In small, flat in-line display tubes having a screen of the order of 75 mm diagonal it is important to be able to deflect the electron beam, say in the frame direction, along paths which are accurately parallel to each other in order to make the subsequent beam deflection processes easier.
In order to do this the display tube includes an electron beam deflector comprising first and second electrode arrangements (30, 40) arranged successively along the electron beam (13) path from an electron gun (14). Each electrode arrangement comprises a pair of resistive plates (31, 32 and 41, 42) extending transverse to the path of the electron beam path and disposed laterally one on each side of the path. The plates (31, 32 and 41,42) of each electrode arrangement (30, 40) are joined at their top and bottom ends and a potential difference is applied across the plates to provide electrical fields (E) substantially normal to the electron beam path from the electron gun (14). The effect of the fields (E) provided by the respective electrode arrangements (30, 40) on the electron beam are equal and opposite so that the angular deflection (a) of the electron beam caused by the first electrode arrangement (30) is cancelled by the second electrode arrangement (40) and the electron beam (13) leaves the electron beam deflector on a path parallel to (or coincident with) the path it entered the deflector. In order to ensure that no additional angular deflection of the electron beam occurs when it crosses the interface between the first and second electrode arrangements (30, 40), the voltages (Vt2 and Vb2) applied to the second electrode arrangement are varied so that the point (B) of entry of the electron beam is at an equipotential with that of the beam.
Other electrode arrangements are disclosed together with an embodiment of a flat display tube.