GB339994A - Frequency setting fork - Google Patents

Abstract

339,994. Electrical Research Products, Inc., (Assignees of Curtis, A. M.). Oct. 4, 1928, [Convention date]. By varying resistance.-In electric ally-operated frequency setting devices of the tuning fork type a valve amplifier is controlled by a tuning fork having pick-up and drive coils characterized in that polarizing windings associated with the coils are supplied from the same source of potential as the anode of the valve. In the circuit shown the tuning fork 10 has a pickup coil 11 connected to the input of valve 12 through the transformer 50. The secondary winding of the transformer 14, the primary of which is connected in the output circuit of valve 12, is divided, a part 15 being connected through a resistance 16 and condenser 33 to the drive coil 13, and a part 17 being connected through a filter circuit 18 and transformer 19 to the amplifying valves 20, 21. The filament and anode currents are drawn from the same source 24, the filament circuit including resistances 26, 27 so that the drop of potential across the resistance 27 provides a negative bias on the grids of the valves. The drive and pick-up coils are provided with polarizing windings 35, 36 connected in series and supplied from source 24 through variable resistances 38, 38<1> and choke coil 37. By a suitable adjustment of the normal current through this polarizing winding the arrangement can be made to compensate for variations in the amplifier efficiency caused by variations in the supply voltage at 24. Magnetic coupling between the drive and pick-up coils is reduced by arranging the coils so that the flux traverses the fork tines in a transverse direction. When the fork is not of magnetic material shoes 5 of magnetic material are mounted on the tines in a manner to avoid damping of the fork. The Specification gives the composition of a steel the elasticity of which changes little with temperature variations.