Restoring a 1951 Jensen F15N Field Coil Speaker for a Fender Tweed Pro

[AJK]

I recently decided to have my 1951 Jensen F15N for my Fender Pro re-coned.

I brought it to a specialist who has re-coned field coil speakers in the past but was concerned about the condition of the coil before he proceeds with the his work.

We disassembled the speaker and removed the voice coil (measured at 8ohms), the field coil (not measuring in ohms but measuring at .9h), and the hum cancelling coil (measuring at .3ohms and .055mh).

Please see image of the field coil and the hum cancelling coil



The specialist was concerned about the field coil not measuring any resistance and thought that it might need to be rewound. Does anybody know the resistance should be for the field coil?

I know somebody who winds pickups who might be able to help me with this but I can't find any schematics on the Jensen F15N that might be helpful to him.

The only schematic I've found is for the amp itself which shows how the particular speaker uses a Triad #1856 Output Transformer to pass through the field coil and then to the tubes (see schematic: http://ampwares.com/schematics/pro_5...-corrected.pdf).

I appreciate any friendly advice on restoring this speaker.

Thanks!

AJK

[Steve Conner]

The field coil should have a resistance of a few thousand ohms. Some digital multimeters get confused trying to measure the resistance of a big inductor. The auto range function hunts endlessly and never settles to a stable reading. If this is the case, try setting the range manually.

[olddawg]

Correct me if I am wrong, but these were very common speakers in the late 40s and early 50s. I see them on eBay all of the time. Not necessarily out of a Fender amp but Hammond organs and such. You might consider buying a couple to cannibalize, reference, or even use if in decent condition. I see them in the $25-50 range all of the time.

[Bruce / Mission Amps]

The only schematic I've found is for the amp itself which shows how the particular speaker uses a Triad #1856 Output Transformer to pass through the field coil and then to the tubes (see schematic: http://ampwares.com/schematics/pro_5...-corrected.pdf).

I appreciate any friendly advice on restoring this speaker.

Thanks!

AJK

Could that schematic possibly be right!!?? Looks to me that that grounded coil would short the entire B+ rail right to ground and blow a fuse.
It would have to be quite a few thousand ohms, to not overload the B+ rail .... right?
I could see it being in series with the B+ rail and creating the actual "magnet" of the speaker...
I have a couple Field coil speakers here... I should measure one out.

[Steve Conner]

Field coil speakers came in two flavours. Low impedance for use in series with the B+ (replacing the power supply choke) and high impedance for connecting between B+ and ground.

[J M Fahey]

not measuring any resistance

What does this mean?
Open? / Short? / Over range ?
What scale was used?

And if it measured 0.9Hy you *do* have a coil there, not an open circuit.

EDIT: and sorry but don't think a pickup winder can help you much in *that* task, a transformer winder (or a burnt electric motor or generator rewinder) is much more likely, considering the coil size and wire gauge.

[AJK]

Today, I brought the field coil over to an amplifier technician. He and I noticed that one of the wires had broken its connection to the copper lead.
He soldered a new wire to the copper lead and took a new reading.
The field coil now measures 7k ohms. 55 Henrys at 100Hz and 23 Henrys at 1000Hz.
I believe think I can bring it back to the Speaker Restoration shop to have the speaker re-coned.
Thank you all for your help!

[Bruce / Mission Amps]

Field coil speakers came in two flavours. Low impedance for use in series with the B+ (replacing the power supply choke) and high impedance for connecting between B+ and ground.

The B+ rail on the early Fender amps was anywhere from 360vdc to 400vdc... if that coil is really 7000 ohms and it appears to be a dead short to ground.... 360v/7K = 51ma
51ma^2 x 7K = 18.5 watts... do you think that is right?
The PTs used back then in the 6L6 amps were frequently wound with HT secondaries of no more then 140ma-160ma.
By the way.... how do you test the actual inductance of that field coil without the 60ma-100ma required to make an EM speaker work?

I'm thinking out loud and I don't have any answers...

[Steve Conner]

Yes, I do think that is right. I've seen similar arrangements in Hammond organ amp schematics. We kind of take powerful, cheap permanent magnets for granted nowadays.

[guitician]

51ma sounds like the Field coil would make a nice self biasing resistor for the 5881's if only the inductance weren't an issue.

[Steve Conner]

Also the fact that it's designed to drop 300V across it with the 51mA flowing, in other words its resistance is too high. For self-biasing, you'd use the lower resistance series type of field coil.

[guitician]

That's what I was getting at...to get the needed inductance to develop the Gauss with a 250 ohm winding would be huge.

[olddawg]

Just thought this might be helpful:
http://item.mobileweb.ebay.com/viewi...d=321099189922

Who knows what he would really take for it. But they are out there if you look.

[Rhodesplyr]

This may be a late response, but I work with these field-coil speakers all the time. I've got several.

7,000 Ohms is fairly high, but Jensen basically supplied whatever kind of coil the OEM wanted.

Hammond typically used a pair with unequal coil DCR. One was 250 Ohms, used in series with a power amp's ground return to develop a negative bias voltage; the other was 5,000 Ohms wired in parallel with the output tubes. The goal was to supply both speakers with the current capacity of a single 5U4 rectifier tube. 700 and 1k Ohm coils are also common as these were used in the M-series spinets.

With the 12" speakers, they usually dissipated between 11 and 14 Watts through the coil. Conventional wisdowm is that the higher DCR field coils are more delicate due to the thinner gauge wire used.

[Rhodesplyr]

I should add that I just re-read that this is a Jensen F15N. The Jensen F15LL, which I have in several Leslie speakers, dissipates 30W over a 2,800 Ohm coil, so 18.5 Watts for a F15N coil is not out of the ballpark. If properly reconed, it should be a great-sounding speaker.