[cpollock]

I'm new here, and am enjoying this treasure of information.  I have been restoring a 37-8, replacing caps, trying to undo some previous "fixes", etc., and am learning a lot. It actually works, but is weak, so I hope to correct the weak links step-by-step.  
I am puzzled by the speaker.  Can anyone explain what the speaker circuit is doing?  The speaker itself is unusual, with a flex section both at the rim and one pinned at the center of the cone. I've never seen that before.   There is a big coil on the back of the speaker, but the output transformer feeds two small wires underneath that big coil,  into what I suspect is the actual speaker coil.  The big coil on the back of the speaker looks like the traditional speaker magnet, but it is not driven by the OT, so I think it must be the inductor shown on the schematic in series with the speaker.   But it isn't wired up consistent with the schematic, so I can't make sense of it. It has 1000 ohm impedance, while the OT has about 400 ohm.
What is that big coil on the back of the speaker do?  This obviously is a Philco speaker, so it must have been widely used.   There are three wires that feed the transformer, one from the +HV (350v) one from the plate of the 6F6G (~270 v).  That makes sense.  There is a third wire that goes to the big coil on the back of the speaker, and it is at about  70 v. 
It is a very unusual speaker, I've never seen anything like it.  Has anyone posted a description of this speaker and its driver? It seems to work, but I'd like to understand it.
Thanks!
Clif

[morzh]

Are you talking of the field coil?
All these early speakers used a DC electromagnetic coil instead of a Perm magnet.
The coil iften doubles as the rectifier filter.

[cpollock]

Well, that explains a few things. Thanks!  I was wondering where the choke was for the power supply, and why three wires were needed for the speaker. Clever stuff!

[EdHolland]

morz is correct, the big coil is the field coil.

The full +350V comes in and supplies the OP transformer for the 6F6. It is also connected to the field coil, and the other end of the coil feeds subsequent circuitry in the radio. This achieves 2 things rather efficiently.

1) The current to the rest of the radio is used to create the magnetism needed for the speaker.

2) The supply passing through the field coil's large inductance is extra filtered, providing a quiet B+ supply for the sensitive stages of the radio.

[morzh]

And the ripple through the coil that could cause hum in speakers is countered by an extra coil called humbucking coil which is a few turns coil that is wound around the field coil and is in series with the voice coil. It is shown in the sch as a small coil symbol in series with the voice coil.

[cpollock]

Thanks, I was wondering about that. It seemed ironic to be using the speaker magnet coil as a choke to reduce ripple for the B+ to reduce 120Hz hum on the sound, only to be sending that alternating current right to the speaker.
I am impressed by all the clever ideas that were developed to optimize these circuits.