Conductive Paint for Inside of Head Cabinet?

[The New Steve H]

I decided to build a head cabinet for an amp I made. It's nearly done. I just put the crocodile vinyl on it.

I'm thinking I might want to shield it a bit. Last time I did something like this, I cut a sheet of aluminum and stapled or glued it to the bottom of the inside of the cabinet. I forget which. The chassis sat on it.

This time I'm thinking of slopping conductive paint around in there. The chassis is heavy aluminum channel, so only the bottom and two ends are open. It would sit on the conductive paint, apart from tiny gaps. I could rig something up to close the ends up.

Good idea or stupid? I probably have some sheet aluminum around here somewhere, but I always like to try new stuff.

I assume tiny gaps are not a problem in what is essentially a Faraday cage, right? The vinyl wraps around the bottom of the cabinet in front and would actually prevent the front of the chassis from making conductive contact with the bottom of the cabinet, but the gap would only be like 1/32" or so.

[xtian]

No help at all, IMO. Read up on Faraday cages.

[Phil_S]

The simplest thing to do is smear some glue on the bottom of the cab and apply a pre-cut sheet of heavy duty aluminum foil -- yes, the kind you find in the kitchen. It will be about as thick as the paint without all the bother of paint (odor, overs pray, dry time, etc.) This is what I do. It works as well as anything.

[xtian]

Well, my point for Steve is that it won't provide any protection from stray radiation unless the cage is without holes, and all the parts of the cage must be electrically connected.

[sluckey]

I like to use 3" wide aluminum duct tape.

[The New Steve H]

In college, what I was taught about shielding enclosures was that the holes have to be several times smaller than the smallest EM wavelength that you want to keep out. The wavelength of 10 KHz is somewhere near 1.5" so I would guess that the tiny slits I would be dealing with would be too small to cause any real problems, especially with 60 Hz noise, which has a gigantic wavelength.

But then I am not a shielding expert.

[TUBEDUDE]

Electrostatic and electromagnetic fields need different strategies to effectively eliminate noise. Normal shielding is for electrostatic fields. I'm guessing Mu metal is still the frontline strategy for attenuating electromagnetic fields.

[The New Steve H]

The Faraday cage, usually made from wire mesh, is the traditional tool for blocking EM waves (time-varying fields). I don't know anything about static fields. I did not realize they could cause tube amplifier problems. I'm not planning to block every possible field; just things I can reasonably expect to make noise or whatever.

I looked the answer up on a physics forum, and it looks like the little gaps in my enclosure will not be a problem. I was too lazy (still am) to do the wavelength calculation using a pocket calculator, so I used an online calculator and got something like 1.3" as the wavelength for 10 KHz. Turns out it's more like 1.2 million inches (oops), so the gaps can be pretty big without letting stuff in.

If things obey Internet theory and not some crazy condition imposed by reality.

[JMFahey]

Even a single sheet of aluminum at the bottom works wonders, apparent gaping holes at ends won't really hurt unless interference is not exactly aligned to profit from in-out holes.

RF shielding is different, specially if a high power generator is lurking inside, because RF wavelengths are tiny, and slots will be .... weeelllll ....... "slot radiators" or "slot antennas" , a well researched subject, but at any audio frequency they are too small.

[img:272:317]http://electriciantraining.tpub.com/141 ... _163_1.jpg[/img]

[The New Steve H]

I remember one of my physics profs in grad school saying he refused to be in the room with a microwave oven because he thought the gap in the door could function as an antenna.

As I recall, he had concerns about irradiation of the man parts.

[Structo]

That paint is pretty expensive isn't it?

I usually just buy a sheet of aluminum flashing from the Depot.

Then either glue it down or staple it down.

Aluminum foil will work but is less durable.

[The New Steve H]

I don't remember what I paid for the paint. I got it a long time ago, to use on a guitar I never finished!

[Leo_Gnardo]

I assume tiny gaps are not a problem in what is essentially a Faraday cage, right? The vinyl wraps around the bottom of the cabinet in front and would actually prevent the front of the chassis from making conductive contact with the bottom of the cabinet, but the gap would only be like 1/32" or so.

would actually prevent the front of the chassis from making conductive contact with the bottom of the cabinet

Unless/until that bottom conductor IS electrically attached to the rest of the box it's not a Faraday cage / gaussian surface / iow doesn't count as shield.

You can get away with some gappage as long as that bottom cover IS grounded to the rest of the box.

[pdf64]

The Faraday cage, usually made from wire mesh, is the traditional tool for blocking EM waves (time-varying fields). I don't know anything about static fields. I did not realize they could cause tube amplifier problems. I'm not planning to block every possible field; just things I can reasonably expect to make noise or whatever.

Shielding with non-ferromagnetic materials only acts on electric fields; magnetic fields should be pretty much unaffected and pass through.

Thanks for questioning it; I too have used the term electrostatic when referring to electric.

I've got a Fuchs head cab here, and it's got Nashua 324A aluminum tape to screen off the inside, where the open face of the chassis sits.

[The New Steve H]

Shielding with non-ferromagnetic materials only acts on electric fields; magnetic fields should be pretty much unaffected and pass through.

That's not really right. An electromagnetic wave has two perpendicular components. One is an electrical field, and the other is a magnetic field. A Faraday cage will get rid of the whole wave, not just the electrical part. The magnetic part can't go on by itself.

A magnetic field that isn't part of an EM wave is different, but I'm not worried about magnetic fields that show up unaccompanied, such as fields from magnets. I'm trying to block things like 60 Hz hum from overhead lights.

Unless/until that bottom conductor IS electrically attached to the rest of the box it's not a Faraday cage / gaussian surface / iow doesn't count as shield.

It would be grounded except for the thin gap under the front, which would be way smaller than wavelengths corresponding to audible noise.