Patent GB-199016-A (GB)
Title: Improvements in radio broadcasting equipment
IPC: H04R21/02
Filed: 1923-05-30
Abstract: 199,016. Western Electric Co., Ltd., (Assignees of Maxfield, J. P.). June 9, 1922, [Convention date]. Thermionic amplifiers, oscillation generators, and modulating systems; measuring and testing apparatus.-Relates to a telephony installation specially adapted for the transmission of broadcast programmes capable of being reproduced by loud-speaking receivers with minimum distortion. The installation comprises a push-and-pull airdamped microphone which feeds low-frequency currents of very small wattage (as compared with an ordinary telephone transmitter) to an audiofrequency amplifier which exalts them to average value, whereupon they are applied to modulate the output of a transmitting unit comprising thermionic amplifiers, variable-impedance tubes, and a high-frequency generator. Lay-out. The microphone is located in the studio marked A and is connected to the firststage amplifier panel marked B. The radio transmitter panel marked D is connected through a relay panel E to the aerial 57. The high and low tension power supply is furnished by batteries 13, 21 and a motor-generator set Mo, GB, GA located in the power room C, and controlled from a power panel F. The motor-generator set may also be controlled from the operator's room G which also contains a switch 91 for actuating the aerial relay E for " changing over " and means, including a loud-speaking receiver 98, to enable the operator to monitor the input to the radio transmitter. Operation. The low-wattage output from the microphone 7 is applied to the first-stage amplifier B comprising three electron discharge tubes 8, 9, 10 energized from the batteries 13, 21, the former also feeding the microphone circuit 5, 6, 7. The output from the panel B passes first to an amplifier L, then to a constant-current modulator M comprising two discharge tubes 62 in parallel, and finally to a radio-frequency generator O comprising two parallel tubes 50 back coupled through the coils 53, 59. The modulated output is fed to the aerial circuit through the coil 53. The low tension supply to this panel comes from the generator GA, whilst high tension is fed from the generator GB. A resistance 75 reduces the voltage applied to the tube L. Resistances 67 and 68 serve to maintain the grids at desired negative potentials relatively to the filaments. Power supply. The system is designed for a radio frequency output of at least 100 watts modulated with low-frequency power of the order of 6 milliwatts. The motor generator set in the power room C comprises a shunt-wound motor MO driving a filament-current generator GA and a high-tension generator GB. The motor is supplied with current from the mains 102, 103 through an automatic starter 105 controlled by push-buttons PB1 or PB2. As the voltage across the motor armature rises, resistances 110 and 111 are successively cut out by means of relays 112, 113, and a resistance 114 is brought into circuit to reduce the holding-current through the relays. Ammeters 200 and 201 measure the space current of the tubes M, 0. Ammeter 203, indicates the grid current of the generator O whilst the aerial current is shown at 202. The plate voltage from the battery 21 feeding the panel B is adjusted by switches H, I, J, K. Filament-heating circuit. Current from the generator GA supplies field current to generator GB and also energizes a time-limit relay 82, which, when the filaments are sufficiently heated, closes contacts 83 to short circuit a resistance 68 which reduces the negative grid potential to normal and thereby allows full space current to pass. The space current flow across the remaining resistance 67 imparts a negative potential to the grids of the amplifier L and modulators M. The filaments of the amplifiers 8, 9 on the panel B are arranged in series across the battery 13, and the circuit comprises fixed resistances 14, 15, a choke coil 16, and rheostat 17. Microphones. The transmitter, Fig. 2, is of the, differential or push-and-pull type. It comprises two carbon buttons 5, 6 arranged on each side of an air-damped diaphragm 7 clamped between two annular rings 151, 152 and tensioned by means of a ring 154 and a series of adjusting screws 155. The diaphragm is adjusted to have a natural frequency higher than 5000 per second. The power output is of the order of 6 microwatts as compared with 6 milliwatts from an ordinary telephone transmitter.