The amplifier is of the plug-in type, has rapid warm-up time, can be constructed with a wide variety of input impedances and frequency response and permits adjustment for dark current so that when the photomultiplier is in the dark there will be zero output voltage. Output impedance is 200 Ω, output voltage is linear up to 45 V and photomultiplier anode currents from 10^-6 to 10^-12 A can be recorded.