You can make a simple Variable Power Supply unit with two Transistors and a Pot. Two high power complementary transistors are used in the circuit to handle high current. Just provide 14 volt DC to the input and adjust a Pot to get 1 to 12 volt DC from the output. It is useful to power circuits while testing.
MJE 13005 is a high power NPN transistor commonly used in CFL lamp circuits and MJE 2955 is a PNP transistor. The biasing voltage of T1 depends on its base voltage set by VR1 and conduction of T2 provides a varying voltage from its collector.