A new series-connected tunnel diode scaler which is free from multiple triggering is described. The tunnel diodes are biased at different current levels by extracting a part of the common bias current from each junction of the chain. These current drains are progressively removed as each tunnel diode forward switches, the bottom tunnel diode initiating reset of the system. A test circuit operated from 0 to 135 MHz with input current pulses of 1 to 2.5 ns duration, the maximum scaling rate being determined by the reset time.