Ground the poles? Ground to cavity shielding?

[gb156]

Hello everyone! I've been visiting here for a while and learning a lot. I can't imagine having trying to learn this stuff without the internet and helpful people such as yourself. Thanks to everyone for sharing their wisdom.

I've successfully made a dozen or so pickups and am happy with their sound, but I'm having some trouble tracking down the source of some hum (even with humbuckers). I know this topic has been beaten to death; and I've tried everything I could find on the subject. The hum isn't loud, but, it's rather annoying. So, I've got a few questions about my shielding/grounding method which I'm hoping some of you can help shed some light on what I'm doing wrong.

First, the pickups themselves. Are the pole pieces themselves supposed to be grounded? Humbucker base plates are grounded, and they contact the pole pieces, so, I'm thinking, yes.

Second, shielding the coil. After winding the pickup, hooking up lead wires and wax potting, I wrap it with electrical tape then with copper foil tape. However, where the copper foil tape ends overlap, I place another piece of electrical tape so the copper foil tape does not contact itself. Is that correct?

Third, shielding/grounding the control cavities. If the entire cavity is shielded (with copper foil or shielding paint) and the jack's ground lug (or jack body/mounting washer/etc.) is in contact with the shielding, is it necessary (or redundant) to connect ground wires from the potentiometers and switches to the jack ground lug if their respect grounds are in contact with the cavity shielding?

Thanks in advance for any help!

[David Schwab]

First, the pickups themselves. Are the pole pieces themselves supposed to be grounded? Humbucker base plates are grounded, and they contact the pole pieces, so, I'm thinking, yes.

Yes.

Second, shielding the coil. After winding the pickup, hooking up lead wires and wax potting, I wrap it with electrical tape then with copper foil tape. However, where the copper foil tape ends overlap, I place another piece of electrical tape so the copper foil tape does not contact itself. Is that correct?

Yes. Is that foil grounded? It should be.

Third, shielding/grounding the control cavities. If the entire cavity is shielded (with copper foil or shielding paint) and the jack's ground lug (or jack body/mounting washer/etc.) is in contact with the shielding, is it necessary (or redundant) to connect ground wires from the potentiometers and switches to the jack ground lug if their respect grounds are in contact with the cavity shielding?

It's not necessary, but I would. I would also have the backs of the pots daisy chained with ground wires. The mechanical contact to the foil is probably good enough, but that connection can oxidize over time, while a nice soldered connection will not.

[gb156]

Yes.

Yes. Is that foil grounded? It should be.

It's not necessary, but I would. I would also have the backs of the pots daisy chained with ground wires. The mechanical contact to the foil is probably good enough, but that connection can oxidize over time, while a nice soldered connection will not.

Thanks for your quick responses! I didn't ground the poles, so that's probably contributing to the hum.

Yes, the foil around the coil is grounded.

Not to open a can of worms, but, does having multiple ground paths create any issues?

[David Schwab]

Not to open a can of worms, but, does having multiple ground paths create any issues?

No. You only have one ground which is at your output jack. Everything else is pretty much at the same ground potential.

Here's how Bourns shows to wire grounds.

[gb156]

Thank you so much for your help!

[Corcia]

First, the pickups themselves. Are the pole pieces themselves supposed to be grounded? Humbucker base plates are grounded, and they contact the pole pieces, so, I'm thinking, yes.

Interesting!! so, if i understood properly, in a single coil pole pieces should be grounded? which would be the correct way to ground the six polepieces in a single coil?

[David Schwab]

Interesting!! so, if i understood properly, in a single coil pole pieces should be grounded? which would be the correct way to ground the six polepieces in a single coil?

Well, they should be, but most of the time they are not. The way it's commonly done these days is to stick some conductive adhesive copper foil to the bottom and grounding that. It's tricky to get a go0d connection on all the magnets though.

Another thing you should do is wind the pickup so that the start of the coil which is closest to the magnets is grounded. This way touching the magnets won't make a big hum like it would if the start lead was hot.

[Corcia]

thanks David!
that's very interesting!!
i've never grounded the poles and not even the coil! may I ask why the coil shielding has to be an open loop? maybe because otherwise it would be another coil but grounded?
by putting in place all this "tricks" the hum reduction is significative? consider up to noe i've only shielded the cavities

[gb156]

Well, they should be, but most of the time they are not. The way it's commonly done these days is to stick some conductive adhesive copper foil to the bottom and grounding that. It's tricky to get a good connection on all the magnets though.

Would you mind sharing an alternative to the adhesive copper foil? I tried the foil last night, and it worked about half the time, but I suspect it will not work at all eventually.

[David Schwab]

Would you mind sharing an alternative to the adhesive copper foil? I tried the foil last night, and it worked about half the time, but I suspect it will not work at all eventually.

I don't make pickups with rod magnet pole pieces, so that's all I can tell you. I did rewind a ken Smith Design Jazz bass pickups once, and grounded the plea this way. It just tok scraping any wax or lacquer off the magnets and rubbing the foil with a pencil. It was kind of hit-or-miss.

[big_teee]

The old P bass pickups sat on a ground plate, but don't know if the magnets actually touched the Plate.
Does grounding the magnets Make the P/U Quieter?
T

[David King]

The P pickup magnets don't touch the back plate because there is a foam rubber "spring" between the two.
Fender never made any attempt to ground magnets. The most effective way I know of is with conductive paint. Nordstrand uses a second piece of 1/16 forbon that he sandwiches over the copper tape forcing it tightly around the magnets where they stick out the back of the pickup. The forbon layers are held together with small brass screws. This seems to work if the copper tape doesn't get torn up in the process but it's still not 100% reliable. You could use a conductive foam or conductive gasketing material that has a fine metallic screen embedded in the surface to hold the pickup up and make a reliable connection between the magnets and the brass shielding plate at the bottom of the cavities. This gasket material is widely used and available from the Chomerics sample page if you want to experiment with it.
As I'm mentioned several times before here the most elegant solution would be to use custom double-sided PCBs with plated through-holes for each pole or magnet with a press fit.

[David Schwab]

I think conductive paint on the back of the magnets is a good idea.

[Corcia]

Why not using a copper wire to ground the poles? my idea would be to use a non insulated copper wire, wrapping a couple of turns around each pole ant then soldering the free end into the GND eyelet; do you think it might work?

[rjb]

Why not using a copper wire to ground the poles? my idea would be to use a non insulated copper wire, wrapping a couple of turns around each pole ant then soldering the free end into the GND eyelet; do you think it might work?

I'm no expert, but I wouldn't trust wire wrapped around the poles to maintain good electrical contact over time.

This magnet vendor says "Alnico lends itself to silver soldering operations in certain circumstances" (see the last paragraph):
www.duramag.com/alnico-technical-info.html

Has anyone here successfully silver-soldered wire to Alnico poles? I tried once, but with no joy... however, it was a BIG honkin' magnet- and I was working outside and had to quit when the rain started.

-rb