Patent GB-268048-A (GB)
Title: Improvements in space discharge tube systems for electric wave signalling
IPC: H04B1/04
Filed: 1925-12-28
Abstract: 268,048. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Dec. 28, 1925. Thermionic generators and amplifiers.-Transmitting systems for wireless telegraphy comprise a low-power control oscillator, preferably controlled by a piezo crystal, coupled in tandem to one or more amplifiers, which are adjusted to the verge of oscillation and are set into oscillation, at the frequency of the control oscillator, by the operation of the signal key. As shown in Fig. 1, the prime oscillator controlled by a piezo crystal 25 is coupled through successive amplifiers 20, 30 and a filter circuit 61 - - 63, to the aerial 65. Each unit is enclosed in screening compartments 50, which are earthed. The prime oscillator comprises a valve 11, with its plate connected through a blocking condenser 17 to one side of a tuned circuit 18, 22. The grid is connected through a piezo crystal 25 and condenser 24 to a tapping on the coil 18, and another tapping is connected to the earthed screen 50 by a condenser 20<11>, thus constituting a high-frequency path to the filament via the condenser 21. The plate voltage is supplied by a dynamo 15 through a choke coil 14, and the filament voltage by a dynamo 67, the filament of the valve 11 being in series with that of the first amplifier 26. The grid is connected to the filaments through a choke coil 12 and resistance 13. The end of the tuned circuit 18, 22 remote from the plate of the valve 11 is connected to the grid of the amplifier 26. The tuned circuit 38, 40 is associated with the valve 26 in a similar way as for the oscillator 11, except that the control crystal is omitted, and the grid condenser 42 is adjustable to enable the amplifier to be set to the verge of self-oscillation. Plate voltage is supplied to the valve 26 through air-cored choke 32 and iron-cored choke 33 by a dynamo 34, the return connection to the filament being through resistances 29, 31 and choke coil 30<1>. These elements are also included in the direct-current grid circuit of the valve 26, and result in a negative bias on the grid. The negative bias is partly removed when the key 70 is depressed to form a short-circuit across the resistance 31, thus causing the amplifier 26 to break into oscillation at a frequency determined by the oscillator 10. The key is shunted by a resistance 71 and condenser 72 for reducing sparking, and is in series with a choke 69, for preventing key " clicks." A resistance 25 is connected in shunt to the plate-filament path so as to maintain a current through the biasing-resistances 29, 31 when the valve is taking little or no current. Ripples from the dynamo 34 are reduced by the choke 30 and condenser 36. By displacing the switch 28 from the contact 28<1> to 28<11>, an audio-frequency generator 27 may be connected in the grid circuit for working with tonic train. Between the tuned circuit 38, 40 and the grid of the second amplifier 43, adjustable resistances 44 - - 44<11> are connected as shown for regulating the voltage applied to that grid, and for matching the impedance of the circuit to the respective input and output impedances of the adjacent valves 26, 43. The output circuit of the valve 43 comprises a coil 57 and two condensers 58, 62 of which the latter is included in the filter circuit 61 - - 63 designed to eliminate harmonics. The aerial circuit includes the condenser 63 of the filter circuit. A reactive connection is taken from the output circuit through a variable condenser 59 to a tapping 60 on the inter-valve coil 38. Grid bias is applied to the valve 43 from a generator 47 through a choke coil 46, and plate potential from an alternator 55, rectifier 54 and smoother 52, 53. The generators 34, 67, 47, 15 are mounted on a common shaft driven by a motor 68. Fig. 2 shows a modified circuit in which the general arrangement is similar to that of Fig. 1, except that an additional amplifying-valve 77 is interposed between the prime oscillator 11 and the regenerative power amplifier 26, and the arrangements for supplying plate and filament current, and grid-biasing potential, are modified. The grid of the amplifier 77 is conductively connected through a resistance 781 to the oscillator grid, and its tuned plate circuit is connected directly to the grid of the power amplifier. The filaments of the valves are supplied with alternating current through transformers 76, 89, 94 the primary coils of which are connected across the phases of a three-phase supply 102. Plate current is supplied in parallel to the valves 11, 77 from the positive brush 78 of a continuous-current generator 1, having three armature tappings connected through slip rings 82 and star-connected choke coils 83 to earth, so as to give a three-wire continuous-current output, with the central wire earthed. The negative brush 80 is connected through resistances 31, 31<1>, 29 to earth. The grid of the amplifier 26 is biased by a connection through choke coil 30<1> and resistance 71 to the negative brush 80, and the grid of the other amplifier 43 is connected through choke coil 46 to the same negative brush. The signal key 103 is connected in series with a coil 104 as a shunt across the coil 30' and resistances 31, 71. Depression of the key reduces the negative grid bias of the valves 26, 43, permitting them to oscillate at the frequency of the control oscillator. Plate potential is supplied to the valve 26 by the winding 2 of a second armature mounted on the same shaft as the winding 1, the negative brush 91 is connected to the positive brush 78 of the first armature 1. For the last valve 43, plate current is taken from the three-phase supply 102 through transformer 95 and rectifiers 97, the plate being connected to the neutral point of the star-connected secondary, the ends of which are connected to the respective mid-points of the secondaries supplying the rectifier filaments. The plates of the rectifiers are earthed to complete the circuit to the filaments of the amplifier valves.