Aluminum or Steel Chassis?

[brucec59]

Fabrication and finish issues aside (assume CNC machinery and paint or powder coat), are there any reasons to choose aluminum or steel for a chassis? It seems most manufacturers use steel. Why?

[olddawg]

Fabrication and finish issues aside (assume CNC machinery and paint or powder coat), are there any reasons to choose aluminum or steel for a chassis? It seems most manufacturers use steel. Why?

Right off of the top of my head I would say weight and rust. However, I can see having problems with aluminum since you cannot solder to it.

[PRNDL]

Steel is sturdier and conducts electricity, which may assist in blocking stray RF.

Aluminum is easier to drill and cut square holes (for transformers). You can solder to it, but you have to scrape the protective coating off first.

Thin aluminum (i.e. Hammond enclosures) is fine, although on tweed style combos I like to put 2 screws in each side to secure it to the cabinet (in addition to the 2 on the top).

[Albert Kreuzer]

Hi PRNDL,

in fact aluminum conducts electricity better than steel and far better than stainless steel (at 300 K temperature):
Aluminum 36,59·10^6 S/m
Steel (iron) 10,02·10^6 S/m
Stainless Steel 1,36 · 10^6 S/m

I agree that steel is sturdier than aluminum and easier to solder, and it is magnetically conductive (not stainless steel).

HTH
Cheers,
Albert

[defaced]

What series aluminum do you plan on using? Different series of aluminum differ in their ability to be soldered.
Ref: http://www.aws.org/wj/2004/02/046/
Given the thermal conductivity of aluminum, I'd use mechanical chassis joints if I chose an aluminum chassis.

Also, how do you plan to close the seams at the edges of the chassis?

[brucec59]

What series aluminum do you plan on using? Different series of aluminum differ in their ability to be soldered.
Ref: http://www.aws.org/wj/2004/02/046/
Given the thermal conductivity of aluminum, I'd use mechanical chassis joints if I chose an aluminum chassis.

Also, how do you plan to close the seams at the edges of the chassis?

I was thinking either .125" 5052-H32 aluminum or 14ga steel (not sure about specific steel spec). The chassis will not be a box but a folded piece without sides, something like this:

 ______________
|              |
|_            _|

With those thicknesses, it should be plenty stiff enough. I would not try to solder to it, so that is not a concern.

Thanks.

[Chuck H]

Fabrication and finish issues aside (assume CNC machinery and paint or powder coat), are there any reasons to choose aluminum or steel for a chassis? It seems most manufacturers use steel. Why?

I'm just guessing but it's probably because steel provides more strength per $$$ than aluminum. You can get aluminum chassis for a little less than steel. But the steel chassis will be much stronger. So if you were a MFG and you needed 500 chassis capable of supporting X pounds of weight, steel would be cheaper than aluminum. If we use Hammond products as an example I think that if they made their aliminum chassis as strong as their steel chassis that the aluminum would be more expensive than the steel.

Pros and cons:
aluminum doesn't solder well.
aluminum is more expensive.

steel rusts
steel is heavy

aluminum is a better conductor than steel.
aluminum has better heat sink properties than steel.

steel is easily soldered to

Overall I prefer aluminum. It's easier to work with for DIYers using drills and hole saws. If you don't want to trouble with a faceplate you can just polish the chassis. And it won't rust. Lots of great amps have been built with both materials.

I do wonder if the magnetic properties of steel present any advantage/ disadvantage compared to a chassis with non magnetic properties. I would think that a chassis with non magnetic properties would be quieter somehow. I mean, If your transformers are attached to the chassis, thats pretty close proximity. Being as the chassis is the ground plane in a tube amp, wouldn't the ground plane magnetically fluxing with the power transformers be a potential source of hum?

Chuck

[tubeswell]

I use ~16 Gauge (1.2mm) Aluminium sheetmetal that I buy from the factory in town - it comes with a white durable peel-off plastic lamination on one side, so you can bend it, tap it, punch it, drill it, engrave etc without leaving scratch or dent marks, then when you peel off the film, its shiny and unblemished. It is about the maximum thickness I can bend by hand. I bend everything up on my benchtop with very primitive tools and pop-rivet end caps on. If you bend so that you only have to hammer the back side, you get quite a tidy job (where it will be exposed to the light of day). It makes for a very rigid and strong chassis that takes really heavy trannies no problem. I use 4 mounting bolts in the top to secure it - doesn't need any other mounting bolts.

[Chuck H]

Very cool. You've posted those pics here before. Or I've seen them somewhere. Seems like a very economical way to go for the DIY. I used to bend aluminum flashing to make custom lamp reflectors for friends (with indoor gardens ). It really isn't that hard to do once you find a method that works. Have you ever had trouble with the riveted corners vibrating?

I also dabble in knife making. So I've polished alot of metal by hand. If you start with the factory finish on your aluminum (with no deep scratches) and sand it with 800 grit until you only see the sanding marks, then 1500 grit (for about twice as long), and then use any metal polish, Wenol, Mother's, etc., you can get a mirror like chrome shine that looks soooo cool. The process takes about two hours. Which is worth it IMO. Maybe I'll get industrious and post some pics on one I just built.

Chuck

[tubeswell]

Yeah they were on the tweed builders forum - The rivets stay in pretty solidly and don't vibrate. I tap/drill the redundant centers out of the finished rivets to make sure. As for the economy, its alot cheaper than the alternatives in this neck of the woods

[jrfrond]

I do wonder if the magnetic properties of steel present any advantage/ disadvantage compared to a chassis with non magnetic properties. I would think that a chassis with non magnetic properties would be quieter somehow. I mean, If your transformers are attached to the chassis, thats pretty close proximity. Being as the chassis is the ground plane in a tube amp, wouldn't the ground plane magnetically fluxing with the power transformers be a potential source of hum?

This really isn't the case, but you are on the rght track. Transformer orientation becomes critical because they can couple inductively through a steel chassis. You can get audible hum at the output even with no tubes in the circuit, just powered up. This is the PT coupling to the OT through the chassis. Other than this advantage, I really do NOT like aluminum.

[Robert M. Martinelli]

My vote goes to Aluminum,
its better ability to conduct electricity virtually eliminates the chances of ground loops, and, as John said, it also eliminates the chances of electromagnetic coupling between the transformers.

Even if it's difficult to weld/solder, I don' t think this to be a big problem - Ground lugs can be tied to the chassis with bolts and washers, and, even if it's true that steel makes for a sturdier chassis, there are thousands of old AC30s around the world that stood the test of time pretty well IMHO ( certainly better than the more recent reissues that were made of steel ) proving that a careful design can overcome all the "mechanical" issues and take advantage of Aluminum's outstanding "electrical" properties.

Best regards

Bob

[MattT]

The ability to solder to it is a total non-issue for me. I never solder directly to the chassis. I think NOT being able to solder might ecven be a good thing...prevents bad practice.

...its better ability to conduct electricity virtually eliminates the chances of ground loops...

I'm not sure I follow you on that one. A ground loop is a ground loop...it's just two paths to ground. Chassis material doesn't have anything to do with that. But I don't usually use the chassis as a conductor as much as some do (i.e. I usually have just two ground bolts...one for the incoming power and one for everything else).

I haven't found a reason to not prefer Aluminum...but steel is fine too.

[Robert M. Martinelli]

What I meant is that aluminum, with its higher conductivity virtually eliminates the chance of ground loops to occur; with "ground loop" I mean two points in a circuit tied to the chassis at two different locations, if the two points are not at the very same potential ( even a few millivolts count ) then disturbances are likely to occur, especially on high gain-high sensitivity amps.
Regards
Bob

[husky]

What I meant is that aluminum, with its higher conductivity virtually eliminates the chance of ground loops to occur; with "ground loop" I mean two points in a circuit tied to the chassis at two different locations, if the two points are not at the very same potential ( even a few millivolts count ) then disturbances are likely to occur, especially on high gain-high sensitivity amps.
Regards
Bob

'Old post but
What he means is that using a chassis to connect grounds is not good practice. Grounds carry audio and there is no way to predict the interaction between them when using a chassis to connect grounds, there is no way to steer the ground. I agree that Audio ground should go to one place at the chassis and chassis ground to the wall should be at another preferable at least a few inches away or much more. The chassis should not be used to connect any other grounds in the audio path.