Advice on Jensen A-12 Field Coil speaker please

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capohk

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Dear Shockpersons

I have a field coil speaker that came to me with a jukebox amp. It has been previously reconed by persons unknown and works, but doesn't have much high-end reproduction. It is currently running off the amp's B+ at ~260VDC as per the schematic, but is not required as a choke in that circuit so I am able to compare it to another 12" speaker - a Weber 12F150 in the same 2x12 cab, and a Weber 12A150 in a single 12" cab. Even as guitar-specific speakers, they are much more full range.

Questions - can the frequency response of the field coil speaker be manipulated by adjusting the applied voltage - ie if I lower the supply, the speaker will reproduce more top end?

Can the electrodynamic magnet set-up degrade over time? Or does a partial short in the coil possibly cause this, could this be rewound?

Is it possible that it has been reconed in a way that cuts off the top end?

It's not a subtle thing - I used the amp and speaker as a shop mono music player for a while and its night and day with the change in speakers. The jukebox would have wanted a full range response to reproduce records, and the amp has controls to attenuate treble and bass, so I'm thinking it can't always have been like that.

Thanks in advance for sharing any experience with these
 
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Wally

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Weber reconed a Jensen F15N for me…great job, great sonics. The speaker was OEM in a 1952 5B2 Pro. I feel certain that they would have a handle on the problem with that speaker.
 

capohk

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Thanks Wally. I’ll message Brian Spane who has helped me with some other stuff in the past. He is great but their shipping to HK is nuts expensive so would probably preclude him doing a recone on this. I have a couple of other speakers that need doing so I might have to learn how to do that myself, but the recone job on this looks pretty good. I’m not sure it’s the problem here.

I pulled the speaker and took some measurements and pics:

7.6 ohms across the voice coil

4.3k across the field coil

Serial on edge of basket A12 C2180-3 PH3520

D8ADB99A-76A3-4480-BE7E-B2EEEED689D5.jpeg
 

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capohk

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From what little I can find on the web, these seem to want to run about 14W dissipation. Using the remnants of my old grey matter, would it seem sensible that 14x4300=60200 sqrt = ~245V, drawing ~57mA?

Of course, I could be completely talking out of my erse, but then until about 2 minutes ago, P=IV was a 30 year distant memory...
 
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Wally

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I did not take note of your location. I am sure that the shipping is prohibitive. I am of little help as to what may be going on there…other than to say that I would install a permanent magnet speaker if I wanted to run the juke box.
Best of luck with it.
 

capohk

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Further research shows that the organ pulls in pairs seem to have mismatched voice coil resistances - one at around 4.5K and another, much lower, at around 500r. This at least tells me that mine is in the expected range.

It is running off of the B+ in the jukebox amp at ~260 on the schematic, but I think probably higher in reality due to wall voltage increase. I'm wondering if the higher voltage is the cause of reduced high-frequency response.

I guess I'll try to find a lower voltage at one of the other power rail taps and try it. Surprised it's so hard to get info on these though.
 
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