The design of a three‐stage high‐gain amplifier for laboratory use in audiofrequency investigations is described. Four‐electrode tubes are used as screen‐grid amplifiers and an amplification of the order of 200 per stage is obtained. The inaccuracy of McDonald's formula for calculation of stage‐gain has been pointed out. The gain‐frequency characteristics are given for power as well as voltage amplification. It is shown that extreme care is necessary in the design of shielding to obtain high‐voltage amplification of the order of 120 decibels as obtained in this three‐stage amplifier.