03-04-2023, 09:25 AM
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.
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.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
