Field-coil speakers

[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?

The F12Ns I've dealt with have 1-3/4" voice coils. They're no more challenging to recone than any vintage speaker, even less so, in some ways, because, if you need to clean the gap, you can actually unbolt and remove the coil and pole piece. Your reconer may have to get out the micrometer and do some direct measurements because the speaker parts places I've dealt with don't have standard parts kits for them. Most any Kapton or Nomex voice coil currently available will handle more power than the original paper voice coils. Keep the field coil power to a maximum of 15W.

They tend to have very strong bass response for twelves, so keep that in mind when choosing parts. In the end, you get a speaker that's very sensitive, dynamic, and well-controlled, more like a speaker with a strong ceramic magnet than a vintage Alnico.

I just built a stand-alone power supply for mine, and I had to add a flyback diode because the field coil's inductance would send the supply briefly negative (-3.5V) at shut-off. It quickly drains a 470uF capacitor charged to 100V (700 Ohm coil).

[Diablo]

The F12Ns I've dealt with have 1-3/4" voice coils. They're no more challenging to recone than any vintage speaker, even less so, in some ways, because, if you need to clean the gap, you can actually unbolt and remove the coil and pole piece. Your reconer may have to get out the micrometer and do some direct measurements because the speaker parts places I've dealt with don't have standard parts kits for them. Most any Kapton or Nomex voice coil currently available will handle more power than the original paper voice coils. Keep the field coil power to a maximum of 15W.

They tend to have very strong bass response for twelves, so keep that in mind when choosing parts. In the end, you get a speaker that's very sensitive, dynamic, and well-controlled, more like a speaker with a strong ceramic magnet than a vintage Alnico.

I just built a stand-alone power supply for mine, and I had to add a flyback diode because the field coil's inductance would send the supply briefly negative (-3.5V) at shut-off. It quickly drains a 470uF capacitor charged to 100V (700 Ohm coil).

Thanks much Rhodesplyr, that's just what I need to know. With the 1.75" voice coil, I have plenty of options for getting more power handling. Could you post a schematic for your power supply?

[Rhodesplyr]

Thanks much Rhodesplyr, that's just what I need to know. With the 1.75" voice coil, I have plenty of options for getting more power handling. Could you post a schematic for your power supply?

You have more options for greater power handling, but remember that it's also a very sensitive speaker, so you'll get a healthy output from it without pumping in gobs of power, which was the original point of them to begin with.

The power supply? Well, I was going to build one, and then I saw this:

2 to 120V DC High Voltage Adjustable Regulator Module Based on TL783 SKU171002 | eBay

For the price (shipped), it was hard to beat. For one speaker, the Hammond 166G80 power transformer is sufficient. If you want to power more than one, you'd need a Hammond 166G100. This is assuming 700 or 1,000 Ohm coils. 250 Ohm and 5k are also common values.

[Alan0354]

Mr. Coil was very defensive at the time. So after a few years, what is the verdict of the sound of his speakers?

[Diablo]

You have more options for greater power handling, but remember that it's also a very sensitive speaker, so you'll get a healthy output from it without pumping in gobs of power, which was the original point of them to begin with.

The power supply? Well, I was going to build one, and then I saw this:

2 to 120V DC High Voltage Adjustable Regulator Module Based on TL783 SKU171002 | eBay

For the price (shipped), it was hard to beat. For one speaker, the Hammond 166G80 power transformer is sufficient. If you want to power more than one, you'd need a Hammond 166G100. This is assuming 700 or 1,000 Ohm coils. 250 Ohm and 5k are also common values.

Thanks, but I'm a little confused with your power supply. Can't you use that adjustable power supply 2v-120v you linked by itself, with 110V AC input? Or are you intending to use the Hammond transformers along with your adjustable power supply? Are the Hammond transformers needed for isolation?

[Enzo]

If you mean connect it directly to the mains????? That is a great way to make it deadly.

[J M Fahey]

To answer that question we should:
1) know *which* are those speakers
2) listen to them

Otherwise the answer does not exist.

[Alan0354]

To answer that question we should:
1) know *which* are those speakers
2) listen to them

Otherwise the answer does not exist.

You seen those Fluxtone speakers?

[Diablo]

If you mean connect it directly to the mains????? That is a great way to make it deadly.

Thanks Enzo for the good safety advice. I was using some half-baked logic that went like this. I was planning to power the field coils with a standalone power supply powered from the AC line voltage thinking I really didn't need an isolation transformer. If the field coil shorts to the speaker frame, I could get a live speaker frame. But, the speaker frame isn't connected to the amp chassis or anything metallic. The speaker connects back to the power amp via the pair of wires on the voice coil. And that pair of wires is isolated by the output transformer from the amp chassis. I'm thinking the only danger of electrocution is if the field coil shorts, and I touch the speaker frame while it's powered up.

[soundguruman]

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

Well, field coil speakers appeared in the late 1930s and were used until the early 1940s...as an alternative to heavy iron magnets.

And, field coil speakers sound much better than regular permanent magnet speakers. They sound "hi fi" and have quite a nice tone.

So, to preserve the original tone, I recommend having the speaker rebuilt if possible.

I would not convert an amp to a regular speaker. I would try to restore the original field coil.

When you find an amp with a field coil speaker, you are looking at the first guitar amps, a piece of history, worth preserving.
and they do sound really, soooooo sweet.

Now, many have said that Leo Fender invented the guitar amp, of course he did not.
And when you find an amp with a field coil, you are looking at an amp which pre-dates Leo Fender's amps.
Extremely Cool, it has maximum mojo.

[trobbins]

If you make the power supply variable, then you may also achieve a form of 'VVR type' function.

[Gingertube]

This was absolutely stunning with my Telecaster, I took it to a mates place for Sunday afternoon jam and now have orders from 2 of the 4 guys there for a clone.
I'm convinced that the field coil speaker is a large part of its charm. Have 6 old Field Coil Speakers collected.
Schematic trace and pictures are here:
Historic Harmonica Amplifier - diyAudio
Cheers,
Ian

[Diablo]

This was absolutely stunning with my Telecaster, I took it to a mates place for Sunday afternoon jam and now have orders from 2 of the 4 guys there for a clone.
I'm convinced that the field coil speaker is a large part of its charm. Have 6 old Field Coil Speakers collected.
Schematic trace and pictures are here:
Historic Harmonica Amplifier - diyAudio
Cheers,
Ian

Could you post up the pictures here? I can't see anything on your link without registering for another forum.

[Gingertube]

Here is the schematic - will chase down the pictures shortly

Cheers,
Ian

[Gingertube]

Let me try once more. The schematic is pretty much fig 15.35b from RDH. Nifty treble boost/cut tone control via feedback around the 6V6 output tube. Missing from the schematic is the 15K grid stop I added to the 6V6.
Cheers,
Ian