Field-coil speakers

[Diablo]

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

So, can I get away with a better filtered power supply using bigger caps and eliminate the hum bucking coil? Or is it a more practical solution to stay with the hum bucking coil? I'm talking about an application of the speaker using a stand-alone power supply, not using the B+ of the tube amp like in vintage designs.

[Mr.coil]

Yes that is correct, "FluxTone Speakers" uses power supplies with no hum, you can to.

MC

[Diablo]

OK, that's good information to know. One less coil to wind.

[hasserl]

Hmmm, FC speakers are supposed to me MORE efficient than PM? I suspected the opposite. I recently picked up an old Vega TA-8 instrument amp that uses a pair of 6L6's for power, and a FC speaker for the driver. The amp sounds fantastic, but the volume is very low, less than a nice single ended 6V6 amp (w/ Jensen C15N speaker). I just assumed the low volume was due to low efficiency of the FC speaker and was making plans to replace the FC with an alnico Weber, hoping to wind up with an amp loud enough to gig with.

Here's a couple of shots of the amp:

[J M Fahey]

Your amplifier could have low volume for any number of reasons.
For one, how much is it actually putting out?
Field coil speakers *can* reach iron saturation point easily, which is not so with permanent magnets.
Anyway, if you need efficiency, ceramics will give you more than equivalent Alnicos, and Neodymium even more so (and in a lighter and smaller package).
Anyway check your amp first.
Beautiful find !!!!

[hasserl]

Of course it will need a thorough going over, along with 3 prong cord, fresh caps, etc. I guess I just assumed things wouldn't be in too bad of shape if it sounds as grand as it does, the only real issue is lack of volume. Can't wait to get this out and use it live. Oh, sounds great with harp too!

[Mr.coil]

Hey, those are real 6L6's...not 6L6-GC's...Those old bottles can't do more than about 10 watts (when new), especially with that small iron. However at 10 watts it should be loud enough. Can you tell if the cone is draggin? You probibly will have weak coupling caps...(Driver to final grid)

BTW, that is an old Rola...about a 10 watt speaker...1" or 1.25" V.C.

Please be careful poking around in there it can sting pretty good!

That output trany could be bad? How many volts AC does it deliver to the voice coil terminals...should be about 8-12VAC.
The field should be about 250-400 VDC (paralell) 70-120 VDC in series. Stick you left hand in your pocket when testing.

Looks cool to me!

I hope you get it going.

MC

[jhelm_waterw]

Stick you left hand in your pocket when testing.

[hasserl]

Actually, it came originally with 6L6G, at least that's what's printed on the chassis. It came to me with one 6L6GC and one steel tube 6L6. I pulled those out and inserted a pair of Chinese 6L6GC with the ST shape, which I just happened to have on hand. I'll check it over when I get the chance, right now I have some customer pay jobs that get priority over personal projects, so it'll be a while yet.

Oh, and I'm familiar with safe work habits. Thanks though, you never know who is reading these threads and it doesn't hurt to to remind folks to keep safety in mind.

[Mr.coil]

Ok that's great...
The only thing is, those new GC;s can, and will draw more current...and that can overtax the high voltage winding in that old iron.

They will change the bias too. If you down size the first cap, (before the field coil), to 2-4 mfd...that will drop a few more volts.

If you can get the b+ under 400 VDC at 70 mills...you could run a pair of 6v6 JJ's They are happy with 375-400 VDC as long as your not over 70 mils for the pair.

Good luck
MC

[Diablo]

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.

Say Rhodesplyr:
Digging up an old thread here, and I was wondering if you have any specifications on the Jensen F12N. I'm trying to buy a speaker or two and was wondering what the voice coil diameter is for those. I'm thinking ahead if I need to recone the speaker, and seeking out a voice coil that can handle a little higher power without burning it up. I assume the cone dimensions are similar to other vintage Jensen speakers with permanent magnets?

[Alan0354]

What is a field coil speaker?

[The Dude]

A field coil speaker uses an actual wound coil for the fixed magnet rather than a magnetized structure with no coil.

[Alan0354]

A field coil speaker uses an actual wound coil for the fixed magnet rather than a magnetized structure with no coil.

[ATTACH=CONFIG]27033[/ATTACH]

So you can change the efficiency by altering the strength of the electromagnet?

[Diablo]

A field coil speaker uses an actual wound coil for the fixed magnet rather than a magnetized structure with no coil.

[ATTACH=CONFIG]27033[/ATTACH]

Yep. An electromagnetic provides the magnetic flux for the gap between the pole piece and the front plate. It's a vintage design that was used before cheap and strong permanent magnets were an available technology. By the end of the 1950s, field coil speakers were history.