Magnetic shielding

[jnb]

How does one make a magnetic shield for a transformer? How close should it be to the core, should there be two shields separated by a gap and how can I avoid making a shorted turn.

Is it OK to have a transformer set flat (with the laminations flat against the chassis)?

Thanks in advance.

[richie00boy]

Basically you make a cage/cover from steel or iron, that goes over the transformer. It would be pretty much impossible to make a shorted turn on an EI transformer unless the laminations actually touched the cage.

[jnb]

Thanks richie00boy, I was concerned about the shorted turn but it is an EI, I'll stand the shield off from the transformer. I plan to mount it directly to the aluminium chassis.

The transformer has a bell on one end only. It was meant to be mounted using the bolts through the laminations. andI am concerned that this means that I won't be able to avoid the fluk pointing toward the valves and I won't be able to mount the other transformers at 90 degrees. Is this a problem?

[richie00boy]

By bell do you mean cover? I'm not sure what you mean by a fluk.

[jnb]

My bad . Yes I did mean cover, and if I was more careful typing it would've said 'flux'.

[richie00boy]

OK. I'm have no idea how you intend to physically lay out your thing so I can't really comment. Maybe you could show a drawing or mock-up?

[jnb]

I've only just started to think about layout and I understand that doesn't make it easier. This is a PT in a pre-amp and will be in a corner adjacent to a corner with a heater PT in it. In the centre of the chassis, wall-to-wall will be regulators and their heatsinks, and at the other end, the amp itself. Hope this helps.

[TechGuy]

There is a Material called "Mu Metal" that is intended for Magnetic shielding, but it isn't cheap nor is it flexiable. Its sold in flat sheets (brittle) I believe non-magnetic metals (Aluminum, Copper) can also provide some shielding. Although You have to be careful not to create a current loop with your shield otherwise it will draw current from your transformer if its very close to the transformer.

Generally EMI is more of a concern for generating noise from a transformer. You can reduce EMI uing an "electrostatic shield", which is essential a band of copper foil wrapped around the axis of a EI tranformer and connected to ground. You'll need to insulate the overlapping area of the foil band to prevent a current loop.


I hope this was useful

[jnb]

Yes, it was. In particular the part about EMI being more of an issue and that it can be fixed with non-ferrous metal shielding.

So does this mean that many parts in an amp are not that susceptible to stray magnetic fields compared with the electrical fields? I mean, I've heard that transformers and coils will interfere with each other by magnetism but is the effect much less of a concern with non iron based components such as capacitors, and what about valves?

[jacco vermeulen]

Besides Mu foil, the stuff is also manufactured as cylindrical covers and in a shape that holds a CRT.

Mu stands for Magnetisch Undürchlässig, a German term.
The foil is very flexible, 0.1 to 0.5mm thick, and requires a delicate glove.
Can be folded within it's elastic range, but bending it creates plastic deformation, making it loose the electromagnetic properties.
Mu is a nickel-iron alloy, the product is heated till red hot in a gas chamber, then rapidly cooled to freeze the homogenous crystal orientation.
Mu foil can be cut, provided it remains perfectly flat adjacent to the cutting line.
Overhere, regular production Mu pan sizes go up to 4.5" diameter and 2.5" height, up to 1mm thickness.
Rolf Gemein has been using the largest size Mu pan to shield the toroids in his Symphonic Line RG type amplifiers for decades, his tubey pre-amplifier model houses an Mu metal covered transformer for the MC stage.

Personally, i hate the stuff and the cost of Mu has more than doubled in 15 years time.
I've had much better and loads cheaper results by double shielding, 1 shield grounded.