Chassis Design

[broken flyer]

Hi, I'm going to start building a tube amp soon and i've got some questions i was hoping someone would have the answers to. I'm building this amp completely from scratch. I'm going to start by making my own chassis then my own cabinet and then ill wire it up. I'v got the cabinet all figure out, it'll be made out of 3/4" pine, then covered in tolex, however im not sure whats the best way to go about the chassis. I was going to make it out of 16 gauge steel and then weld it with a MIG welder, however it was brout to my attention that my dys for cutting out the holes for the components might not go threw 16 gauge steel, so now im considering aluminum or maybe just some thinner steel. maybe 20 gauge steel or so. Have any of you built your own chassis before? if so, out of what and how thick? If i opt to do aluminum how thick should i make that? Any helps appretiated, thanks in advance.

[soundguruman]

16-14 gauge steel. Aluminum chassis are flimsy and don't last. Yes, weld the steel together, then have it triple galvanized, after all the holes are cut.
There are plenty of components designed to mount in a thick steel chassis.

[defaced]

Hi, I'm going to start building a tube amp soon and i've got some questions i was hoping someone would have the answers to. I'm building this amp completely from scratch. I'm going to start by making my own chassis then my own cabinet and then ill wire it up. I'v got the cabinet all figure out, it'll be made out of 3/4" pine, then covered in tolex, however im not sure whats the best way to go about the chassis. I was going to make it out of 16 gauge steel and then weld it with a MIG welder, however it was brout to my attention that my dys for cutting out the holes for the components might not go threw 16 gauge steel, so now im considering aluminum or maybe just some thinner steel. maybe 20 gauge steel or so. Have any of you built your own chassis before? if so, out of what and how thick? If i opt to do aluminum how thick should i make that? Any helps appretiated, thanks in advance.

People who don't work with steel on a regular basis find that because it's so much different than working with wood that it's " hard". If you take a drill bit and blast it full RPM on a piece of wood, you get a hole. You do that with a piece of steel, and you get a seriously dulled or broken bit. Normal every day sheet steel is no harder to work with or drill than wood, the rules are just different. I have used elcheapo Harbor Freight step drill bits to drill octal sockets in 16g steel. When I was done, the bit was as sharp as when I started. Use pilot holes, drill slow (RPM), use oil, clamp stuff down. If you're not getting a chip out of the hole, stop, you're doing it wrong.

If you do have the chassis galvanized after you fab it, account for the thickness of the galvanizing when choosing your hole sizes.

[soundguruman]

WELD some grounding lugs to the chassis, one at the power supply, one at the preamp ground, one at the power AC safety ground, before galvanizing.
Don't try to solder grounds to the galvanized chassis. Avoid, if possible, screwing ground lugs to the chassis. The soldering will produce toxic fumes, and a superficial ground connection.
Get your grounds welded deep into the metal.

[J M Fahey]

Hi Broken Flyer.
I have been home building my own chassis at home, for around 40 years.
Have metal shears, folding and punching brakes, etc.
In fact, in this very moment, I'm fabricating 20 front and back panels for 100W SS amps.
By tonight they will be finished, painted, silkscreened and punched.
If I had ordered them, I would have to pay a lot and wait at least 15 days.
For small scale production I found aluminum much more practical.
2mm is thick enough.
Fold a C (or U, it´s the same) shaped chassis and add reinforcing ends.
If TIG or MIG available, solder them along the edges (best); if notPop rivets are better than nothing.
For a single chassis with minimum tools you can use hardwood ends.
The *big* advantage is that it does not rust, so you are not *forced* to galvanize or blast/phosphatize and paint.
And it can be easily worked with "wood" or lighter type tools.
I´ll take some pictures and post them tomorrow to show you how easy it is.

[soundguruman]

Aluminum chassis collapse under the weight of the transformers.
Take a look at Marshall prototypes that were built 40-50 years ago on aluminum chassis.
They look like a pile of melted butter.
Now look at the steel chassis that were built 50 years ago, or longer. Bingo.
If I am going to invest a huge amount of time and effort into building a guitar amp from scratch, I'm going to invest in materials that withstand the test of time.
Aluminum also oxidizes, and the resistance increases with time, which means the grounding fails.

Aluminum is easy, cheap, and fast to work with, but it's a terrible long term investment.

Nobody said that building an amp to last was going to be cheap or easy. Do it right the first time.
"there is always time to do a job fast, but never time to do it over." -Gene Berg

[J M Fahey]

MAYBE I work better than Marshall, then.
My own aluminum chassis handmade in 1969 are still going strong .
And all 10000 others built since.
No oxidation or bad grounding problems either.
While similar age iron chassis (including very well made triple plated Fenders) show different degrees of rust, from random spots to useless heaps of rust.
Of course I shouldn't be taken *too* seriously, I'm based just on personal experience, not Internet folklore or drunk ruminations.

PS: I'd LOVE to see some pictures of Marshall "pile of melted butter".
So much that I'd spread it on my morning toast, go figure.

[Steve Conner]

Aluminium isn't as strong as steel, so you simply have to use a thicker gauge if you want it to support heavy transformers. Marshall probably used thin, weedy stuff, especially since they were prototypes and they probably couldn't care less if they lasted.

Steel and aluminium both can corrode if unprotected. Steel much more so. Steel chassis have to be painted, chromated or nickel plated to stop them from rusting up and looking terrible, but you can get away with leaving aluminium untreated.

Here is my modest contribution to the debate, made of 3mm thick aluminium plate. I think it is some high strength alloy like 6082 or 7075. As far as surface treatment goes, I used fine sandpaper to give it a brushed effect, and that was it. It still looks nice 10 years later.

[Chuck H]

+1 for aluminum. The observation that aluminum "oxidizes" is true enough. But it implies that steel chassis don't?!? I'm pretty sure rust is caused by oxidation. And there have been easily as many ground faults on solder blob steel chassis as there have been on screw grounded aluminum. Just ask ANY tech that doesn't include "guru" in his personal moniker. I build many amps on the normal .044 aluminum chassis sold by Budd and Hammond. They are only a few years old but my personal experience tells me there isn't going to be any problems. My personal amp is built in an off the shelf Hammond chassis and I've been beating the tar out of it for five years with NO evidence of problems. I think soundguruman is just smaking based on some photo he saw. Or something like that. Any excuse to offer input. I'm guilty of that myself. Even recently. I mistakenly believed info from a book that I had no personal knowledge of and hadn't researched sufficiently for myself. It was embarrassing.

JM was more graceful. But I would also be interested in factual documentation of the inferiority of aluminum.

[defaced]

The observation that aluminum "oxidizes" is true enough. But it implies that steel chassis don't?!? I'm pretty sure rust is caused by oxidation.

Both materials oxidize. Actually all metals will oxidize depending on the circumstances. The difference is that *usually* (meaning in typical inland air environments) the oxide layer that covers aluminum is passive (Al2O3). Passive means once it forms, it doesn't continue to attack the metal. If you've ever seen aluminum with big patches of white powdery stuff, that's aluminum oxide that's done a little more damage than we typically see. Rust is the third oxidation state of iron, Fe2O3. Rust is an active oxide layer and that is the reason why once something starts rusting, it continues to rust.

JM was more graceful. But I would also be interested in factual documentation of the inferiority of aluminum.

As Steve pointed out, it's not a material issue, it's a strength and thickness issue. Neither material is inferior to the other, they're different and to be used for different things. A chassis is a such a light duty item (in the grand scheme of manufactured goods using each material) that it really is not hard to make one out of either material. The thing is one has to design (meaning do the loading and strength of materials calculations) to properly evaluate the options. It can get pretty deep or very simple depending on what assumptions you make and what your goals are.

[J M Fahey]

To shorten an endless discussion, here is an equivalence table published by the guys at
Sheet metal materials
which *should* know what they are talking about.

Please make a little mental gymnastics, column formatting gets lost with simple cut and paste, align columns with corresponding headers.
In any case, the 2nd column gives steel thickness, the 3rd one aluminum thickness needed for *SAME* strength.
40% more, nothing to write home about
For those in doubt, check the original page mentioned above.

Aluminum sheet vs steel sheet

The following table gives a quick point of reference when you need the approximate thickness of aluminum sheet to use in replacing steel sheet.
The designated aluminum thickness will give you about the same stiffness.
Or, putting it another way, the deflection will be about equal.
As a rule of thumb, plan on using an aluminum sheet about 40% thicker than steel.
Since aluminum weighs only 1/3 as much as steel, this means that the equivalent aluminum sheet will weigh only half as much as the steel sheet it replaces.


Approximate stiffness equivalence:

Steel LB/SF Steel Thick Alu Thick Alu LB/SF
.975 .024 .032 .452
1.22 .029 .040 .564
1.47 .035 .050 .705
1.80 .044 .063 .890
2.44 .059 .080 1.13
2.56 .062 .090 1.27
2.86 .070 .100 1.41
3.66 .089 .125 1.76
4.88 .119 .160 2.25
5.49 .134 .190 2.68
7.33 .179 .250 3.53

[soundguruman]

Aluminum does collapse, now you know.
It takes years for it to happen, but slowly over time, due to the weight of the transformers, it caves in.
If there are no heavy items mounted, then it's probably acceptable.

Considering conductivity, we (electricians) are no longer allowed to install aluminum in residences.
The conductivity breaks down over time and it runs hotter and hotter.

[broken flyer]

Thanks for all the replies. I have delt with metal before and have a shop full of tools, including mig and tig welders as im building this at my school, but i have punchs at home i was planning on using to put the holes for the components in the chassis and thats why i was was considering aluminon as well as i was thinking i could just leave it bare/just polish the face if i did aluminum. I don't want to spend the money and time to have it plated as by the time i had that done i could just go buy a pre-made chassis. I am more comfortable working with steel as far as welding goes, but right now im undicided how i should go about this.

[Steve Conner]

Someone ought to tell the power companies who use aluminium cables for their overhead lines. (with steel cores for strength mind you.) There's nothing wrong with the metal itself, the problem starts where it is screwed into terminal blocks. Because it is so soft and forms oxide easily, the connection loses clamping force, air gets in, oxidation starts, and you have another house fire.

Only a newbie (or a hardcore vintage-ist) would use the chassis as part of the audio ground system anyway. You connect the green wire to the chassis for safety, and the audio circuit has its own ground bus (made out of copper!) that connects to the chassis in one place. If you aren't sending audio ground currents through it, it doesn't matter what it's made of, as long as it conducts enough to be usable as an electrostatic shield. (See Fender's aluminium foil pasted inside the wooden cabinet.)

But if you really must use an aluminium chassis as a ground bus, those little toothed washers always did the trick for me.

[J M Fahey]

Aluminum does collapse, now you know.
It takes years for it to happen, but slowly over time, due to the weight of the transformers, it caves in.
If there are no heavy items mounted, then it's probably acceptable.

*IRON* does collapse.
Titanium too.
Like *everything* else.
All it takes is going over its tensile strength limit.

*IF* tensile strength limit is exceeded, (any) material collapses, slow or fast.
IF NOT*, it does NOT collapse. Simple as that.
At least in the Physics world .

How do you avoid that?
By *calculating* and *designing*.
Which says that for any acceptable Iron design, simply using 40% thicker Aluminum is EXACTLY THE SAME.

Considering conductivity, we (electricians) are no longer allowed to install aluminum in residences.
The conductivity breaks down over time and it runs hotter and hotter.

Tell that to Power Companies in the whole World, including USA, which rely more and more on Aluminum conductors in the heaviest Power transport problem there is in the known Universe, a.k.a. cross Country and Transcontinental Power grids.
So far , none of them has noticed the effect you mention.
Maybe they should hire you as a Consultant.