[MrFixr55]

Hi David,

Is your goal to restore this radio or you gutting it to add new electronics? That is your decision point. If gutting it, it is much easier to add new speakers. That would likely be cheaper than restoring the original speaker. It is too much work to design a power supply (if you are not restoring the chassis) for the speaker. The field coil resistance is about 1100 Ohm, and likely have a voltage drop of between 50 and 100V. Building a 50VDC Power supply and putting the field coil across it would draw approximately 5 watts give or take. The original circuit works by passing all the radio's "B+" current through the voice coil. Between the current drawn by Filter Cap 11A (yes, caps do draw current when fed with pulsating DC) and the current drawn by the rest of the tubes (mostly the output tube but the others also add up), this current is what magnetizes the voice coil. Unfortunately, the voltages at Cap C45 and C11A are not stated on the schematic. Knowing these voltages will tell us the voltage drop across the coil and the power supply needed. This is why I am recommending subbing a permanent magnet speaker. This is also safer, as you are not bringing out high voltage from the chassis.

My take is to do double duty. Work on restoration, but if you have access to the proper tools, create a new sound board for the same size speaker but 2 smaller full range speakers, these preferably tilted out. The big speaker would be a subwoofer and the small ones, left and right channels. (Save the old one in case you want to go back to a pure restoration.) As you get along with actually restoring the chassis, you can either install the original speaker or "up the capacitors" and wire an 1100 Ohm 20 W resistor between the 2 filters, where the field coil would be. Whether using the original speaker or a new speaker, a god output transformer is needed when actually using the original radio chassis to drive the speaker. The additional stereo speakers will not figure into the original radio circuit; they are totally separate. The large speaker can be switched from whatever modern subwoofer amp is driving it and the radio chassis. This can either be done by a switch or by a relay activated by powering on the radio. The switching circuit should put some sort of dummy load across the amp not used by the speaker.

The internet is filled with various packaged stereo amps and "plate" subwoofer amps that work quite well. Unfortunately, like everything else, these are Chinese. I have used Pyle speakers and Kinter amp units as amps in various rooms (fed by "line level" audio distribution. The Kinters are only about $12.00 each plus power supply. and are supposed to be 25W RMS per channel, Class D. Pile also makes compact amps and receivers for a little more $$. Lepai makes units with FM Stereo built in, but I have not tried them. These units are unobtrusive, and most have mounting tabs. Some even have IR remote control. All are very compact. This stuff is all available on Amazon and eBuy.

This scheme gives you the "freedom" to modernize the unit, but leaving the chassis in for looks, and actually working on restoring the chassis and radio back to original (save all the parts removed) if you or a potential buyer wants it that way.

I will post more on this subject if Pholks are interested.

[morzh]

And. Modern speakers, let's face it, better than the speakers from that time, even the best of them, that sound very nice, like those from 37-116, 14" speakers, they sound really well, but still a modern speaker will outperform it.

That is, I mean, if, like Mr. Fixr theorized, you might be planning to gut the radio and make it into a modern sound device.

[MrFixr55]

Hi David,

More info on the wiring for your original speaker and how 3 wires are used for 2 circuits.

As stated in previous posts, the field coil doesn't have a power supply per se, but it is in series with the "B+" supply.  The field coil serves as a choke in the filter system of the B+ Power Supply.  The more current drawn by radio circuits, the more power is delivered by the magnet. 

There are 3 wires going to the speaker: green / white, white and green.  Looking carefully at the plug that is part of the speaker assembly, you will see that the white wire pin has 2 wires coming out of the same pin. one wire goes to the output transformer primary.  The other white wire goes to the field coil.  The white wire is the output of the field coil, back to the chassis. (See below).

The green /white wire is the "input" from the rectifier cathode and C45. 

The white wire is main B+ output going to the audio output transformer, but also back to the chassis and C11A where it is further distributed to the rest of the radio B+ requirements. 

The green wire on the speaker plug is the conductor between the output transformer and the output tube plate.

The secondary of the output transformer connects to the speaker's voice coil. You will see however, that there is another coil in series with the speaker voice coil. This is called the "hum bucking coil" and is used on electrodynamic speakers (speakers that have a field coil electromagnet instead of a permanent magnet field). This is shown in the second attachment, showing the Audio Output stage.

Attached below are 2 shots of the schematic where I hoped to filter out all of the mumbo jumbo and make this clearer. These shots show the field coil connections and the Audio Output transformer connections.  Hopefully this helps:
   
   

[DavidK]

I greatly appreciate all the time and effort you put in to my question!  Thank you!  It will take me a while to study my components now given your description, but I'm getting the feeling (which was my initial assessment) that I probably should persue modern speakers...which also has its challenges as I'd rather "harvest" them from someone's unwanted equipment instead of buying new.

I did purchase a $50 Pyle PCAU25P Amp so I could play around with Bluetooth (plus USB, Aux, and FM).  This (as everything "modern") is stereo so I've been pondering using two 6" mids even though it would "undo" the L/R stereo effect...or a 3.1 setup.  Right now I'm in the "analysis paralysis" stage trying to play "catch up" on a subject I've come to realize is a bit challenging for my aging brain!

My radio may have had some issue when I got it and my initial "investigation" may have made it worse, so it may be my "lessons learned" before touching my other two!  

[radiorich]

Hello David,
I agree with mike because you need to relize that speaker needs to be sealed between the front baffle mounting board and the gasket on the speaker .
keeping the sound produced from the back of the speaker to front of the cone.

Sincerely Richard