Field-coil speakers

[Diablo]

Thanks Gingertube, looks very cool.

[Rhodesplyr]

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

I think Enzo made my point. Also, with a little bit of step-down, everything runs cooler. 122 VAC produces a higher DC voltage than 80 VAC.

[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).

Rhodesplyr, more questions for you on the Jensen F12N.
I have one sitting on my table now, and was wondering if you could provide some disassembly details?
I was surprised to see the back-iron horseshoe is welded to the front plate.
I see only three bolts below the cone, just outside the spider OD.
Is the iron pole threaded into the back iron, or is it pressed into it?
I'm thinking about reconing and also the possibility of eventually rewinding the field coil.
I assume the only way to remove the field coil is to first remove the iron pole.
How is the front plate mounted to the speaker frame? The three screws?

[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).

Rhodesplyr, more questions for you on the Jensen F12N.
I have one sitting on my table now, and was wondering if you could provide some disassembly details?
I was surprised to see the back-iron horseshoe is welded to the front plate.
I see only four bolts below the cone, just outside the spider OD.
Is the iron pole threaded into the back iron, or is it pressed into it?
I'm thinking about reconing and also the possibility of eventually rewinding the field coil.
I assume the only way to remove the field coil is to first remove the iron pole.
How is the front plate mounted to the speaker frame? The four screws?

[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.

OK, I have one of these ebay power supplies on the way, and will order a step down transformer - 166G80, which is 80 VAC output at .5 amps. So if I have two speakers with 700 ohm coils, is there any reason I couldn't power both field coils in parallel? Each field coil should pull a max of 13 watts, so it's drawing 26 watts total, + whatever losses are in the voltage regulator. Am I wrong in assuming that a single voltage regulator and 40 VA transformer can handle the job?