Abstract: A direct current power supply circuit for a photomultiplier tube comprises a step-up transformer with a secondary winding connected to rectifier means feeding output terminals. The primary winding in series with a transistor switch is connected across a d.c. supply source and the switch is controlled by pulses from a pulse generator which is switched on to produce pulses and hence to switch current in the primary winding when a control signal opens a gate in a feedback circuit of the pulse generator. The control signal is derived from the photomultiplier circuit and the circuit operates with the gate opening and closing as necessary to obtain the required voltage. The control signal may be derived from across an anode resistor of the photomultiplier tube or across a resistor in series with the dynode resistor chain according to the characteristics required.