GB739355A - Improvements in or relating to devices for producing pulsatory x-rays - Google Patents

Abstract

739,355. Supply systems for X-ray tubes. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Jan. 25, 1954 [Jan. 28, 1953], No. 2159/54. Class 38 (4). To avoid excessive screen-grid current in the intervals between successive discharges of pulsed X-ray tube 2 of the tetrode type excited by periodically applying a cut-off bias Vgl to its control grid 6 to interrupt abruptly current flow through an inductor 3 connected in the anode circuit of the tube, means are provided for so controlling the internal im. pedance of the tube between discharges that the screen-grid current is maintained at a low value. In one arrangement, Fig. 6, this current limitation is effected by a high-valued resistor 28 in series with the voltage source Vg2 for the screengrid 7, undue voltage rise at that grid during the discharge period being prevented by a diode 29 which connects the grid to a relatively lowvoltage tapping on the source. In another arrangement the screen grid is connected directly to its voltage source and a control voltage of waveform suitable to maintain the screen grid current at low value is applied to the control grid of the tube, this control voltage and the periodic bias voltage being applied over diodes to prevent interaction. In the arrangement of Fig. 9, a valve amplifier 16, 21, responsive to voltage changes across a lowvalued resistor 13 in the screen-grid supply circuit, provides an output voltage which is applied over diode 11 to the control grid of the tube and which is of the correct waveform to maintain the screen grid current substantially constant. The periodic bias voltage Vgl is applied over diode 8. In a modified circuit the resistor 13 is connected between the tube cathode and the negative side of the screen-grid current source Vg2, a cathode resistor being included in the anode current circuit. The ratio between the screen-grid and anode currents is then equal to the ratio between these resistors.