This power supply was designed for ham-radio use and has been in operation for over 10 years.
Its design is very simple and practically immune to RF. It is assembled from discrete components, being the most costly element a 600VA toroidal transformer, which can be replaced by another with different characteristics, provided it has a voltage of between 17~20V and the right power for our needs.
The reference voltage is obtained from a 7.5V Zenner diode (6V8 ~ 8V2) applied to Q2 which compares the voltage output by the resistive divider formed by R5, VR2 and R6, controlling the Darlington amplifier Q3 which is responsible for driving the output stage consisting of driver Q4 and 4 NPN power transistors 2N3771, which can be mounted directly to the heatsink without being isolated, because its collector is connected to ground.
The source has maximum current limiter (Q1) adjustable via VR1 and overload indicator LED (DL2) and total protection against short circuit, with zero consumption in case of short circuit, can be maintained in this state indefinitely without danger.
(Keep in mind that due to the high power of the PSU a occasional short with a high length or small section cable that has sufficient strength not to exceed the maximum current of 40A. Might not be considered a short circuit and cause the PSU supplying a high amount of power).
The power rectifier diodes (D1, D2) are recommended metallic threaded type with the cathode grounded, mounted on a large enough heatsink isolated from the chassis. For the power transistors using a heatsink as big as possible, without being isolated as discussed above, may be part of the aluminum chassis of the box. We also recommend the use forced ventilation with a thermostat.
I wish you every success.