Patent GB-448101-A (GB)
Title: Improvements in or relating to thermionic valves
IPC: H01J21/12
Filed: 1934-09-27
Abstract: 448,101. Thermionic valves and valve circuits. RODDA, S., 175, Ladysmith Road, Enfield, Middlesex. Sept. 27, 1934, No. 27775. [Classes 39 (i) and 40 (v)] A circuit arrangement comprises a thermionic valve having a cathode, a control grid, an anode and at least one additional electrode, means for applying a signal between the control grid and cathode, means for applying to the anode and the additional electrode suitable positive potentials with respect to the cathode and means for altering the bias of the control grid for the purpose of gain control, the construction and disposition of the electrodes of the valve being such that the anode current-control grid potential characteristic of the valve passes through a point of zero slope in the region in which the control grid is negative with respect to the cathode, the slope being negative over a range of negative grid voltage below (more negative than) this point and positive over a range above this point, the characteristic being approximately parabolic in the neighbourhood of the point. It is shown that a characteristic of this shape reduces distortion by cross-modulation and demodulation, without reducing the range of mutual conductance variation. In the form shown in Fig. 1, the control grid, screen grid and anode each comprise two flat rectangular portions, one on each side of a flattened indirectly heated cathode 1. An earthed screen surrounding the electrodes has two faces 11, 12 apertured and coplanar with the anode members and two faces 13, 14 leaving a gap between the screen grid members 6, 7 and the screen. As the negative bias on the control grid 4, 5 increases, the normal electron flow to the anode through screen grid is reduced but the increased focusing effect of the control and screen grids enables electrodes from the narrow faces of the cathode to reach the anode by passing round the screen grid. A rise in the characteristic is thus produced in a certain region of negative grid bias, the screen current at the same time falling to zero. Further increase in the negative bias causes the characteristic to drop. In a modification, Fig. 3, the focusing action is increased by providing the screen grid with extensions 20, 21, preferably of sheet metal, having slots 16, 17 therein through which the electron beam is concentrated with increasing negative bias, to fall on auxiliary anodes 22, 23 connected to the main anode. In a further modification, a cylindrical screen surrounding the cathode has four slots, the electrons being concentrated upon these by four control rods arranged parallel to the cathode in each quadrant of the screen. An anode plate opposite each slot receives a beam of electrons. In order to improve the slope of the characteristic in the region of low grid bias, a valve section with focusing control may be combined in parallel with a valve section having normal space charge control. The curve 33, Fig. 6, represents the characteristic of the focusing section, and the curve 34 that of the normal section, the resultant characteristic of the combination being shown at 35. As shown, the cut-off point of curve 34 is preferably arranged to coincide with the minimum of curve 33. The wound grid of the normal section may be disposed on longitudinal extensions of the focusing control rods, when the same cathode may be used for both sections. Alternatively separate cathodes may be used. The flow of secondary electrons to the screen grid from the anode may be reduced by earthed or low potential screens clear of the main electron stream, as described in Specification 423,932.