* Always, always, always build the cabinet around the chassis, and the chassis around the circuit. This goes a long way towards preventing mishaps like the scenario above.
The basic idea can be seen here:
The front and rear rails are made of U channel. I was able to find some with a 2" Web (the span across the back), and a 3/4" leg, about 1/8" thick. The rails are then wrapped with a piece of 0.060" aluminum sheet bent to form a "C" shape, with the ends of the "C" forming the side walls. I then had a flat panel cut of the same thickness to enclose the chassis. I suppose that this may be overkill, and you could just use an open top box with a wire screen for shielding, the way Fender did.
The key to a strong box is keeping the front to back dimensions down, since
all your torsional strength comes from the U-channel.
My box proved surprisingly sturdy. Since I was paranoid about the the
strength of the sheet, I used oversized washers (about 3/4" #6) on the
transformer bolts to distribute the load. For a really heavy transformer,
use thicker walls, or you may want to try mounting it through a thick mounting
plate that mounts on the U-channel. I've also considered running the mounting
bolts all the way through to the top. If the transformer is on top, rather
than hanging, you might want to use a stiff spacer so that the bolts
distribute the compression load. None of this proved necessary with the
transformer I used (about 4 or 5 pounds).
This method has a couple of nice things going for it, even if you don't have to roll your own chassis. It's not terribly expensive, not much more than buying a prefabricated box, maybe less in some cases. I got the U-channel for free, and the sheet metal work only cost me about $20 (US), for a box measuring 16.25" x 7" x 2". Secondly, it allows you to drill all of your holes in the front and rear rails before assembly - which may be nice if you don't have a floor drill press. Thirdly, presumably you're building from scratch because you have a warped sense of fun, as I do. This adds an easy item to the list of things built from scratch. I've not gotten so bad yet as to wind my own transformers though.
I generally prefer to work in aluminum - you can generally work it with the tools you already have in your woodshop. It's also obnoxious enough to solder that you'll want to keep chassis ground connections to a minimum - a good idea anyway. (The best method is drill a hole, put a toothed washer on the outside, and a toothed solder lug on the inside, and crank the nut down hard enough to get some deformation.) Ken Fischer uses aluminum chassis for his Trainwreck Amps as well, citing aluminums diamagnetic properties in reducing sources of inductive coupling of 60Hz hum into your circuit. You can see a couple of pictures from my amp, The Fatlady built using this style of chassis, on my projects page.
Ray Peterson (rpeterso@City.Winnipeg.MB.CA) adds:
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