Patent GB-254496-A (GB)
Title: Improvements relating to generators of electric oscillations
IPC: H03B19/10
Filed: 1925-06-18
Abstract: 254,496. Western Electric Co., Ltd., (Western Electric Co., Inc.). June 18, 1925. Thermionic generators and amplifiers. - Thermionic generators are adapted for the production of a constant frequency by employing a single tuned circuit connected at three points to the valve electrodes, and by inserting in the grid connection a condenser of such capacity that in association with the resistance connected between the grid and filament the voltage on the grid is maintained accurately in phase wih the plate current. The oscillator is enclosed within a metallic shield suspended by springs. The oscillations may be multiplied in frequency prior to modulation and transmission. Referring first to the general scheme of the circuit, a constant-frequency oscillator 1 is variably coupled to an amplifier 7, which feeds a distorting valve 8 having its output circuit tuned to the second or other harmonic of the fundamental frequency. This harmonic is passed on to a second distorting-valve 17 which selects a higher harmonic and feeds an aerial through a power amplifier 26. The harmonic-producer 17 also serves to modulate the transmitted wave, having a speech-amplifying valve 22 connected to its plate supply circuit in the manner known as choke modulation. All the valve filaments are supplied in parallel from an A.C. source 12, and the grids are biased by a source 31 and separate potentiometers 34, 36, 39, 44, each grid circuit including a filter 32, 37, 40, 45. The plates of the valves 1, 7 are supplied from a source 3, and the plates of the remaining valves from a source 13. Telegraph signals may be sent by operating a key 48 controlling the circuit of a frequencychanging coil 47 coupled to the circuit 2 of the oscillator. This circuit is connected in the usual way to the three electrodes of the valve 1, and the grid condenser 29 and resistance 28 are so proportioned as to cause the grid potential and the plate current to be accurately in phase. The generated oscillations are fed through a non- inductively wound potentiometer 6 to the grid of amplifier 7, with chokes 33, 9 in its grid and plate circuits. The grid of this valve is so polarized by the biasing source 31 and potentiometer 34 that it never becomes positive during operation. A condenser 11 connects the plate of valve 7 to the grid of the harmonic producer 8, which is polarized by the source 31 and potentiometer 36, through a choke 10. The bias is adjusted so that the grid becomes positive during the cycle, and so distorts the wave-form. A tuned circuit 15, 16 connected to the plate and filament of the valve 8 is tuned to the desired harmonic and has low impedance for the fundamental. The harmonic is fed to the grid of valve 17, which has its plate circuit 19, 20 tuned to a higher harmonic. The plate supply for the valve 17 is fed from the source 13 through a speech-frequency choke 23 and a high-frequency choke coil 18, and the choke 23 is also included in the plate circuit of voice-current amplifier 22, the valve 17 thus serving as modulator and harmonic producer. By-pass condensers are connected across the high-tension and grid-biasing sources 3, 13, 31.