When an interval between a Gm electrode and a cathode is broadened with
temperature, a potential change similar to a case that a Gm electrode
voltage is lowered is generated. If a cathode voltage is constant, a
quantity of electrons which pass is decreased whereupon a screen of a
cathode ray tube becomes dark. A Gm electrode voltage control circuit is
provided to a Gm electrode voltage source for control an applied voltage
to the Gm electrode by time information from a time-measuring circuit such
that the change of the interval between the cathode and the Gm electrode
is corrected during a period of time between the time the voltage is
applied to each electrode of a Hi-Gm tube and the time the change of a
size of the interval between the cathode and the Gm electrode is
saturated.