Aluminium vs steel chassis

[Rob11966]

Hi,
As mentioned in an earlier post I am building my first power amplifier from scratch. I have decided upon the Dynaco St70 (availability of parts, relitive simplicty of construction + wanted to build a stero unit) but I am setting up my own layout and design. So far I have built a cardboard mock-up and I am about to take it to a sheet metal worker.
Here's the question - according to Morgan Jones, 'steel is not suitable for valve amplifiers'. Is this an absolute rule or is steel OK provided that the transformers are mounted with something between them and the chassis.
Cheers,
Rob

[sorenj07]

Steel is bad. .1" aluminum is good.

[Gordy]

Originally posted by sorenj07
Steel is bad. .1" aluminum is good.

Why, please?

[p_ete2003]

I've used both and aluminum is a lot easier to work with.

[zigzagflux]

I use steel. I like the magnetic shielding it provides from the transformers and chokes above. You could have eddy currents in said chassis, but the field is still shielded from the signals below.

I would think it important to electrically isolate the transformers mounted on it. I use rubber washers (with the additional vibration control) with teflon shoulder washers. Each device then gets bonded to chassis in one point. This avoids circulating currents.

I'm not sure why Jones said this, though there may be some context to it that we're missing. If it has to do with induction, there are usually procedures and installation methods that avoid or minimize it.

Ease of installation has nothing to do with the choice, in my book. My punches and drills go through both Al and steel.

[nhuwar]

Steel chassis can effect inductors. It can get magnetized which is bad. And yes it's alot harder to work then aluminium.

Nick

[zlast]

Steel is stronger and cheaper.

Consider your final product...are you planning on painting or some other method of finishing? Aluminum can be left unfinished and look great, steel needs some sort of protection to keep from rusting. Also consider HOW your chassis is going to be assembled, nuts,bolts,rivets,welded,screws - this might influence the material you use - and the gauge. You may want to discuss these points (and cost) with whomever you hire to build the chassis...

[Gordy]

The notion of circulating currents in a steel plate is valid. I am not sure about how strong a field would be required to induce such a field, or how easy it would be for that field to couple on to other components.

Steel would provide superior shielding, whereas aluminium would provide no opportunity for inductively derived circulating currents, and be physically easier to work.

Humm, an interesting choice...

[kevinkr]

I have wrecked more than a couple of punches on steel chassis obviating any savings in cost between the two.

Aluminum is a much better thermal conductor that steel, and this helps when using the chassis as a heatsink for regulators and power resistors.

Steel is a better magnetic shield, but is no more effective against electrostatic fields.

Steel is stiffer and stronger when heavy weights are an issue.

Steel rusts, aluminum doesn't.

Steel is a little easier to paint, that said I have aluminum chassis painted a decade ago with no issues.

Hammond sells aluminum chassis that are pre-primed and easy to paint.

Some people allege that a given circuit sounds better in a non magnetic chassis as opposed to steel - perhaps this due to magnetic fields in conductors interacting with the chassis. (aka increased parasitic inductance.)

HH Scott used AL chassis for both thermal and sonic reasons based on the comments made by former employees. (And some of their literature as well.)

With careful design either material ought to be fine, AL is generally easier for the typical diyer to handle with simple hand tools.

I like copper even better, but it's too expensive, really hard to work because of it's softness, (Clogs drill bits, end mills, punches, etc), greater thickness is required for a given load on the top plate, needs to be well protected from oxidization, and is prone to bending and scratching. However nothing looks better than clear coated copper IMHO..

[nhuwar]

I have always been parshall to silver plated copper then clear coated It looks good. But stainless would do a good job as long as you dont bend it or work it to much as 300 seires becomes magnetic from cold working. It could feasably polished to a mirror finish for a top plate for a amp.

Nick