Adding shielding to amp chassis with an open side?

[Rhodesplyr]

I've seen a few older guitar amps where people have added metal window screen to shield a side of the amp where the chassis was originally open (as far as EMF is concerned).

How does window screen stack up to a continuous sheet metal for hum reduction? I know someone who does sheet metal fabrication, so I could use either method.

Thanks!

[Enzo]

Window screen works, one of the enginers can probably speak to it better, but I think the mesh size compared to the wavelength to stop is the operative factor of effectiveness. At anything remotely a concern for our amplifiers, the mesh is opaque.

ALuminum foil glued to the cab works, screen stapled to the cab works, metal sheet works. Peavey uses a conductive black paint. What is also important is making contact electrically between chassis and whatever. Ever see a chassis and along one edge there will be a small machine screw screwed through a hole that otherwise doesn;t secure anything? The point of that screw is that the head will bear into the screening to insure contact.

[doctor]

Window screen works, one of the enginers can probably speak to it better, but I think the mesh size compared to the wavelength to stop is the operative factor of effectiveness. At anything remotely a concern for our amplifiers, the mesh is opaque.

ALuminum foil glued to the cab works, screen stapled to the cab works, metal sheet works. Peavey uses a conductive black paint. What is also important is making contact electrically between chassis and whatever. Ever see a chassis and along one edge there will be a small machine screw screwed through a hole that otherwise doesn;t secure anything? The point of that screw is that the head will bear into the screening to insure contact.

There is good explanation of the shielding effect: see Wikipedia,
title “Electromagnetic shielding”.

Conductive grid or paint and aluminum foil are effective against high
frequency electromagnetic field having mostly electric component.

High permeability material is effective against low frequency
electromagnetic field having mostly magnetic component.

In particular it’s clear why microphone transformers are always into a
permalloy box (power transformers may be shielded by conventional steel).

[pdf64]

To stop magnetic hum, a ferrous type material, or special magnetic alloy such as Mumetal is required.
Other types of conductive materials will help to cut down electrical interference. However, complete screening doesn't seem to be necessary; leaving one face unscreened may not cause a problem, screening a previously unscreened face may not result in any immediatley perceivable benefit.
Pete.

[Bill Moore]

I asked this very question to a fellow who is a Fender encyclopedia. I've never owned an original blackface, and have newer amps with the thick foil. He said Leo used the same aluminum screen that you can buy at Ace.
Thanks Enzo, for the purpose of the screw that holds "nothing".

[Chuck H]

As noted above, aluminum foil glued to the cabinet box works well as long as there is an electrical connection. Also noted, this electrical connection can be made by a screw "that doesn't secure anything". Just a construction tip... I use heavy duty aluminum foil and rubber cement. I try o keep the foil as flat as possible. As much for good shielding (smooth is supposed to be best) as durability. If you get good contact to the wood with few wrinkles the HD foil is very durable.

To further the discussion... I know there are magnetic advantages to ferrous metals. Aren't there supposed to be particular advantages to copper??? You can get copper foil and screen pretty easily. Any advantage? And is aluminum inferior to both because it's both non ferrous AND not copper? I think I read that copper doesn't even need to be electrically grounded to block EMF.

[Rhodesplyr]

The only thing I can add here from personal experience is that Hammond used two types of shields for the underside of organ (B-2, C-2, B-3, C-3, A-100, etc...) preamps. The more common type was foil-coated cardboard, and the other was steel, the steel shields being used in cases where the preamp was hung upside-down. Of the two, the steel gives a lower noise floor. The steel shields were used in upside-down preamps because the paper shields could sag and short to the turret-board terminals.

[Enzo]

I don't worry about magnetic shielding. The hum and noise amps like to pick up are largely radiated electrical noise, like when you bring your finger near a grid pin.