Can you measure the DC resistance of the cabinet at the different ohm settings (4/8/16)? Maybe the cabinet is miswired?

Look what the cat dragged in

The transformers are working great for this, I'll be posting the complete "radio shack" power supply soon!

I tried to design a "radio-shack" amp before. It's almost possible, but would be more expensive than jusr ordering better stuff.

I would recommend using the zener diodes in conjunction with the thermal switch, otherwise the transistors will roast long before the thermal switch opens. (see 1980's Fender SS amps).

Another update:
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html
(might have to reload the image)

Did some better parts selection, otherwise not too much different.

You can use zener diode limiting (look at some Fender SS schematics). This arrangement may not handle a short for as long, though.

I did something along these lines a few years ago: TIP142 darlington, 70V transformer, and a 220-ohm emitter resistor. Don't remember the details, but I think a darlington might work the best in this situation.

Well, I'm not done, but I posted a couple more pics here:
http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

I could have done some things better, so I might redo some of it. The final build will have the 4-stage JFET preamp shown on the breadboard, and the usual tone controls.

Updated the schematic to match my build, and added a few notes.
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html

(Updated 11/02/09)

-When wiring panel-mount fuseholders, it's important to wire them correctly. The hot line from the switch goes to the back terminal, and the side terminal goes to the transformer primary. This prevents the hot line from being connected to the cap/exposed metal if someone forgets to unplug the unit before attempting to change the fuse. Also make sure the fuse comes out with the cap, and doesn't stick in the hole.

-It's always a good idea to use a hold-down footswitch when probing around a live chassis. This way your foot will release the electricity in case of a shock. Harbor freight sells these switches for like $10. (Two kinds are available, make sure it's the hold-down kind not the on-off kind.)

-Always wrap electrical tape or use heat shrink around mains connections, and wire-tie them in enough places to keep them secure. Tie things in such a way that if the ends come loose, they can't fall down and hit the chassis. Also, loose AC wires can get pinched between lids/covers/etc and create a shock hazard.

-Make sure the green ground wire from the AC cord is attached using a star-type lockwasher, maybe some lock-tite also. Grind any paint off the inside surface where the ground wire is to be mounted. It needs to be secure and in conduction to the chassis.

-Extra screws/nuts left in the chassis can cause shorts. Hold the chassis upside-down and shake it hard to (hopefully) get rid of them.

Please add to the list...

I tested the amp today, although it's sorta tacked together at the moment and not at all pretty. I designed it to be compatible with a pedal circuit, so that's what I used for a preamp. Sounded pretty good distorted, but I still need to run a clean tone and see how that works out.

My NPN and PNP transistors were mismatched, so there was about 2V offset. I adjusted one of the emitter resistors on the first stage to compensate, but I really think all complementary pairs need to be matched. (Not all the transistors, just each NPN/PNP pair.)

The power supply uses the two radio shack transformers, two bridge rectifiers wired in series, and two 4700uF filter caps & some bleeder resistors.

Edit: Tested the short circuit protection with a screwdriver across the speaker leads, and it works good. Did a little more play testing and am happy with the sound. So it's a matter of wiring some things nicer and tweaking the bias a little.

Put another pic up of the board/heatsink. Just a layout, will solder the parts this weekend. You can see the protection circuit on the right-hand side.

http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

Thought I would try it this way, since the transformers are from RS, and only $10 a piece. Also, I think it will bump the output some.

Did some work on the chassis today:
http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

I found some huge extrusions awhile back, which I cut the heatsink out of. That gets bolted to the chassis with heatsink compound between, sorta making the whole chassis part of the heatsink.