PURPOSE: To obtain a light source tube by bringing the potential of an anode electrode same with that on a fluorescent face while setting the distance between a current control electrode and a cathode and the hole diameter of the current control electrode into specific values thereby eliminating the deflection system of the electron beam.

CONSTITUTION: A fluorescent face to be coated with a cathode 4, first grid 1, anode electrode 3 and a phosphor subsistance is arranged in a cathode ray tube. Furthermore a graphite film 7 is applied on the tube wall between the electrode 3 and the fluorescent face 5. The distance LG1K(mm) bbetween the cathode 4 and the grid 1 is set such that 1<LG1K<3 while the hole diameter d1(mm) of the grid 1 is set such that 1<d1<3. High voltage Eb is applied through the electrode 3 or a contact 8 on the fluorescent face 5 and the graphite film 7, while D.C. voltage EC1 close to the ground potential is applied on the grid 1 and the modulation voltage EK having the relation EK≥EC1 is continuously applied to the cathode 4. In such a manner the electron beam 6 emitted from the cathode 4 is projected on the fluorescent face 5 with the required spot diameter.