A few shielding questions

[BiBi]

Hello,
I am having a bit of a dilemna about shielding guitars at the moment. The problem started when a friend asked me to rewire his stratocaster for him. It is an 90's mexican version which he had shielded all the cavities with tin foil. When I finished rewiring it back to stock Fender style I was surprised, no really surprised at how quiet the guitar was. All single coils but very little hum when my hands were off the strings. So of course I wanted to do that to my guitars too...
Problem is that I have a homemade Telecaster that uses a humbucker in the neck position, p90 in the middle and a single coil at the bridge. I have tried to shield the cavities and then rewired it according to this diagram : http://deaf-eddie.net/drawings/strat-series-2.jpg
Strange, but now the p90 is dead quiet, and the humbucker and single coil are noisier than before regardless of the series/parallel setting.
Looking at the Telecaster, I can't understand how to isolate the bridge pickup from ground, or even if this is necessary? As well, the humbucker is not connected to the pickup guard but screwed into the body cavity. I tried to separate this from the shielding but to no effect on the noise.
My understanding of the shielding is that the pickups should be isolated and that the wire's shield should only be connected at one end.
Any help would be greatly appreciated.

[David Schwab]

Which pickups are being switched into series?

The thing to look out for is when you put two pickups in series, the wire that was once ground is now hot. That return wire might have been attached to the metal parts, like the baseplate or poles on the humbucker or P-90 pickup. If it is, it might now be hot when the pickup is in series. Then you get major hum.

The Tele bridge pickup has the steel plate on the bottom. That is often connected to the (-) on the pickup. So the grounds have to be separated from the pickup return wires.

If the humbucker is being switched into series, make sure it has 3 or 4 conductor cable. It can't be vintage shield and hot. The P-90 and Tele need to have the grounds separated from the commons also.

[BiBi]

Thanks for the reply.
The switching as I understand it allows for a variety of in-series options neck with middle, middle with bridge, all three together; however, the fact is that the bridge and neck pickups hum even in the "standard" wiring. Interestingly, the humbucker in the neck position and the tele bridge pickup both have shields (the humbucker only has two leads not four however). Originally the shields were connected to the ground in the control cavity. I separated those to provide shielding and because it prohibits the use of series connection.
I guess I have a difficult time understanding why the circuit really cares which is the hot and which is the negative wires? The signal is AC after all? Other than phase between the pickups how would it matter which end is connected where if separated from the metal body of the pickup?

[David Schwab]

If you are still getting humming, then you have something wired wrong.

So when you say you separated the shields, do you mean that the Tele pickup's bottom plate has its own ground wire and is no longer connected to one end of the coil? And for the humbucker, the metal baseplate (and maybe cover... don't know if it has one) is now not connected to the coils? You used multi conductor wiring?

The circuit doesn't "care" about the polarity of the pickups. The issue with series wiring is not having normally grounded metal parts becoming hot when in series.

The Tele pickup is going to hum because it's a single coil. The humbucker will only hum if either you have the two coils wired incorrectly, or you have a ground connected to hot.

Re check your wiring.

[BiBi]

Hi David,
Thanks for sticking with me on this one. I drew up a little diagram for myself and maybe it makes things clearer for you to see what I am doing wrong. The humbucker in question only has two connector wires presently-- I assumed probably incorrectly that it could be looked at as one coil now. Like the diagram states, the shields on the Tele pickup and the humbucker are connected only at the pickup not to circuit ground. Although, and this is one of the problems I am having in understanding this, if the bridge shielding is connected to the control plate, then the shield from the bridge pickup is also connected. Ditto for the humbucker. Perhaps I should disconnect these shields from the pickups and connect to circuit ground instead?
I still find it strange that the P90 pickup is dead quiet, I'm not sure whether to chalk that up to the shielding, phase reversal or dumb luck...

[David Schwab]

One thing I don't understand... you say "neck and bridge pickup shields are connected at pickup only". Are they not grounded? They need to be grounded.

Perhaps I should disconnect these shields from the pickups and connect to circuit ground instead?

Yes, that is what I was saying. They need to be disconnected from the pickup (-), and run to ground with another wire. The humbucker is not an issue, because one side of the humbucker is always grounded. But it has to go to grounded or it will hum. So you can leave the humbucker alone, but ground it!

The Tele bridge pickup will need to have the (-) end of the coil disconnected from the baseplate, if they are connected together. Sometimes they are, sometimes they aren't. The baseplate needs to be grounded, and that also grounds the bridge.

Then check the P-90. They have metal baseplates and the screw poles are grounded. If they are connected to the (-) of the coil, you have to separate them. But make sure all the shields are grounded.

Other things... don't worry about doing some kind of star grounding. You don't need that in a guitar. Solder your grounds to the backs of the pots, not the switch. The switch also needs to be grounded, and can also go to the back of the volume pot. All the pots' grounds should connect together.. from one pot to the other. Then run a ground from the pots to the output jack. Any foil shielding needs to be grounded to the back of the pots.

You say "output jack shield connected at jack only". What is the "output jack shield"? Is it shielded cable? The other end of the shield needs to connect to the ground plane, which is the pots, etc. You can't have the jack's ground not connected to the rest of the circuit. Don't worry about ground loops because they can't happen in a guitar because you have only one ground. That's a big myth spread around by websites like guitarnuts.

You say "cavity shields tied together at control plate separately" No, ground them all together and to the back of the volume pot, or any convenient ground (like the tone pot, etc.). Don't rely on the metal control plate to make ground connections. Use wires. You want all your shields to be like an enclosure. You are making a Faraday cage. You don't need to ground them separately.

So the hum is likely caused by improper grounds. I think you have grounds that are not connected together, or ungrounded shields.

[BiBi]

Ah o.k. I was trying to differentiate between shield and negative connection to make things clearer... A lot of good that did!
Anyways I took your advice and grounded the shields to the common ground on the switch-- I'm too lazy to un star ground the thing now!
Anyhow it works like a charm-- thanks a million for all the help!

[andy89]

Hey Bibi,

I have a shielding question as well and maybe you could help me out with it. I have heard of foil being used but is it really just house hold tin foil? Also, I have heard of a special kind of paint being used. So you know anything about this paint (what is is called, where I could get it)? Right now there are just empty wood cavities. How did you install your foil? Could I see a picture if possible. Thanks for any help you are able to give!

[BiBi]

Hi Andy,

Yes, I used household tinfoil. There is also copper tape which is meant to be better, but I couldn't find any. There is also spray conductive paint, but I can't imagine it being so much better. I used a rubber cement to attach the tin foil although I didn't do the pickguard which I think would need a spray glue or something similar to make the foil lay flat.
I looked at the guitarnuts site actually for help originally, which although David doesn't like their opinion of star-grounding probably has good information on shielding the cavities... Hope that helps.

[andy89]

Thanks! The guitarnuts site was a lot of help with the shielding questions. Thanks for turning me onto it!

[David Schwab]

Thanks! The guitarnuts site was a lot of help with the shielding questions. Thanks for turning me onto it!

Just beware that some of their info is flawed. There is no reason at all to use star grounding. Their premise is that it avoids ground loops, but grounds loops are impossible in a guitar because it only has one ground; at the output jack. You need two different ground potentials for a ground loop. There is no loop with one ground. Also they show to wrap foil around single coil pickups, but this will create a closed conductive loop, and you will lose high end.

You can use aluminum foil, but it's impossible to solder to it. I like the copper foil from Stew-Mac because the adhesive is conductive, so you don't need to solder two pieces when then over lap. Aluminum is actually better for shielding in some ways, and you can use the copper foil to stick to the aluminum and then solder to it.

I often use thicker aluminum foil from craft stores for shielding.

[Tom Phillips]

...Aluminum is actually better for shielding in some ways...

That's interesting. Can you elaborate or point me to a reference. I've been considering some shielding projects and have been wondering about the trade offs between Al & Cu.
Thanks,
Tom