sheilding properties of brass

[Groover]

Does brass provide the same electrical/electronic shielding effect as steel or aluminum, in the case of chassis construction?

[R.G.]

Does brass provide the same electrical/electronic shielding effect as steel or aluminum, in the case of chassis construction?

The shielding qualities of a metal depend on its conductivity and permitivity. IIRC brass's conductivity is less than aluminum's, more than steels, for the same thickness. But it's never the same thickness. Aluminum is the best conductor of the three, and usually used in thicker sections. Brass is way more expensive than aluminum, so in commercial applications the minimum thickness is always used. Steel is a much worse conductor than aluminum, but stronger, so thinner sections are used. But steel is the only one of the three which has a significantly higher permeability than free space, so it offers better shielding from low frequency magnetic fields.

So - shielding? Yes. Exactly the same? No. Good enough for most purposes? Yes. Affordable in big quantities? No.

[Diablo]

Steel comes in two broad types for a chassis - plain carbon (typically chrome plated) and stainless steel. Most stainless steel sheet is austenitic SS. This means it's non-magnetic, and has very low permeability, so it won't be a good magnetic shield, and is roughly comparable to aluminum and brass for magnetic shielding. Plenty of amps have been produced with an aluminum chassis, so magnetic shielding is obviously not a necessary criteria for a tube amp chassis. If a magnet sticks to your chassis, it is a magnetic shield.

[R.G.]

Steel comes in two broad types for a chassis - plain carbon (typically chrome plated) and stainless steel. Most stainless steel sheet is austenitic SS. This means it's non-magnetic, and has very low permeability, so it won't be a good magnetic shield, and is roughly comparable to aluminum and brass for magnetic shielding.

HOLY COW! You can afford stainless steel for amp chassis?

Plenty of amps have been produced with an aluminum chassis, so magnetic shielding is obviously not a necessary criteria for a tube amp chassis. If a magnet sticks to your chassis, it is a magnetic shield.

There's one other issue to consider about magnetic/non-magnetic materials. Ferromagnetic materials can be (!) magnetic shields because they have a high "conductance" for M-fields. Magnetic fields would rather be inside the iron-stuff than outside it by a factor of the relative permeability, that being several *thousand* times for many iron allows, more for special stuff like some iron-nickel-cobalt things and mu-metal.

This is good for shunting M-fields away from sensitive stuff. But it's also good for shunting M-fields from transformers INTO sensitive stuff. It depends on the flux path the chassis sets up. Generally this is set up by the mechanical needs of the chassis. By luck, many amps are quiet anyway, but some can have hum that's impossible to get rid of without going to a non-magnetic chassis.

The devil is always in the details.

[Diablo]

Weber has recently switched to stainless steel for their chassis, because they were having trouble finding a good chrome plater for the plain carbon steel chassis. The late Ted Weber did some testing comparing SS to plain-carbon steel and verified that the SS chassis didn't increase the hum. But, like RG stated, the devil's in the details, and it's doubtful that Ted hd the time to check out all of their amp designs. They make a LOT of different kits.