Understanding field coil speaker

[trem]

This is the first time i have looked at one on a schem. not sure if this is a common way of doing it or not.

How is this creating a magnet for the speaker.?
It is all located on the primary side of the OT.
Is the choke magnetizing the voice coil with DC.?

Is there a Video that explains the basics of this.?
I have read a few things but i am still somewhat confused.

Thank You

https://el34world.com/charts/Schemat...lco_510_1b.pdf

[Helmholtz]

The field coil acts as an electromagnet powered by the DC supply current.
At the same time the coil is used as a filter choke, reducing ripple voltage.

[g1]

The field coil is physically part of the speaker but separate from the voice coil. A coil wrapped on a iron core forms an electromagnet. The voice coil acts in a magnetic field whether the field is from permanent or electro magnet.
Like Helmholtz said, it can perform double duty by also being used as a filter choke. It is feeding the OT primary (and the other supply nodes down the line).

[trem]

Thank You Both ..... When a choke is also acting as a magnet for a speaker does it need to be rated higher.
In just round and arbitrary numbers....... Would you need a 6H instead of a 4H if your choke also needs to be a magnet.?

[Helmholtz]

When a choke is also acting as a magnet for a speaker does it need to be rated higher.

There might be some misconception.
The field coil is an integral part of the speaker. Its wire wraps around the center polepiece of the speaker.
So, when powered, the field coil is the speaker magnet.
Speaker efficiency depends on turns' number and DC current through the coil.

As the field coil (+ ferrous core parts) has inductance, it can additionally be used in place of a filter choke (within limits).

[mhuss]

The down side of the inductor double-use is that you'll have B+ DC voltage on two of the four speaker terminals; these are rarely marked or adequately shielded on the old amps that do this.

[trem]

There might be some misconception.
The field coil is an integral part of the speaker. Its wire wraps around the center polepiece of the speaker.
So, when powered, the field coil is the speaker magnet.
Speaker efficiency depends on turns' number and DC current through the coil.

As the field coil (+ ferrous core parts) has inductance, it can additionally be used in place of a filter choke (within limits).

I did not understand that basic concept..... Thank You

[Tom Phillips]

The attached document contains some nice reference info about field coil speakers. I saved this a while back when I was curious about some extra connections on a field coil speaker installed in an old amp I was repairing. Those "extra" connections turned out to be a humbucking coil as described in the document. The humbucking feature is not present on all field coil speakers.
Field Coil Speaker Components.pdf

[Justin Thomas]

So, these things have been a mystery to me for a while. Good thread. So the field coil simply replaces the permanent magnet in the speaker, and there's still an OT needed (or at least usually used)? And the OT is not "required" to be mounted on the frame?

Jusrin

[Helmholtz]

Good thread. So the field coil simply replaces the permanent magnet in the speaker, and there's still an OT needed (or at least usually used)? And the OT is not "required" to be mounted on the frame?
Jusrin

Yes and yes.
The field coil is just a big solenoid used to generate the necessary static or DC magnetic field. Everything else, like voice coil and OT are like "normal".
So one might say it replaces the permanent magnet, but actually it's the other way round as field coils were used before suitable permanent magnets were available.

Interesting to note, varying the field coil DC current varies the speaker SPL, so could be used as a sound attenuator.

[Tom Phillips]

...So the field coil simply replaces the permanent magnet in the speaker, and there's still an OT needed (or at least usually used)? And the OT is not "required" to be mounted on the frame?

Yes and Yes. I don't believe that field coil speakers were ever used in solid state amps so I'd say that an OT was always used with a field coil speaker. I suppose that someone could rig up a way to use a field coils speaker without an OT but it just wouldn't be commercially practical.
Except:
It is possible to energize the field coil with a dedicated power supply and one could then control the strength of the electro magnet with a variable power supply. This would provide a way to control the efficiency of the speaker. The goal being the ability to vary the overall sound level of the amp. There is a company that produced such a product but I don't think it caught on significantly. Probably because a single 12" speaker sold in the $800 range.

[The Dude]

You don't see field coil speakers in SS amps because, well before solid state amps came into being, it was cheaper to use a fixed magnet rather than an electromagnetic speaker. It would certainly be possible to build a SS amp with a field coil speaker, but why? A good part of innovation is cost driven. Of course, another reason might be that SS amps don't generally have the high voltages that tube amps do.

[trem]

So, why was the OT ... sometimes ... mounted on the speaker frame in older amps.?

[Helmholtz]

So, why was the OT ... sometimes ... mounted on the speaker frame in older amps.?

Why not?
They were small enough.
I think it was just convenient.

[The Dude]

Yep. "Why not?" is probably as good of an answer as any. They only thing that sucked about OT's mounted on speakers is the extra work to change a speaker.