making conductive paint??

[ndog]

Yeah, 60Hz is a real pain to shield against. But there are two related things going on here, EMI and magnetic shielding.

Typically you want a conductive, grounded shield for 60Hz, RF, and similar. This is what is done with the copper tape, right? Does anybody not ground the shield?

Even the Stewart-MacDonald conductive paint description mentions connection to a ground.

Has anyone used copper or brass screen pushed into the cavity. It might work if it conforms to the shape and would be easy to ground.

Speaking of ground, where does the ground connection actually ground, through the amp cable?

[Bob411]

Yes ground outer shield of pickup wire, through control cavity, becomes the outer shield of the instrument cable, to amp. If you have a tremolo, it branches off to ground the "claw spring holder thing" on the back, so your strings are ground too. When you are done , and put the volume pot in the guitar, the outside is going to touch the copper tape, so that gets grounded. So mine is grounded, but it wasn't by design.

On the guitar I made, I bought copper tape. Was too cheap to buy the paint, which seams so much easier. Anyways I remember from shopping, some of the tapes state that the adhesive is conductive, to make the shield you are making "one whole thing".

No idea why one piece of tape needs to conduct to the next piece of tape.

[pjackson92]

http://cgi.ebay.com/Guitar-Conductive-Shielding-Paint-Acheson-Electrodag_W0QQitemZ230396449458QQcmdZViewItemQQpt ZGuitar_Accessories?hash=item35a4b2d2b2#ht_720wt_1 075

[ndog]

Thanks Bob411, that makes sense. As for the tape to tape conduction, electrically you would want it to make as seamless a shield as possible.

pjackson92 - That looks like good stuff, fairly low resistance. I wonder how that compares performance wise to copper tape or something with very low resistance.

[stratotak]

For the guys who said they have some of that krylon magnetic primer ordered..After you have tested post back if the stuff works..

[mrbrown49]

Originally Posted by Bob411

No idea why one piece of tape needs to conduct to the next piece of tape.

Shielding isn't about making a barrier or force field that noise can't penetrate. The noise isn't blocked and it doesn't disappear. It's redirected to the circuit ground by the shielding. If the shielding doesn't conduct to ground it is entirely ineffective.

[customtele]

Copper screen? I have miles of it! Being hopelessly inflicted with packrat syndrome can be a good thing. I'm getting ready to assemble my camp guitar, but it's getting humbuckers so I really don't need shielding. Plus the pots go thru the wood. The copper screen would make it so thick I wouldn't get a nut on the pot shaft. But this is worth a try on something like a strat cavity.

[pjackson92]

Originally Posted by stratotak

For the guys who said they have some of that krylon magnetic primer ordered..After you have tested post back if the stuff works..

yeah same here i need to know if it works so i can use it on my strat build next summer

[Bob411]

Originally Posted by mrbrown49

Shielding isn't about making a barrier or force field that noise can't penetrate. The noise isn't blocked and it doesn't disappear. It's redirected to the circuit ground by the shielding. If the shielding doesn't conduct to ground it is entirely ineffective.

I didn't think it disappeared. I just thought it would bounce the other way. Like the satellite dish bounces the waves to the head of the dish.

Anyway, you are right. I'm still trying to understand this. The Faraday cage is like a bathtub, and the ground is like a the drain?

Here is Wikipedia, about Faraday cages. At end of first paragraph, also see electromagnetic shielding.

http://en.wikipedia.org/wiki/Faraday_cage

I know I'm wrong, but I still can't wrap my brain around why it can't just be blocked. Why does it need to be redirected?

[customtele]

I think the key word is dissipation. You can't get rid of an electrical charge. It has to go somewhere. If my brain is wrapped around the whole concept you don't really generate electricity. You merely gather it up in a single place by using a generating device. Same type of thing.

[ndog]

Having read through that link and a bunch more, it looks as though in principle a Faraday cage does not need to be grounded.

To me, not ground a shielded circuit seems counter-intuitive.

[WRGKMC]

"Having read through that link and a bunch more, it looks as though in principle a Faraday cage does not need to be grounded.
To me, not ground a shielded circuit seems counter-intuitive".

The guitar interior is grounded for the same reason the guitar wire is grounded. If it isnt, the hot wires acts like an untuned radio antenna and picks up the strongest EMF (Electro Magnetic Force) (Radio Wave) around you which is the AC frequencues passing through the air. These waves strike the ground shielding and get harmlessly carried to earth.

In other words the shielding prevents the EMF waved from getting to the Hot wires and generating an AC noise signal.

Whats key to remember here is what generates a signal isnt magic.
You either pass a wire or coil through a magnetic fields or the field through the wire.

You can visualise it this way. Think of wire sticking up out of a pool of water. With the water calm, theres no movement. Create a wave by throwing something into the water. The waves will lap against the wire up and down. Thats the action of an AC wave generating a signal in the wire.

Next, put the wire in a pipe and have it in the pool. The wave will lap against the pipe not the wire inside the pipe. The pipe is of course grounded and prevents a signal being generated inside the wire.

This is the other key item. It takes a MAGNETIC WAVE to generate a signal. Once the MAGNETIC WAVE hits the grounded pipe it changes from a magnetic wave that passes through the air, to an ELECTRICAL SIGNAL.

Lastly a wire conducting a signal does generate a magnetic field, bit its generated around the conductor not within. If you read up on Flemmings right hand rule for generating current you'll see the wire within is parallel to the ground and within the core so the EMF has nearly no ability to generate a signal in the core wire.

http://en.wikipedia.org/wiki/Fleming's_right_hand_rule

Oh unless a fariday cage is grounded, the EMf passes right through it. We're not talking about static electricity. Wire mesh can block EMF as well when its grounded but the holes have to be the proper size to block EMF. In the case of shielsing a guitar we want to block ALL EMF which is a wide spectrum, not just microwaves which is a specific diameter and therefore can have holes because because its blocking only a specific frequency of radiation.

[ndog]

Empirically, I fully agree that the shielding should be grounded.

My understanding of the Faraday cage is that it works because of charge redistribution creating canceling fields in the interior. That's why several sources claim the interior is shielded regardless of whether it is grounded. Several sources cite the example of ungrounded foil lined shopping bags being used to by shoplifters to steal items with RFIDs without setting off sensors.

As far as shielding against magnetic fields the standard procedure is to use a Mu metal enclosure. The Mu metal reroutes the magnetic lines of flux within the enclosure, they never make it through the enclosure. The Mu metal enclosure would not need to be grounded. But how big of an issue is magnetic interference in an electric guitar? It's rare to find an application that requires magnetic shielding.

[WRGKMC]

Well you're not worried about high voltage fields no magnetic waves. The pickups have magnetic waves that expand out quite a ways from the pickups but they are steady state, not alternating like a radio wave is.

What you want to do is simply shield against radio waves. They are mostly untuned radio waves but AM and other emf noise from motors, CB radios, remote controlls, CRTs, cellphones can all generate noise in an amplifier circuit. If you were able to see how many radio waves that pass through the body every second it would blind you. The heaviest waves are AC because all the wires in a building act like radio transmitters. The Ground also conducts half the waveform as well.

Since a 60Hz radio wave falls in line with the audio spectrum between 20 and 22Khz. The 60hz hum and its overtones are the most prevelant amplified in unshielded circuits. There are other radio waves that can be amplified as well. I have a strat at home that can pick up local AM radio stations quite well. I could get rid of it completely if I wanted with a few tricks.

I could install shielded wire to the pups or use some iron chokes etc. To isolate the area or the frequency you simply power up an RF generator used for tuning radio coils and sweep through the spectrum and figure out where the hole in the shielding is and what the peak window for the emf is using a probe. You can either design a tuned circuit to null the offending frequencies by putting them out of phase or you can just try iron chokes in different areas till you minimise the output.

I have mine wired for multi phasing and want to keep it single wires with the TX Specials in there. The Gold anodized pickguard gets rid of most of the hum so the radio stations are kind of cool in my case. They arent heard unless you got all the overdrives kicked up and listen hard. Dont do that very often though. they usually get shut off between songs.

[Bob411]

Thank you for the explanation. Also thanks for the heads up about the " If you were able to see how many radio waves that pass through the body every second it would blind you"

Not only do I need a tinfoil hat, but I need it grounded.

[customtele]

Bob, the only reason you'd need a tinfoil hat is if you hum.

[WRGKMC]

Ha thats funny bob.

Believe it or not, I used to have a grounded bunker when I used to build radios as a kid. I had a section of basement as a with all of the walls coated with alliminum foil and grounded. Then I'd run my short wave antennas out and the whole deal. Guess you could say the whole room was a fariday cage.