A heat treatment oven (1) defines a plurality of inner chambers or zones for receiving Cathode Ray Tubes stacked in two rows and vertically orientated with front faces of tubes facing one another. The oven includes heating means in the form of gas burner or electric element (3) for heating fluid medium, in this case air. The air is circulated around the Cathode Ray Tubes by means of an impeller (5). The oven includes first and second control means for varying the temperature of the fluid medium and the velocity of the fluid medium introduced.