Bench Power Supply: Regulated, with Adjustable Current Limit and a Pentode Pass Tube (Gately, Forbro, US 3,243,690, 1966)

In a case, wire the power cord through a switch and a fuse to the primary of a transformer with four secondaries, and put a neon lamp across the primary to show the supply is on. One secondary (6.3 V) goes to two terminals for outside use, one heats the pentode, one is the main supply and one the bias supply. Ours: the case. Not given: the other winding voltages.

Rectify the centre-tapped main winding with two rectifiers and smooth it with a capacitor. From the bias winding make two more supplies: one through two rectifiers with by-pass capacitors to a smoothing capacitor, for the pentode's screen and the reference circuit; the other through one rectifier and a capacitor, for the pentode's grid bias. Not given: the values.

Put the pentode in series with the positive output: plate to the main supply, cathode through two current-sensing resistors (one fixed, one adjustable) to the + output terminal; suppressor tied to the cathode, screen fed through a resistor. Build the reference: a Zener fed through a resistor feeds a second Zener at constant current, giving a fixed voltage. Against it set two resistors and the voltage-control variable resistor: the output voltage is proportional to that resistor's setting. Not given: the tube type and values.

Build a five-transistor amplifier that drives the pentode's grid to keep the bridge balanced: a PNP emitter follower, an NPN stage, a PNP emitter follower with a resistor-capacitor roll-off against oscillation, a PNP driver whose base resistor has a capacitor across it for quick response, and an NPN clamp whose base is set by a potentiometer. Feed the voltage across the current-sensing resistors to the driver's base through a diode: above the limit it takes over and turns the pentode down. Choose the fixed sensing resistor to limit at about 125 % of rated current, trim with the potentiometer, and use the adjustable one to set lower limits. Not given: the transistor types and values.

Fit a meter with a four-pole, three-position switch. Position 1 reads output volts through a multiplier resistor; position 2 reads current across a shunt in the negative line; position 3 (spring-return) puts a load resistor across the output and swaps a fixed resistor for the voltage control, so you can turn the limit control until the current limits at the value you want, without touching the voltage setting. Release the switch and use the supply.