US2838662A - Circuit-arrangement producing sawtooth currents - Google Patents

Description

E. JONES ET AL 2,838,662

Filed Aug. 25. 1953 AGENT IN VEN TORS All Pod 10000 l r; v

5M4)! Jolvzs'. KENME m EDNA/PD MART/A! CIRCUIT-ARRANGEMENT PRODUCING SAWTOOTH CURRENTS June 10,1958

ilnite CIRCUIT-ARRANGEMENT PRODUCING SAWTOOTH CURRENTS Emlyn Jones, Horley, and Kenneth Edward Martin,

Rochampton, London, England, assignors, by mesne assignments, to North American Philips Company, Inc. New York, N. Y., a corporation of Delaware This invention relates to circuit arrangements producing sawtooth currents by means of a screen-grid tube whose anode circuit comprises the series-combination of an inductor and a capacitor, a diode being connected in parallel with the series-combination of part of the inductor and the capacitor, and the cathode of the diode having a relatively long heating up time.

In such circuits a high peak voltage (of the order of 4 kv.) appears on the cathode of the diode and with the type of diode normally employed and available hitherto it is necessary to derive the filament current either from a winding associated with the inductor, or from a small mains transformer, the secondary of which is suitably insulated since the heater-cathode insulation of this type of valve is insufficient to withstand the high potential difference if the heater is included in the normal valve heater chain.

Diode valves have recently become available, in which the heater-cathode insulation has been improved to an extent which makes it possible to include the heater in the normal filament supply to the other valves of the receiver, but this improvement is, not unnaturally, accompanied by a cathode heating time which is relatively long in comparison with the other valves of the receiver. This in turn gives rise to the serious disadvantage that when using a screen-grid tube the anode remains unenergised, after switching on the receiver, until the diode conducts so that during this period substantially the whole of the emission current of the tube goes to the screen grid with the result that the maximum permissible wattage dissipation for this electrode is greatly exceeded. The object of the invention is to remove this disadvantage.

The circuit arrangement according to the invention has the feature that means are provided for applying a negative bias to the control grid of the screen-grid tube and, moreover, means for reducing or backing off said bias when the diode conducts.

Preferably the negative bias is derived from the line oscillator valve or valves to avoid reduction of the eiiective H. T. line potential and the means for reducing or backing ofi the bias may comprise a connection, through a resistor of suitable value, between the control grid of the screen-grid tube and the appropriate end or a tapping point on the inductor included in the anode circuit of the screen-grid tube.

In order that the invention may be more clearly understood and readily carried into efiect it will now be described with reference to the accompanying drawing in which Fig. 1 shows a line time base circuit.

Fig. 2 is one form of a negative bias circuit.

Fig. 3 is a second form of negative bias circuit and Fig. 4 is a third form of negative bias circuit.

Referring now to Fig. l, a line time base circuit includes an oscillator (not shown) driving a valve 1 of the tetrode or pentode type having its cathode connected to the common earth line of the receiver through the usual bias resistor 2, and its anode to one end, or a tapping point States Patent" goes to this electrode.

holds the control grid of the output valve at a sufliciently 2,838,662 Patented June 10, 19 58 on an auto-transformer winding 3 to which the line deflector coils (not shown) are also connected. The other end of the winding is connected to one pole of a capacitor 4 whose other pole is connected to the H. T. line of the receiver. A diode 5 has its anode connected to the H. T. positive line and its cathode to a tapping on the auto transformer winding.

The filament of this diode is included in the normal heater chain including the other valves of the. receiver. The screen grid of the line time base valve is connected directly to the H. T. positive line, if necessary, through a' voltage dropping resistor 6.

The circuit as so far described follows but in accordance with the present invention the grid leak 7 associated with the control grid of the line output valve instead of being returned to the earth line, is connected to a negative voltage source so that the grid is normally biased negatively by an amount sufficient to reduce the current flowing to the screen grid to a safe value when the voltage on the anode is such that there is no or substantially no current flowing in the anode circuit.

The control grid is further connected through a resistor 8 of suitable value to that end of the auto-transformer Winding which is connected to the capacitor 4.

The circuit works in the following manner. When the receiver is switched on the oscillator tube and the screengrid tube become energized, that is to say the cathodes after a short interval commence to emit electrons and the current potentials appear on the electrodes of these valves as soon as the H. T. supply becomes available, with the exception that there may be no voltage on the anode of the valve 1 due to the fact that the diode 5 is of the type having a relatively long heating time. By this is meant a heating time longer than that of the screen-grid tube and of the order of 30 seconds or more. Since the screen grid of the valve is connected to the H. T. line, in the absence of the grid bias, the whole emission current of this valve However, the negative grid bias negative potential while the diode is warming up to prevent too large a current flowing to the screen grid. As the booster diode commences to conduct, a positive voltage appears at the end of the transformer winding con nected to the capacitor 4 and this backs-01f the negative" crystal rectifier or thermionic diode 12 as shown in Fig. 2

but this method cannot be used with D. C. supply to the receiver since the bias required for the screen-grid tube when this is driven by an oscillator stage may be in the region of 50 volts. It is not feasible to obtain this from a resistor in the negative H. T. lead as the H. T. line potential would be reduced too much.

In this case the bias may be derived by rectification of the frame scanning waveform appearing at the primary of the. frame output transformer 9, as shown in Fig. 3, where tube 13 is the output tube of the frame time base circuit. A crystal rectifier 10 or thermionic diode may be employed and fed from a tapping in the primary winding 9 which is so chosen that the permissible peak inverse voltage of the rectifier is not exceeded. The current drain of this bias system is too small to afiect the operation of the frame time base. a i

In the case of self-oscillation line time bases thebias may be obtained from the grid 14 of a suitable blocking normal practice oscillator as shown in Fig. 4 and in this case no other rectifier is necessary.

What is claimed is:

v 1. A circuit-arrangement comprising a tube having at cathode, a controll grid, a screen grid, and an anode,. an inductor and a capacitor connectedin series combinatinn, a source of voltage, said series combination being connected between said anode and a terminal of said source of voltage, means connecting said cathode to a remaining terminal of said source of voltage, a diode connected in parallel with the series combination of said capacitor and at least a part of said inductor, said diode comprising a thermionic cathode requiring a relatively long time to heat up to operating temperature, means connecting said screen grid to said source of' voltage, a source of negative grid bias voltage, direct-current conductive means connected to apply said bias voltage to said control grid, said bias voltage having a value to reduce the screen grid current to a safe value in the absence of anode current, and direct-current conductive means connected to produce normal operating bias at said control grid only when said diode cathode reaches operating temperature.

2. A circuit-arrangement comprising a thermionic tube having a cathode, a control" grid, a screen grid, and an anode, a tapped inductor having, an end thereof connected to said anode, a source of voltage, a capacitor connected between a terminal of said source of voltage and. the

a a 838,662 a 3 4 remaining end of said inductor, means connecting said cathode to a remaining terminal of'said source of voltage, means connecting said screen grid to said source of voltage, a thermionic diode having an anode connected to said source of voltage and a cathode connected to said tap on said inductor, the cathode of said diode requiring a longer time to-warm up-to' operating temperature than is required for the cathode in said tube, a source of negative grid bias voltage,. a direct-current conductive impedance member connectedbetween: said control grid-and said bias voltage source, said bias voltage having a value to reduce the screen gridi current to a safe value'in the absence of anode current. in said tube, anda direct-current conductive impendance member connected between said control grid and a point: on said inductor whereby normal operating bias is automatically produced at said control grid only when said diode cathode has warmed up sufficiently to permit anode currentto flow in said tube.

ReferencesCited in. the file of this patent UNITED STATES PATENTS 2,063,025 Blumlein Dec. 8, 1936 2,212,217 White et al. Aug, 20, 1940 2,503,745 Kerkhof -a Apr; 11, 1950 2,543,428 Wendt et al Feb. 27, 1951 2,565,392 Neuworth Aug. 21, 1951 2,760,110 Janssen Aug. 21, 1956

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