chassis material choice

[Mick Bailey]

But, we're talking about a chassis here, not conductors. Incidentally, aluminium is a far better conductor than mild steel, iron, or stainless steel and the soldering of aluminium was advanced during WW2.

I don't consider aluminium and its alloys to be weak per se. The construction and gauge have to be consistent with the expected loads. Plenty of structural assemblies use aluminium alloys; aircraft, cars, motorcycles etc. Extraction and refining of aluminium generally places it at a higher commodity price than steel, so that's why manufacturers like steel, unless there is some compelling reason why aluminium needs to be used - usually weight saving.

The home constructor, generally with a limited selection of benchtop and hand tools, can't be compared with manufacturing plants with CNC punches, laser cutters, CNC folders and automated welding processes. Aluminium is much easier to work with hand tools. Again, the success or otherwise of any construction comes down to design and acceptable limits; if you make an amp the thickness of a biscuit tin you're asking for trouble. Don't forget that there are numerous extruded sections that can be readily assembled into an amp chassis (along with some thicker sheet for the top) capable of bearing a person's weight without distorting.

An aluminium SVT is perfectly feasible, but the weight of aluminium would likely be the same as for a steel chassis, but at a much higher material cost. So from an engineering and economic standpoint there wouldn't be any reason to use aluminium.

[potatofarmer]

I don't consider aluminium and its alloys to be weak per se. The construction and gauge have to be consistent with the expected loads. Plenty of structural assemblies use aluminium alloys; aircraft, cars, motorcycles etc.

In all fairness, an airplane going through tens of thousands of fatigue cycles while slowly corroding is a piece of cake. Maybe someday they'll figure out how to make it strong enough to hold a 10 pound weight three inches above an indoor table top. I tried to use a 7075 ingot as a paperweight once - shrapnel everywhere.

[Mick Bailey]

I can't figure out what you're referring to - can you explain?

[nickb]

... the weight of aluminium would likely be the same as for a steel chassis, but at a much higher material cost....

Borrowing from air frames, I have never tried it yet, but I have wondered about bracing the aluminum chassis with carefully placed extruded "U" or "L" sections inside, where they can't be seen. It seemed that I might be able to get the strength up while keeping weight and cost down.

[nickb]

..how great aluminum wire is...

Just BTW, I heard the other day that the city of Miami is ripping out the copper street light wiring and replacing it with aluminum due to theft.

[Mick Bailey]

Borrowing from air frames, I have never tried it yet, but I have wondered about bracing the aluminum chassis with carefully placed extruded "U" or "L" sections inside, where they can't be seen. It seemed that I might be able to get the strength up while keeping weight and cost down.

Morgan Jones covers this in his "Building Valve Amplifiers" book. Channel and angle sections are widely available from non-ferrous stockholders.

[defaced]

Borrowing from air frames, I have never tried it yet, but I have wondered about bracing the aluminum chassis with carefully placed extruded "U" or "L" sections inside, where they can't be seen. It seemed that I might be able to get the strength up while keeping weight and cost down.

Absolutely. It's called the section modulus. Different cross sections behave differently when mechanically loaded. For example, take the I beam. Its shaped the way it is because it places a large amount of mass furthest from the neutral axis (center point of bending), which means more material must be stretched or compressed in order to bend the frame. This creates a very stiff and strong member as compared to say a solid square that has the same amount of material. If you really want to get into this, it's covered under the topic of Strength of Materials.

Material, while it has great importance on a design, is not the only factor in how the end product performs in service. This is why things are *engineered*, not just designed.

[potatofarmer]

A poor attempt at humor. I work in aluminum research so I find the assertion that it's useless laughable. I guess that didn't need to be said though.

[Steve Conner]

Well, it is laughable. Airplanes, racing car wheels, frames for mountain bikes and motorbikes, all sorts of high performance structures are made out of aluminium every day. Building a chassis to hold a couple of transformers is easy.

Aluminium is softer and weaker than steel, but you just make it thicker to compensate. If I remember right, a good alloy like 6061, 6082 or 7075 is about half as strong as a good steel, but more like one-third the density, so you can beef it up to the strength of a steel structure and it still ends up lighter.

The old Marshalls did collapse, but I think that is because Marshall used thin aluminium of a soft grade with no internal bracing, and then they got 50 years of abuse on the road.

[Mick Bailey]

The first Marshalls were hand folded aluminium. Looking at the crispness of the folds I would guess 16SWG which was popular in the 60s for larger DIY chassis construction. Radios and test gear used 18 or even 20 gauge. The radio and TV construction magazines of the day produced detailed chassis drawings for the home constructor. 'Half-hard' sheet which was work hardened during the rolling process was the favoured material. It would make sense that operating out of the back of a shop would need straightforward construction methods to make the first Marshalls, but when moved into production steel would be more economical.

The JTM45 in the Tube Amp Book doesn't appear to have sunk. Maybe its been restored.

[Steve Conner]

I got my facts wrong. Aluminium alloy is one-third the density of steel, but the strengths of both materials vary widely.

Pure aluminium is very weak, it has only about one-fifth the yield strength of cheap mild steel. I suspect aluminium electronic chassis are made out of something like this.

An alloy like 6061 is just about as strong as mild steel, but considerably more expensive. If you spend money on a better grade of steel, you can get it much stronger too. 4130 chrome-moly steel (as used for BMX bike frames, machine gun barrels and the like) is about two and a half times stronger than mild steel.

[J M Fahey]

The first Marshalls were hand folded aluminium. Looking at the crispness of the folds I would guess 16SWG which was popular in the 60s for larger DIY chassis construction. Radios and test gear used 18 or even 20 gauge. The radio and TV construction magazines of the day produced detailed chassis drawings for the home constructor.

I've been building , including own aluminum chassis, since the late 60's , so I can answer that doubt.
Thinner gauges were used (1mm and 1.25mm , roughly SWG19/SWG18) in projects, but for *small* ones.
The typical one was, say, a shortwave regenerative receiver (1 tube), a small transmitter, an amateur to broadcast band converter (2 or 3 tubes), nothing heavy.
For an Audio amp with heavy transformers, a thicker gauge was used, at least 2mm /SWG14 .
FWIW my very first commercial MI product (I had been making "Psychedelic Flash" disco units and Disco lights before) was a small Guitar Treble Booster (similar to Clapton's Dallas Rangemaster) , housed in a store bought aluminum mini box, about cigarette pack size.
And yes, in that small size, it was around SWG20 .
My first commercial tube amp was a Fender Bassman (Blackface) clone, built on a "U" folded aluminum chassis, SWG18 with end reinforcing blocks (very similar to the first Marshall chassis) made out of a very hard wood.
Not "regular " wood, it is called "quebracho" (literally: break axes) .
It sagged somewhat under transformer weight, so next chassis were made out of 2mm/SWG14 semi-hard aluminum: problem solved.

'Half-hard' sheet which was work hardened during the rolling process was the favoured material.

You got it right.
Today I have custom made aluminum sheets, I can order them in 3 tempers: Hard, semi-hard, and soft.
Hard is for stamping rigid flat machine parts: levers and such; semi hard is meant for bending or slight stamping (recesses to hide bolt heads, "ears" to hold PCBs, etc.) , this is the one I use; and soft which is *pure* aluminum meant for drawing, deep forming and lathe turning over a hard core (pots/pans/pizza pans/etc.).
I use this last one to make "Vintage looking" magnet covers, for those customers who "listen with their eyes" and want "VOX Blue/Gold" lookalikes. Oh well.

It would make sense that operating out of the back of a shop would need straightforward construction methods to make the first Marshalls, but when moved into production steel would be more economical.

That's the point.
Having *big* hydraulic presses, Iron is easier and cheaper, can be spot soldered to build complex shapes, and galvanizing costs peanuts.
All of this impossible for the home constructor.

The JTM45 in the Tube Amp Book doesn't appear to have sunk. Maybe its been restored.

Or rather, it never sunk

I got my facts wrong. Aluminium alloy is one-third the density of steel, but the strengths of both materials vary widely.

Pure aluminium is very weak, it has only about one-fifth the yield strength of cheap mild steel. I suspect aluminium electronic chassis are made out of something like this.

No, you got your facts right, and yes, aluminum comes in a lot of widely varying types.
Pure aluminum is very soft indeed and nobody uses it, except the pot and pan makers I mentioned above.
Pots and pans kitchenware often carry a small label stating "pure aluminum" .
Well, they mean it
Specially because it will be in contact with food (health reasons), but also because salt and acids inevitably present in food attack any alloy much more than the pure one.

FWIW there's a simple test for aluminum sheet, hit it with a coin or key while hanging it from a corner: semi-hard "sounds like aluminum" ; hard "sounds clangy, closer to a cymbal" and soft "sounds dead like lead".

I often buy second hand metals at recyclers, and obviously they don't have the slightest idea about alloys or specs.

[soundmasterg]

Most of the vintage Voxes were built using steel for the power amp and aluminum for the preamp so there is that approach too...

Greg