Bucking Coil Polarity - Printable Version
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Bucking Coil Polarity - RGK20m3 - 09-22-2025
Anyone who has recommendations on how to determine the polarity relationship between the voice coil, the bucking coil, and the field coil in a field coil speaker?
I rewound the field coil due to corrosion and was unable to determine the direction wound, when completed I connected the voice coil red to the bucking coil tan, which left the voice coil tan and bucking coil red, which were connected to their original output transformer terminals. Everything seems fine and there is zero hum at the speaker with the volume control at minimum, which would indicate to me the correct polarity relationship as the field coil ripple and audio stage hum are constant regardless of the volume control position.
I did notice that there is some hum present at high volume settings such as when tuning in a really weak station at night. Only noticeable above about half rotation…
Normal, or should I try flipping polarity of one of the coils? Seems it wouldn’t matter which one….am I overthinking or missing something?
RE: Bucking Coil Polarity - radiorich - 09-22-2025
Hello RGK,
Connected one way the hum-bucking coil will reduce hum, the other way it will increase it! There will always be some residual hum.
Sincerely Richard
RE: Bucking Coil Polarity - RGK20m3 - 09-22-2025
As I stated, there is zero hum when volume is down, only above half volume can you hear it. Does this sound normal, or should I try swapping coil wiring….
RE: Bucking Coil Polarity - MrFixr55 - 09-22-2025
Hi RGK,
What make / model is the radio? Did it play before you rewound the filed coil? Was the hum there then?
It is quite possible that the hum is from another source. With the radio off, carefully check lead "dress". Ensure that the audio leads and coupling caps for the volume control are as far away from the power cord as possible. Make sure wiring for the tube filaments are as close to the chassis as possible and other coupling caps and resistors are as far away from the filament circuit wiring as possible.
If the 1st Audio tube has a grid cap, adjust the cable so that it as close to the chassis or tube shield as possible. Some sets, especially some Philcos have that cable run inside the tube shield.
Further troubleshooting can be done. If this is a transformer powered set, unplug the IF tube. Is the hum less? if so, then the hum is coming from before the audio stage. Reinstall the tube and unplug the converter. Compare hum level to the previous test.
Hope that helps.
RE: Bucking Coil Polarity - Arran - 09-22-2025
I don't think that the phase (not polarity) of the field coil winding is a factor, it's what they call "bulk wound" and it's two main purposes are as a choke, and as an electromagnet, and has 100s of turns going from right to left, and back again. The hum bucker (some schematics mark it as Hum Neut coil) is normally connected in series with the voice coil, but out of phase with it, not every field coil speaker has one though, it looks like the Philco 42-380 does not, although it may have but may have been left off the diagram. If there is minimum hum at low volume I think that the hum bucker is connected correctly, the hum at high volume is likely coming from the audio amp itself and not the power supply.
Regards
Arran
RE: Bucking Coil Polarity - morzh - 09-22-2025
Humbucker is simply wound opposite to Voice coil; the Field coil does not matter.
RE: Bucking Coil Polarity - tbone - 09-22-2025
When in doubt reverse the connections and see how it affects the hum. More hum means it’s wired incorrectly. You could put a scope across the voice coil to see the difference.
RE: Bucking Coil Polarity - RGK20m3 - 09-22-2025
I thought about hooking up the scope; one trace from the bucking winding, the other from the field coil to see if they are out of phase.
And yes the coil polarity would change the N and S of the pole, “right hand rule” and all that. I would not think it much matters if the cone moves outward or inward on a positive pulse though. Even though the coil is random wound, all the turns are in the same direction. I believe that reversing the field coil wires would also change the phase of the ac component induced in the pole.
I have about 2.6v of 120hz at the output of the field, and about 4mv at the bucking coil, which seems to totally eliminate any hum from the speaker with the volume control at minimum.
Radio is a Philco 38-7XX with a H31 speaker.
RE: Bucking Coil Polarity - RGK20m3 - 09-22-2025
The other observation is that the voice coil is wound in a conventional single layer parallel to the pole piece, but the hum bucking coil is a flat ribbon conductor wound in a single layer radially ( at 90 degrees to the pole piece). Must have known something I haven’t figured out.