Abstract: A cathode ray tube, having a cathode and first, second and third grid electrodes, is biased on to allow an electron current to flow from the cathode to the second grid electrode during a first time period. A first voltage, which is proportional to a sum of gas plus leakage currents flowing in the third grid electrode circuit during the first time period, is applied to a first terminal of a capacitor having first and second terminals. The capacitor is charged to the first voltage by momentarily grounding the second terminal. During a second time period following the first time period, the cathode ray tube is biased off causing cessation of the electron current flow from the cathode to the second grid electrode and of the gas current to the third grid electrode. A second voltage which is proportional to the leakage current flowing in the third grid electrode circuit during the second time period, is applied to the first terminal of the capacitor.