Patent GB-207817-A (GB)
Title: Improvements in and relating to thermionic tubes
IPC: H03B5/10
Filed: 1923-11-30
Abstract: 207,817. Trautwein, F. Dec. 4, 1922, [Convention date]. Thermionic generators; thermionic valves, arrangements and connections of.-The grid and plate circuits of a back-coupled thermionic tube are connected through a common resistance Rv shunted by a condenser Kv, Fig. 4, the resistance being so dimensioned relatively to the constants and variables of the tube that the hysteresis loop of the oscillatory current, and of the anode current as a function of the grid voltage is eliminated. In Fig. 2 curve 1 represents the normal grid-volt plate-current characteristic of a tube, whilst the curves 2 - - 5 show the effect of inserting coupling resistances of two, four, six, and eight thousands ohms respectively. One advantage of this arrangement, when used for reception, is that the variation of grid potential is created by the amplified plate currents, and not by the relatively weak incoming signals. The combined effect of the resistance Rv and condenser Kv also sets up a superposed low-frequency oscillation, which is determined by the time constant of these elements and by the damping of the oscillatory circuit. When the arrangement is used to generate high-frequency oscillations, the superposed low-frequency due tc the elements Rv, Kv modulate the carrier wave at audible frequency. The relations between the steady and oscillatory plate components as the grid voltage is varied, for different values of coupling resistance, are shown in Fig. 5. Curve 1 represents the conditions when no coupling resistance is used, whilst curves 2 and 3 show the effect of resistances of 4000 and 8000 ohms, respectively, shunted by a capacity of .01 mfds, on the circuit shown in Fig. 4. Curve 3 has practically no hysteresis loop so that speech modulation direct on the grid becomes feasible, whilst maintaining a comparatively tight reaction coupling. Within the cross-hatched area a process of " automatic modulation " occurs, the oscillations being automatically quenched at a definite frequency. The relation between a direct voltage applied to the grid and the steady plate current with a tight reaction coupling gives a steep characteristic with sharply-defined upper and lower limits as shown in Fig. 6. This can be utilized with known methods of indicating the peak values of alternating-current voltages, as well as in the general case where a small applied impulse is required to release a comparatively large current of definite value. By suitably adjusting the normal grid potential of such a back-coupled tube, the reception of undesired signals and atmospherics can be limited or suppressed. Where the oscillatory circuit L, C, Fig. 4, is tuned to the incoming signals its effective damping is decreased. This in turn affects the note of the superposed lowfrequency arising from the resistance Rv and condenser Kv, and can accordingly be utilized to distinguish " spacing " signals from. " sending " signals and as a substitute for, or in combination with a local heterodyne, either to operate a telephone or a relay. A method of utilizing a resistance-coupled generator so as to produce simultaneously two sustained oscillations of different frequencies is also described. The Specification as open to inspection under Sect. 91 (3) (a) comprises also the application of the invention to a two-key telephone repeater for opening and closing the speech channel in both directions without introducing distortion effects. This subject-matter does not appear in the Specification as accepted.