GB534294A - Improvements in thermionic tube amplifiers - Google Patents

Abstract

534,294. Valve amplifying circuits. ELECTRICAL RESEARCH PRODUCTS, Inc. Oct. 20, 1939, No. 28341. Convention date, Nov. 22, 1938. [Class 40 (v)] A sound recording system employing a negative feed-back recorder comprises a microphone 14 feeding bus-bars 12, 13 energy from which is fed by means of a transformer 22 to an amplifier 11 comprising push-pull pentodes 29, 30 and electron beam valves 45, 46. The output from valves 45, 46 is fed to the driving coil 16 of a recorder which has a coil 19 for providing a negative feed-back to an amplifier 21 which is coupled to the input of amplifier 11 through condensers 64, 65 and resistances 32, 36. The amount of negative feed-back is rather less than that necessary to produce a flat frequency characteristic over the whole frequency band as this produces high frequency instability, and the resultant drooping low-frequency end of the characteristic is corrected by suitable proportioning of condensers 64, 65 and resistances 24, 32, 26. 36 shunting the inputs to valves 29, 30. The amplifier 11 is provided with internal negative feed-back paths 47, 48 from the anodes of valves 45, 46 to the cathodes of valves 29, 30. To improve the quality of the amplifier 11 the screen grids are energized from a point 58 between resistances 59, 60 forming a bleeder circuit across the main supply. By suitably proportioning the resistances 59, 60 the screen potential is caused to vary such that the sum of the screen and plate currents is constant, thereby keeping the cathode current and grid bias for each tube constant. This arrangement gives greater stability of the negative feed-back and lower harmonic distortion. Switches 31, 33, 35, 37, 70, 77 and 78 are provided for use when the recorder is not of the negative feed-back type. In this case. the amplifier 21 may be switched out or may be connected in the signal circuit for extra gain.