Field-coil speakers

[balijukka]

portable field coil

I've been thinking lately the possibility of making a field coil that you could could put on top of a permanent magnet like a cap, then dial in the dc current so that the created magnetic field is opposite to the permanent field, so reducing the sound output.
This would have no permnent harmful effect on the speaker but you could choose the noise level according to the situation.

jukka

[J M Fahey]

Ah !!! but you *are* dissappointing me !!

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.

WHERE - IS - THE - MOJO -?
What is the Marketing Department going to do with you ?
I promised no to use 4 letter words, but here you have it:
You think like an .................................................. Engineer !!!

[Albert Kreuzer]

I promised no to use 4 letter words, but here you have it:
You think like an .................................................. Engineer !!!

That's an 8 letter word - twice as bad.

[Steve Conner]

Around these parts it's a compliment

[yerbluez]

Field coil speakers - Jensen F 10 U

Hi all,

I recently purchased a vintage gibson tube amp (BR 6) and it has an older Jensen F10 U field coil speaker... I can't seem to find these anywhere. I believe this speaker is damaged as it can't handle the lower bass notes on a guitar without rattling. Do any of you or does the original poster know anything about where I can find one of these? Ebay only had the 12" ones and I don't know if the amp can run that.

Thanks

Sean

[GibsonLover]

Sounds like your field coil is energised if you get some good noise out of it. If all else is good with the amp, it may just need a recone. Gently move the cone in and out with your fingers and try and detect any rub. Make sure the excursion is just as far in as out. If not, recone it. I just had my F15N done, so I'm sure parts are still available. Northland Loud Speaker in Mineapolis still has smooth cones. Call around. A recone is a better idea than looking for another one that is just as old, and would probably need the same treatment. Spend your money once, and you're good for 50 more years.

[yerbluez]

Hey thanks for your reply. I had similar advice from the local music store and I now feel a lot better about the whole deal. It definitely sounds nice except for that rattle. I'll check out that place you suggested.

thanks

Sean

[Mr.coil]

Field coil speakers

First off...on your sign up page it asked a random question..."what brand of amp was Leo Fender associated with" I assumed that you were looking for something other than Fender ... so I put Musicman. Much to my surprise it said I was wrong...Really? So I went back and put in Fender...and voila....bing bang. here we are.

FluxTone Speakers seems to have done a fair amount of Spectrum analysis on these items, and has a nice line up of new ones for sale, and a fair amount of real info on the subject.

[Enzo]

Hi, Mr.Coil, welcome to our forum.

Forums like this get a lot of spam traffic and robot readers. A question like the fender one is completely meant to be obvious to a human, but not easily answered by a spam robot. it is a way to keep spam traffic out of the forum without having to delete it all manually the next day. Otherwise we get to see offers for AMAZING DEALs on cell phones or whatever.

[loudthud]

It is my understanding that Leo didn't have much to do with the MusicMan amps.

[Diablo]

You folks have me interested in cobbling up a field coil speaker, so I've been doing a little googling. I 'd like to recreate something similar to the Jensen F12N with 700 ohms DCR, powered with around 100V/142mA. To get 700 ohms from standard gauges of magnet wire that can stand that current density, I come up with a minimum of #35 gauge wire (.0056 inch) at 2174 feet. If I step up the gauge to #34 (.0063 inch) to handle more current for a safety factor, I calculate 2734 feet of wire. Of course, if I increase the gauge and corresponding length, I'll be increasing the flux density for the same current. Do my estimates seem reasonable for the F12N? Does anyone know which wire gauge they used on the F12N? I also have been thinking about the magnetic circuit, and it seems that you really can't get to unusually high flux densities in the air gap because the steel front plate, backplate and center pole will saturate at around 12,000 - 15,000 Gauss - the exact number depends on the composition of the steel used.

[J M Fahey]

I think you have it pretty accurate.
It should work.
Starting from the end, I agree that it's very difficult to go over 15 Kilo Gauss, specially since we use regular soft iron (think SAE1010) , not any fancy alloy.
Anyway, anything over 10000 Gauss is going beyond 99% of commercial speakers; 12000 is top of the top; 15000 is almost unheard of.
Your speaker will be *very* efficient.
The basic calculation, from a "practical" point rather than a magnetic one, is to consider how much wire you can fit in the "window" you have, less insulation plus some tolerance to be able to slide the coil in and out the pot.
It's a little like designing guitar pickups, not that much magnetic calculation involved, you just fill it up with wire and pray for the best.
Your inside dimension will be somewhat over the polepiece diameter, the outside one somewhat less than the outer tube (if you can get a SAE1010 tube) *or* you will have to make a "square" yoke out of flat iron.
You can make the coil between 2 and 3 inches long.
That defines a window with a given surface, transformer design charts show how many turns of any given diameter can fit in a given window surface; it's hard to fit more than 80 or 85% of the theoretical amount.
Go for the highest diameter, most turns you can, you already know it will be the maximum physically possible, later you will be able to downgrade if overdesigned, not upgrade for lack of space.
Your speaker will be very expensive, but it's an experiment, not a commercial product.
Around 1985 I made, together with some friends, a couple monster 30" speakers, 4" coil, field coil magnet.
It was literally earthshaking but a commercial disaster.
Back to the "iron", finding the approppriate tube will ease your work a lot, because it plus the plates can be lathe turned; working with flat iron will require expensive milling.
These days I'm going crazy trying to find a "slice" of 12"/14" tube for a bigger magnetizer than the one I already have.
*Nobody* wants to cut a 6" slice for me from a "new" tube, they want me to buy the 20 ft piece (crazy); and going the surplus route, I only find used pipeline/gasline tube made of a very strong steel, chock full of wonderful additives such as nickel, chromium, and who knows what else, quite unsuitable from a magnetic point of view.
Oh well, nobody said it would be easy.

[Mr.coil]

After tons of testing we found anything over 15 watts, (in the field coil) is a waste of resources. Just like the terminator, We have charts and detailed files...the significant difference is "how wide is the gap". anything over .060 is too much, and if you get under .030 Then your name must be .."James B Lansing"...Welcome back!


Most critical of course is...can your motor take a 3 foot drop? without bending the basket, or becoming misaligned.

Mr.coil

[Diablo]

More questions about the field coil speaker and filtering out AC hum. I understand that the Jensens had an additional humbucking coil in between the backplate and the field coil. The humbucking coil is in series with the voice coil, but wound in the opposite direction. Do all field coil speakers use these humbucking coils? Can an inexpensive power supply be built with enough filtering to eliminate the humbucking coil? The field coil is a big choke. Why doesn't it effectively filter out all the AC?

[Mr.coil]

Those hum-bucking coils were to neutralize the "hum" introduced into the magnetic circuit..VIA power supply noise.

Remember, the "field coil" was smoothing out that ripple in those crude power supplies.

Rippled DC went into the field, was current regulated, and what came out was relatively smooth. Smooth enough that the modest caps 2-4 mfd. could finish out the rest of the job.

However some of that 60 hz stayed behind in the magnet....So they put some back..."out of phase"...hence Hum-bucking.

MC