Patent GB-985444-A (GB)
Title: Improvements in or relating to electro-mechanical transducers
IPC: H03B5/30
Filed: 1959-11-05
Abstract: 985,444. Valve oscillator circuits; valve supply circuits. P. BEER. Feb. 2, 1961 [Nov. 5, 1959], No. 37558/59. Headings H3F and H3W. Various valve oscillators are used to drive an oscillatory circuit 19 comprising a piezo-electric resonator, from which useful mechanical energy is derived, and an inductor which, together with an optional parallel capacitor, forms with the capacitance of the crystal a parallel resonant circuit resonant at the mechanical resonant frequency of the crystal. In the circuit of Fig. 5, feedback derived from the potentiometers R6, R7 is applied to the grid of triode V1. A feedback voltage which is independent of frequency is maintained by choosing C7 such that C7R7=C<SP>1</SP>R6 where C<SP>1</SP> is the stray capacitance across C6. The anode of V1 is capacitycoupled to the grid of V2 and the cathodes of the two valves are coupled by a capacitor C5. The output of the oscillator may be modulated at mains frequency by using an H.T. power supply such as is shown in Fig. 7. In the circuit of Fig. 8, the resonator circuit 19 forms one arm of a bridge, the other arms of which consist of fixed resistor R15, variable resistor R16 and a non-linear resistance consisting of a lamp LP1 in series with a resistance R14. Two opposite points on the bridge are connected by leads 59, 60 to the terminals of output transformer T6 and the intermediate point is connected to a feedback lead 66 including a phasecompensating capacitor C19. By means of a variable resistor R17 an adjustable feedback voltage is applied to the control grid of valve V3. The circuit is protected against overload in four ways. If the bias supply disappears, bias present relay RE releases and opens contacts RE1 1 which disconnect the primary winding of HT transformer T7 and the coil of interlock contactor RC. This will release and, through contacts RC1, RC2, disconnect the primary winding of the EHT transformer T8 and the coil of the HF switching contactor RD. If V4, V5 or V6 draw excessive grid current, overload relay OLA operates and when contacts OLA1 open the coil of the main contactor RA is disconnected, thereby switching off the whole apparatus. If V4 draws excessive anode current, this will cause operation of overload relay OLB which is provided with a holding coil extended through normally-open contacts OLB2 to an auxiliary D.C. supply provided by a bridge rectifier MR8 connected to filament transformer T5. Contacts OLB2 complete a holding circuit for the relay and contacts OLB1  break the circuit to the coil of the contactor RB which releases. Contacts RB1, RB2 break the circuits of the bias transformer, the HT transformer and the EHT transformer. To re-set and release relay OLB, switch PBA is operated and released, followed by re-operation of " mains-on " push switch PBB. If V5, V6 draw excessive anode current, overload relay OLC, which functions in the same manner as OLB, is operated. A valve heater time-delay TD connected across the secondary winding 53 of the filament transformer T5 operates after a predetermined delay and contacts TD1 extend a circuit through normally closed contacts OLB1 to an operating coil of contactor RB. Switches RB1, RB2 close to apply bias voltage which, however, exceeds the cut-off voltage since contacts RD2 and RD3 are still closed. Thereafter, relays RB and RE operate in turn to switch on the HT and EHT supplies respectively. When switch PBD is closed manually (PBC, also manual, is normally closed), relay RD is actuated and operates switches RD2, RD3, whereby operational bias is applied to the valves. Contacts RD1 and RA1 provide holding circuits against the release of push switches PBB and PBD.