Abstract: A color tube so arranged as to check the operation of an electron gun reserved for a particular type of visual display such as a red warning signal and therefore not normally employed for independent operation. The arrangements contemplated preferably include direct checking for continuity of the corresponding filament circuit by means of a separate output lead from said filament, as well as checking of the emission of the reserved-gun cathode by inserting an additional anode which is intended to form a diode with said cathode. Testing by means of the additional external connections thus formed can take place by detection of current flow.