How much hum can a humbuck buck?

[loudthud]

I have a Gibson SG from the 70's. The original pickups were potted with what looks like black epoxy. With a higher gain amp, those pickups give an acoustic feedback because the top of the cover vibrates. I tried some other pickups, finally settled on a Duncan 59 at the bridge that had wax added under the cover. It doesn't feedback, but the hum is a little louder than one of the originals at the neck. Is this just an imbalance in the coils or something you have to live with? The Duncan pickup has the traditional single coax. The hum is silenced if the Volume control is all the way off.

Has anybody tried balancing coils with a trim pot?

[Chuck H]

Can't say I've ever tried a trim pot. It seems to me you would need to identify the higher output coil WRT hum (since you obviously can't ADD output to the lesser coil) and parallel something like a 1M pot across it. Makes sense. It would surely alter the tone of the pickup, but you know that.

I think most PAF type humbuckers (like the SD '59) are wound with symmetrical coils. The pickup guys will correct me on this if I'm wrong So I have to think that the '59 is a high-ish output for that type of pickup (or a least the bridge model) and that is the reason for the louder hum.

[Mike Sulzer]

Has anybody tried balancing coils with a trim pot?

.

The two humbucker coils in series resonate with the combination of their self capacitance and the cable capacitance. A trim pot will mess this up and alter the tone as Chuck said. You might end up with something you like, but I kind of doubt it.

A humbucker cancels (approximately) hum from magnetic fields. You still have the possibility that electric fields are the problem in some circumstances. This is not so easy to determine sometimes. With the wax under the cover, does it still have good electrical connection with the base? I suppose that you could test this by touching one end of a wire to the cover and the other to the jack or other good ground. I really doubt that this is the problem, but it cannot hurt to try.

[copperheadroads]

Has anybody tried balancing coils with a trim pot?

Isn't a Jazz Bass wired like that ? rw/rp
not trim pot but no switch & 2 volume & 1 tone pots .... rw/rp .
quiet when 2 coils are on at even volumes ,roll one off & you induce hum ..............
I would try to dig a hole through the epoxy of that pickup & wax pot it & hopefully it will be done .

[loudthud]

Isn't a Jazz Bass wired like that ? rw/rp.

I have a bass like that and couldn't stand it. Re-wired it with a Master Vol and a Blend pot. The Blend pot has a center detent and minimum hum is just slightly off center. But I don't play bass through an amp with 100dB of gain at full Volume.

I was hoping some of the winders here could offer anecdotal evidence of what it takes to get the best hum cancellation. I'm sure some of them get the exact turns count equal on both coils.

I would try to dig a hole through the epoxy of that pickup & wax pot it & hopefully it will be done .

No way I'd try that, but I did notice that the hum is lower when both pickups are on.

[Mike Sulzer]

Isn't a Jazz Bass wired like that ? rw/rp
not trim pot but no switch & 2 volume & 1 tone pots .... rw/rp .
quiet when 2 coils are on at even volumes ,roll one off & you induce hum ..............
I would try to dig a hole through the epoxy of that pickup & wax pot it & hopefully it will be done .

Why does wax prevent feedback but epoxy does not? Do not both prevent the cover from vibrating?

[copperheadroads]

Why does wax prevent feedback but epoxy does not? Do not both prevent the cover from vibrating?

Simply .....when a pickup becomes micro phonic .whether the cover or coil or other problems a fix would be to repot it .
so hopefully a dunk in the wax pot might get some wax in the trouble area ..& probably the simplest solution .

[J M Fahey]

Why does wax prevent feedback but epoxy does not? Do not both prevent the cover from vibrating?

Don't have it before my eyes but I suppose in one case the coils were epoxied by putting them inside some kind of ruber mold and epoxy was poured in; afterwards they were removed and a metallic cover was applied ... it can and will vibrate, while it's common to dip the whole assembled microphone in hot wax, cover and all, let it soak fully, even under vacuum, and then wipe excess wax off from the outside ... what you can't do with epoxy.

Also wax becomes very liquid at relatively low temperatures and it stays so for as long as you wish, while epoxy usually is thicker, even if warmed, and has a definite "pot life" before it gels, way before it actually hardens.

I guess that might explain the difference, they are not applied the same way.

[Mike Sulzer]

Don't have it before my eyes but I suppose in one case the coils were epoxied by putting them inside some kind of ruber mold and epoxy was poured in; afterwards they were removed and a metallic cover was applied ... it can and will vibrate, while it's common to dip the whole assembled microphone in hot wax, cover and all, let it soak fully, even under vacuum, and then wipe excess wax off from the outside ... what you can't do with epoxy.

Also wax becomes very liquid at relatively low temperatures and it stays so for as long as you wish, while epoxy usually is thicker, even if warmed, and has a definite "pot life" before it gels, way before it actually hardens.

I guess that might explain the difference, they are not applied the same way.

That makes sense; I was thinking of how I did it as a teenager back in the sixties: 1. Mill out cover from solid metal. 2. Turn up side down. 3. Put in pickup coil. 4. Dump in epoxy. 5. Pump out the air. 6. Wait 12 hours.

[big_teee]

You might try removing the adjustable bobbin screws.
Wax pot with the screws out.
Some wax may go around the bottom of the bobbin through the screw holes, and maybe some will penetrate at the top of the bobbin as well.
Then put the screws back in while the pickup is still warm.
I build my humbuckers with the coils well taped, then solder the cover on with the pickup seated against the cover.
Then I pot with the cover on.
IMO this gives a harmonic rich pickup with no Microphonics.
I've not had anyone complain with microphonics since I've been potting with the cover on.
I suspect the epoxy on the bottom is not a full saturation like hot liquid wax.
GL,
T

[loudthud]

First, I want to apologize to any wookchucks who may read this.

One of the original pickups is pictured below. I removed one of the screws and it appears that there is potting material between the bobin and the cover. The SD pickup is not completely potted, about 1 or 2 CC of wax was melted into the cover just before it was installed.

My concern is the hum. I use this guitar to demo my amp. It has tons of gain and is very low noise. I want to create an OMG moment when I flip the amp on and hit the first note.

[J M Fahey]

OK.

Those are 2 different problems.
To minimize microphonics/squeals you want everything mechanically strong, so flushing down to the latest cranny with wax is good.
Normal epoxy is quite thicker, but there is a special kind of Polyester resin which is both crystal clear, very liquid and you can vary accelerators so it cures in 12 hours or more; it's used to make those clear cubes where , say, a flower or a butterfly or some other delicate object is enclosed.
You might try it.

Problem with hardened rosins, Epoxy or Polyester, is that they harden too much and irreversibly, bad for the adjustable poles; wax is far more forgiving and, worst case, can be remelted.

Now for humbucking, you want both halves of the pickup perfectly balanced, so pay extra attention to that turns counter, it's not just filling the bobbins with as much wire as can fit ... but you guys already know that.

I guess coils must also be magnetically balanced, not sure how having solid slugs on one and adjustable ones on the other may affect that; I love experimenting and would put an unmounted humbucker close to, say, a transformer or a fluo reactance to induce extra magnetic hum and then adjust polepieces, I'm quite certain that will allow very fine tuning.
Any takers?

[Mike Sulzer]

OK.


I guess coils must also be magnetically balanced, not sure how having solid slugs on one and adjustable ones on the other may affect that; I love experimenting and would put an unmounted humbucker close to, say, a transformer or a fluo reactance to induce extra magnetic hum and then adjust polepieces, I'm quite certain that will allow very fine tuning.
Any takers?

Balancing the screws with the slugs is less important than you might think because of the open circuit. (As discussed earlier, there is a limit to how much the steel can do.) If you have a proper setup, you should be easily able to hear the hum increase as each screw is removed, but the effect of just a turn or two is pretty small in my experience.

Also, there is not much point in trying to achieve really good magnetic balance unless you are really sure you have gotten the electrical hum very low.

[Steve A.]

There was a post here 10 or 15 years ago that observed that in at least one case there was better humcancelling when a ceramic magnet was used, perhaps because the ceramic magnet conducted no current.


Steve A.

[J M Fahey]

I remember reading long ago (it must have been in this very Forum,where else on Earth is so much pickup knowledge and experience concentrated?) where "somebody" suggested wiring a few extra turns on one coil, rough mounting the pickup, connecting it to an amp and start removing turns from the overwound one until minimum hum was found.
Sounds like too much trouble for any kind of production model but might be justified to build *one* "shop reference" pickup, so you have a real world reference as to what to aim at and not go crazy trying to improve on it.