It is a simple circuit regulated power supply, based on the known LM 723, that drive a transistor Q1 [2N3055]. The regulation of voltage, of expense becomes with potentiometer R1 from 0v-30v DC roughly. In order to we achieve 30 V, will should the transformer of supply TR1, it gives all the current that it asks the load, differently the output voltage it will be found in the levels of 26 V roughly. Essential is the use of a good heatsink for transistor Q1, as well as good quality of potentiometer in the place of R1.
circuit from http://users.otenet.gr/~athsam/power_supply_30.htm