MIMF

I am working on a Les Paul Special for a friend, and I've run out of ideas. The guitar has two P-90 pickups.

The owner reports that it USED to be quieter, but it developed a HUM at some point.

The hum is not the "usual" P-90 hum, in my opinion (to be fair, I've never owned a guitar with P-90s, so I have a limited reference). On a scale of 1 to 10 - where "1" is a humbucker-equipped LP, and "10" is putting your finger on the tip of your guitar cable - this is about a 5. The hum is not objectionable when preamp gain is low. But with modest gain, the hum makes the guitar unusable. When he plugs in to a DI at a gig, the sound guy immediately says "Hey, where'd that HUM come from?".

I've checked all the grounds with an ohmmeter. I've brought all the grounds to a single point on one pot. And I've replaced the shielded cable from the control cavity to the pickup switch, and to the output jack. Grounds are good on the pickup switch and bridge. Still the hum persists - no change at all.

The owner insists that the guitar used to have far less hum, so I'm ruling out things like cavity or pickup shielding. Nonetheless, I've experimented with some improvised shielding ... with no improvement in the hum.

I feel like I'm missing something. But what?

You may have already checked for these, but here are a few shots in the dark...
1) Does it still hum when plugged into a different amp, the guitar jack of a personal mixer, or a computer MI interface?
2) Does it still hum when all pots are turned down?
3) Even though the grounds measure out properly, is there any leakage from the jack tip to ground, other than the resistance of the pots and PUs?
4) Does the hum change drastically when no metal parts of the guitar are being touched?
5) Does the hum change drastically when the player is oriented away from interfering devices (CFL lights, computer monitors, Tesla coils, AC cables etc)
6) Is the guitar cable really old and/or really really long?
7) Does the hum still exist when plugged into a passive (transformer) DI rather than an active (op-amp) DI?

Any one of these might lead to an AHA! Good luck and keep us posted.

Have you checked the resistance from the pole-pieces to both ground and signal? Please post the results, I'd guess that's where your problem lies.

Alan:

1) Yes, it hums with various amps, in various locations. Conversely, I can plug my humbucker-equipped LP Studio into the same amp, and it is dead quiet at the same amp gain settings.
2) Turning either VOL control all the way down ELIMINATES the hum (with pickup switch in center position).
3) At output jack, measuring 3.9K from tip to ground, with VOLs full clockwise, p/u switch in center. Rotating either VOL control counterclockwise varies resistance to a high of 7.0K, then down to 0.0 ohms at full counter-clockwise.
4) Hum does NOT change when you touch bridge, strings, or p/u switch nut. Ohmmeter confirms a ground connection at both bridge and p/u switch nut.
5) There are minor variances in the hum when I move the guitar around. But at its best, the hum is still very objectionable.
6) Short cable, high quality. Also tried three different cables to be 100% sure. No change.
7) Hum level seems to be the same when plugged into an amp (I tried three), or when plugged into a passive DI connected to a mixer console / PA.

Greg:

Both pickups show 0 ohms from pole pieces to ground at output jack.

And 3.9K from signal (tip) to pole piece (ground).

I am starting to think that this is as quiet as these pickups can get ... but that makes the guitar nearly unusable. Maybe a noise gate is the only solution?

I'd say that there is something wrong with the wiring. Is it shielded wire, or single? It might be something that isn't up to par in that wiring. Can you post a photo?

Since the photo was taken, I have brought all of the pot ground to a single point - the pot closest to the output jack. At this point, I also tied in the jack ground, and the shield grounds from the pickup switch.

I also ran a new shielded cable from the output jack to the pickup switch, and tied its shield to the same ground point.

Brian, were those measurements taken with the pickup selector in the middle? And what value pots are they? 300k? If so, that seems about right.
It must be a cold solder joint, I can't think of anything else that would suddenly develop.

Would these pickups be reverse wound, reverse polarity? It's easy to check with a compass and easy to flip the magnets in one of them if not.
Have you checked the bridge/string grounding? I find that's at the root of the "suddenly got noisier" problem 90% of the time.
The other 10% of the time the hot and ground leads got reversed during a jack replacement.

OK on re-reading your test results, this detail seems significant:

2) Turning either VOL control all the way down ELIMINATES the hum (with pickup switch in center position).

Something is fishy there for sure. Does the guitars still play when one of these Volumes is all the way down??
Make sure the pot wipers (center lug) are connected to the selector switch sides and not the output jack.

After some hand-wringing, I finally soldered the shielded wire from one pickup directly to a 1/4" jack. Plugging the pickup directly into an amp results in exactly the same objectionable hum. I get the same result from the OTHER pickup. Correct me if I'm wrong - but this would seem to confirm that these pickups are just subject to hum.

I confess, I'm stunned at the LEVEL of the hum. It makes the guitar essentially unusable in a high-gain situation. If I try to play with and overdriven tube amp, the hum is overwhelming.

I'm going to re-assemble the guitar and let the owner know that there's nothing more I can do. It's a pity, because the P-90s sound great.

David - the VOL pot behavior is a result of the wiring scheme. The VOL pots are wired across the pickup outputs, and the wipers are wired to the pickup switch, then the output jack. So if EITHER pot moves the wiper to ground, with the pickup switch in the center, the output is effectively shorted to ground. And the hum goes away.

Thanks to everyone for your help. Greatly appreciated.

Ok, then I suggest that those pickups are perhaps poorly made, or wound too hot, or something. See if you can borrow another P-90 from someone and wire it to your amp. You don't even need to install it in the guitar- just see if it hums just as much. You could also take the guitar to any music store and compare its hum to other guitars in the store. maybe you need to get some better P-90s.

Brian, with the pickups disconnected from the rest of the circuitry, what is the resistance reading of the coil, and from hot to the polepieces? I still have a feeling that the windings have shorted to them. If that's the case then they could be re-wound, rather than them just being poor pickups.

If it's a Gibson and this is the first owner then it should be covered:

Your new Gibson instrument is warranted to be free from defects in materials and workmanship for the life of the original retail purchaser, subject to the limitations contained in this warranty.

The fact that either volume turns the guitar off tells me it's been modified so probably not covered.
Pop the PU covers off and determine what they're wound with and who made them in the first place. A P-90 pickup is sort of the model T of pickups, very easy to rework and hard to imagine what could have gone wrong. Internal shorts within the coil will give you low DC resistance readings so that would be helpful to establish. If it's clear that someone already rewound them then there's nothing lost it doing it again correctly with the right gauge and insulation type for the period.

David, I know that you work mostly on basses, and basses are usually wired so that the volume controls work independently, if the instrument has more than one. On Gibson guitars like these, it is normal to have the volume controls work the way Brian describes. When the switch is in the middle, each control will turn the whole thing down.

4) Hum does NOT change when you touch bridge, strings, or p/u switch nut. Ohmmeter confirms a ground connection at both bridge and p/u switch nut.

This doesn't make sense.

I'm starting to suspect the amp.

Mark,
Thanks for straightening me out on the Gibson volume wiring. I never would have suspected that but I've not played a Gibson guitar enough to notice.

Brian -
What finally happened with this guitar?

AP