DeArmond "Guitar Mic" Resoration

[guitician]

How hot do I wind it? / Magnet Polarity?

OK, I've got the wire and I'm prep'n the bobbin for wire. I was thinking about how much wire to put on the bobbin. I've looked on auction sites for "Guitar Mike" pickups and saw one the was around 12.5K ohms. That seems like a lot of winds. I bought the .045mm wire, so I may be able to get that high of an impedance, but what is optimal? Also, I've got some Neo magnets to charge the alnico. Does anyone know how the seperate magnet pieces are polorized? Is it determined by the materials natural grain? It seems that they charge better in one direction, north/south along the thin side, rather than the broad side. Since the wire is wound around the thin side, it would "couple" to the wire better, when there are more lines of force along that edge.

[David Schwab]

OK, I've got the wire and I'm prep'n the bobbin for wire. I was thinking about how much wire to put on the bobbin. I've looked on auction sites for "Guitar Mike" pickups and saw one the was around 12.5K ohms. That seems like a lot of winds.

You are forgetting they used very thin wire, so it has higher resistance per foot. You can't use DC resistance as a gauge of how many turns there are. You can easily get to that resistance with like 8,000 turns of a thin wire like 45 or 50.

[guitician]

Correct, DC ohms is useless if the gauge is not known. I have the same gauge wire that came off the pickup. I guess I can just put as much wire on as will fit, and go from there.

[guitician]

Magnet Polarity

Spence, I photoshop'd your note on this pickup showing how I have the magnets polorized. The smaller end piece wants to be the reverse of the longer one, so they aren't opposing each other. I'm guessing this is how they were originally. I wanted to leave the assembly taped as they were, but the old tape broke and they came appart. The 1/4 inch edge is north/south. Is this even an issue, since there will only be one pickup on the archtop anyway, the "out-of- phase" thing won't matter. Thanks

[David Schwab]

Spence's drawing shows the North pole was up, not on the side. You have to have them both pointing up, not sideways, and not two different directions.

The magnetic pole has to face the strings.

[guitician]

If theres' no pole pieces, does it matter?

I was thinking, if the magnet itself is all that is sensing the strings vibration, what differance does the North/South polarity make in the functioning of the pickup. The induction pieces may help with the opposing field between the two magnets, if they are kept in the same orientation. It seems that as long as there is a field where the strings vibrate, the coil will pickup the vibrating string. Or is there something I'm not seeing? Thanks

[David Schwab]

The magnet is the pole... the magnet has poles. Think about Fender pickups. The poles are magnets with the poles all facing the strings the same way.

With a humbucker the two poles of the magnet are facing the steel poles which face the strings. And the coils are surrounding each pole.

If you turn the magnet sideways half the coil will be in the north half of the field, and the other will be in the south. In the middle there is no field. Plus you have the other half of the magnet reversed.

You may get sound, but not much. Spence showed how to orient the magnet.

[guitician]

North/South in the same coil cancel out then

OK, I see how having a reversed polarity magnetic field in the same coil would induce opposing currents that would cancel out a common signal. I guess I'll try to recharge the magnets with North/South along the broadsides. Thanks

[David Schwab]

OK, I see how having a reversed polarity magnetic field in the same coil would induce opposing currents that would cancel out a common signal. I guess I'll try to recharge the magnets with North/South along the broadsides. Thanks

Why do you have to recharge them? Did you turn them on their sides, or did you recharge them that way?

Keep in mind that certain magnets have an orientation, and that's the way you should charge them.

Why not just turn them back the way they belong?

[guitician]

The original alnico seemed weak. The larger magnet was half as strong as the smaller one. I used two Neo blocks to charge them. It's something that I've never done before. I just moved them around in the strong field between the two Neo's untill they were strong again. When I checked the N/S orientation, that's what I had. I guess I just need to orient the magnets in the field differently. The actual placement of the magnet in the pickup is always going to be the same as original.