need to get an 83 mercury vapor rectifier (NOS of course); some silk-insulated copper wire, preferably pre-WW2 Japanese silk, have your ironmonger hammer some of that fancy new silicon steel plates for you, get some good old walnut or oak for the cabinet and ........................................ heeeeeeeeeeeeeeyyyyyyyyyyyyy !!!!!!!!!! don't kick meeeee !!!!! , I thought you wanted to keep it Vintage !!!!!!!

[Steve Conner]

Safety is definitely an issue. Don't forget to ground the speaker frame, and maybe a separate cabinet is a bad idea. Or do it like Fluxtone, power supply in the cabinet with a line cord hanging out the back.

Thinking outside the box, why not get a burnt out one, rewind the field coil
for 24v and run it off a SMPS.

Also, what if they just sounded like the early neodymium magnet speakers? People hated those. High flux density gives you great efficiency, but low Q and hence a thin bottom end if you're not careful. Although, I don't see the F15LL having a thin bottom end.

[Rhodesplyr]

J.M., I hate to disappoint, but my five minute power supply consisted of four diodes, a resistor, a capacitor, and 80VAC from the Variac. Worked like a charm.

As for the B+ being dangerous, I suppose it is, but I'm reminded of a recent article where researchers polled parents about what sort of things they worried about with regards to their children and then compared that list to a list of ways in which children actually tend to get seriously hurt. The two lists had little in common. I'll take high voltage hazards any day over people trying to drive their SUVs and yak on the cell phone at the same time.

[GibsonLover]

"Is that a dead F15N or a dead F12N? "
F15N. I have two Gibson GA-75's from the early 50's, one with the field coil and the "newer" model with the P15N. I got a price to recone them both for $60 each. I'll jump on that. I though I'd just replace them, but those early 15's are like hens teeth. 20 years from now, some other lucky bastard will have these amps to enjoy...