Hammond field coil speaker

[mike60510]

I pulled a speaker from a Hammond M-2 organ. The speaker uses a field coil rather than a permanent magnet. According to the schematic, the field coil is 700 ohm and the voice coil is 8 ohm. Additionally from the schematic, it shows 85-volt at the field coil. I hook up the 8 ohm leads to an amp and played through the speaker and was disappointed with the results. I'm assuming that the field coil would boost the volume.

So, to use the speaker in an amp cabinet, I would need a 85 volt power circuit. Please let me know if I'm understanding this correctly.

[J M Fahey]

You need to power the field coil, period.

Otherwise you have no magnet.

IF schematic suggests applying 85 volts DC to it, so be it.

Please post the schematic such as to minimize misunderstandings.

Also post some speaker pictures to see whether are other issues or you/we are missing something.

Sometimes field speakers had an extra "anti hum winding" and you might or might not have to connect it, depending of power supply quality.

[mike60510]

You need to power the field coil, period.

Otherwise you have no magnet.

IF schematic suggests applying 85 volts DC to it, so be it.

Please post the schematic such as to minimize misunderstandings.

Also post some speaker pictures to see whether are other issues or you/we are missing something.

Sometimes field speakers had an extra "anti hum winding" and you might or might not have to connect it, depending of power supply quality.

I didn't realize that this forum provided the ability to post pics and pdf's. Both are includedM-2 Schematic.pdf

[nosaj]

Here was my journey into the same thing But i made it an extension cabinet where you can vary the power supply effectively make something like a fluxtone speaker. Pretty cool once it was done.
http://music-electronics-forum.com/t39353/
Nosaj
PS also hope you kept the vibrato scanner , you can make a cool tremelo project with it.

[mike60510]

No I was not able to get the vibrato scanner. I only garbage picked the amp and the speaker. Just wondering if anyone has had experience using a speakers of that type and if its worth the effort.

The speaker have no manufacturer label only the numbers 285146 painted along the outer edge.

[nosaj]

No I was not able to get the vibrato scanner. I only garbage picked the amp and the speaker. Just wondering if anyone has had experience using a speakers of that type and if its worth the effort.

The speaker have no manufacturer label only the numbers 285146 painted along the outer edge.

To me it was, I got to learn something and have a speaker cabinet I could dial volume down without loosing power tube overdrive. Learning is always worth the effort.

nosaj

[mike60510]

Do you have the speaker mounted in a separate cabinet with power supply or did you design a circuit in your amp to power the field coil speaker?

[nosaj]

Separate Cabinet with the power supply in it. It is set as a 8 ohm speaker so I can hook it up to any cab that can use 8 ohm.

nosaj

[g1]

Have a look at fluxtone speakers to see how much money nosaj has saved with his similar "adjustable" speaker.

[J M Fahey]

Very cool.

So now it's clear that the field coil needs some 10W dissipation.

The Hammond schematic shows 2 versions: current one dropping 85V across a 700 ohm coil, or 100V (early version) with 100V across a 1000 ohm one, both 10W.

A 60V transformer will give you ~84V all day long; since current draw is small, a 24 or 30VA unit is fine, anything above just a bonus.

I'd use a 2200uF x 100V cap for filtering , and 4 x 1N5404 or a 4 A bridge pack, just for safety, all cheap parts, except maybe the transformer, but explore ??Apex?? transformers and their wide range of inexpensive toroids.

Add a 250mA fuse to the secondary side, just in case.

[mike60510]

How did you determine the 10W dissipation? I'm trying to get better at reading and understanding the schematic. Also, if you were to get a 60 volt transformer, how would you adjust the output to provide 85 volts?

Thanks!

[mike60510]

Sorry for the low level of knowledge, but you mentioned "adjustable" speaker? Can you help me understand why you said adjustable. Does this relate to changing the transformer output voltage or adding capacitors in the circuit?

[Tom Phillips]

Sorry for the low level of knowledge, but you mentioned "adjustable" speaker? Can you help me understand why you said adjustable...

Please let us know if you have read the thread listed in post #4 above. Also browse FluxTone Guitar Speakers to see a commercial product that uses variable voltage drive to a speaker field coil. Together these resources will give you a good idea of where the guys are going with this discussion. If you still have questions then we can add to the information. The variable voltage is not required to get your speaker working. It is a bonus if you wish to build a variable sensitivity speaker. That could give you a lot of bang for the buck.
Cheers,
Tom

[J M Fahey]

How did you determine the 10W dissipation? I'm trying to get better at reading and understanding the schematic. Also, if you were to get a 60 volt transformer, how would you adjust the output to provide 85 volts?

Thanks!

Power dissipated= Voltage squared divided by resistance , written as:
V*V/R or V^2/R

Inb this case Voltage is that present from end to end of the field coil and R is the DC Resistance of the coil (what you measure with a multimeter set to the Resistance scale)

* coil # 1 (what you have)=85V * 85V / 700 ohms= 7225 V^2/700 Ohms = 7225/700 W = 10.32 W

* coil #2 (early ones) : 100 *100 /1000 (W) = 10000/1000 W = 10W

There is no magic involved in amp design, if you learn Electronics basics.

Start by downloading Jack Darr's "Repair your Guitar Amplifier" as a very straight and useful introduction.

Did I say "no math? "

After that, the Sky is the limit

[mike60510]

If I was to try and use this speaker with an AO-35 conversion, where would be the best place to pull off voltage to power the speaker? Can anyone point me to a sample schematic to base a design on?

Thanks again
Mike