A new type of trigger circuit for either positive or negative pulses is described in which the sensitivity or threshold is not dependent on the contact-potential variations in valves. A threshold of less than 100 mV with a stability of better than 2% can be achieved under practical operating conditions. The circuit can be used as a normal trigger circuit with less accurate threshold down to 5 mV. The conductances of a diode used in the retarding-field region and two other valves passing constant currents are the main factors which determine the sensitivity. Theoretical calculations of the sensitivity are given for the general case, and some experimental confirmation of the theory is also presented. The minimum pulse width for operating the circuit is shown to be of the same order as that for conventional pulse-amplitude discriminators.