Building it
I used two 12 Vac transformers found in the trash and which belonged to a halogen lighting system. I have connected the outputs in series so that I get 24 Vac which is then rectified by a bridge and smoothed by a couple of capacitors. I have secured the bridge and the capacitors directly to the case as can be seen in the photos.
I made the case with flat aluminum sheet which I scored, bent and cut myself. The components which are bought new I have mainly bought in China when I go there and are incredibly cheap. The two instruments (V & A) on the front panel are in this category. Power supply The Volts instrument is calibrated to 50 V and I will have to change the series resistor so it shows 30 V at end of scale and I need to change the scale itself which may be a tad more complicated. I have made the new scale using Inkscape.
In all the power supplies and other lab instruments I make I like to place AC mains output bases wired directly in parallel with the incoming power input. This way I can use any in the back of the unit to daisy-chain instruments and I am never short of places to plug instruments into. The ones in the front are useful for plugging in temporary tools or equipment under repair. In this unit I had plenty of space so I put two in the front and two in the rear.
Power supply Power supply Power supply back Power supply front Power supply front

I printed a template of the front and then taped it over. As I drill holes and place components I cut the paper away. For some unknown reason the green and red LEDs appear white in the photo even though their colors are perfect in reality.
Power supply

Recovered from the junk I had two 2N3055 mounted on a heatsink and I mounted the assembly on the rear of the case. Probably a single 2N3055 could have handled the load but using two means they can handle the load and the heat much better. The assembly had both 2N3055 in parallel but I fitted a couple of emitter resistors to better balance the load. I made the resistors with a short length of resistive wire and I fitted them directly on the assembly itself. TR2, BD135 is mounted inside the case so that it uses the metal case itself as a heatsink.
This is the circuit I designed and tweaked and which seems to be working fine. TR1 through TR4 form the voltage limiting circuit and the output voltage is adjusted using P1. TR5 through TR9 form the current limiting circuit and the current limit is adjusted using P3. I used a current mirror circuit as a current sensor and I am impressed by how sensitive it is. I could have used a resistor but at 4 A it would be dissipating something like 2.5 W. Not that it is not doable but I just wanted to get some practice with a current mirror. Here is the schematic:
The red rectangle represents the board where the components will be mounted. Everything outside the rectangle is mounted on the case. All wires on the board carry very low currents and all wires which carry high currents are mounted on the case. I built the circuit on perfboard and there were countless changes and modifications made along the way. Now that the design is final I guess I could design and build a PCB but I dislike this job. I need to get together with someone who would share projects so I would do the design, building, testing, etc. and another person could do the PCB and some other jobs.

All wires go to a connector on the board and this way it is easy to disconnect and work on the board. The connector is made with a standard 16 pin DIL socket and a plug that fits into it. The numbers on the schematic identify the pin number on the socket and the color of the wire. This is, of course, just for my own help and does not affect the circuit at all.