US2129088A - Thermionic valve circuit arrangement - Google Patents

Description

INVENTOR DOUGLAS FRANK GEORGE Kg v ATTORNEY Sept. 6, 1938. D. F. GEORGE THERMIONIC VALVE CIRCUIT ARRANGEMENT Filed May 2a, 1956 bib-h in hbhbbbb) 1111111 1111111'! Patented Sept. 6, 1938 h UNlTED f -ST'ATES PATENT orrlcc Douglas Frank George, Hutton, England, assignor to Radio Corporation of America, a corporation of Delawarev Application May 23, 1936, Serial No. 81,361 In Great Britain May 24, 1935 4 Claims. (Cl. 25027) This invention relates to thermionic valve 'cirotherwise known under the registered trade mark cuit arrangements and has for its object to pro- Thyratron. Such a tube is normally prevented vide an improved safety circuit arrangement from being conductive by bias potential applied whereby the chances of damage to'a valve due to to its grid and derived from the grid circuit of the '5 flow therethrough of excessive current caused by valve to be protected, the arrangement being such a failure in an associated circuit is substantially that when the grid current of the valve to be pro reduced or eliminated. tected drops substantially the grid potential of The invention though not exclusively limited the Thyratron device changes to an extent, and thereto is particularly applicable to radio transin a direction, sufficient to cause the said device to mitter and like high frequency oscillator valve cirbe'come'conductive, and thus bring down the po- 10 cuit arrangements. tential of the screen grid of the valve to be pro- If, in a radio transmitter or like apparatus, a tected. thermionic valve be connected in a high frequency The invention is illustrated in the accompanycircuit-either as a high frequency oscillator or ing drawing which shows diagrammatically one as a high frequency amplifier driven by a preembodiment thereof. 15 ceding oscillator'of usual convention arrange- Referring to the drawing, a high frequency ment, substantial grid current will fiow under noramplifierfor example in a radio transmitter 1 mal oscillator conditions. If, in the case of a comprises a screen grid valve l the control grid high frequency amplifier valve the oscillatory 2 of which is connected to the cathode point 3 drive fails, orin the case of ahigh frequency osthrough a grid resistance 4. The screen grid 5 20 cillator valve some high frequency component in of the valve is connected to the anode ll of a gas the circuit breaks down, the valve will no'longer filled grid controlled discharge device l whose present its normal back electro-motive force, and cathode point 8 is connected to the cathode point in consequence, will take. an abnormally heavy 3 of the screen grid valve I. A suitably chosen anode current which will in time damage the tapping 9 upon the grid resistance t is connected 25 valve, e. g. will cause it to become sof or lose to the grid ill of the grid controlled discharge deits cathode emission. As a further consequence, vice 1. The anode ll of the valve H is conthe normally substantial grid current will cease nected through a coupling condenser l2 to one end to flow. The present invention utilizes this latter of the usual parallel tuned output circuit l3 the effect to provide an automatic'safety action preother end of this circuit being connected to the 30 venting the continued flow of abnormally high common cathode point. High frequency oscillaanode current due to a breakdown. tions from a suitable master or other drive In carrying out this invention, the valve to source (not shown) are applied, through asuitable be protected is provided with a screen grid (i. e. coupling condenser M to the control grid 2 of the it is a valve having a grid between the control valve l and anode potential from a source (not 35 grid and the anode, e. g. a so-called screen grid shown) is applied to the anode H of the valve l valve, pentode or other multi-grid valve) and through a choke l5. Normal relatively high means Operated automatically in dependence p screen grid potential is applied to the screen grid the grid current of the said valve is provided for 5 of the valve I from the anode potential source by 40 substantially lowering the screen grid potential means of a high resistance potentiometer con- 40 of the valve below normal if the said valve ceases sisting of two series resistances it, it, one It) to pass normal grid current. It has been found of which is connected between the potential source that if the screen grid potential of a valve havand the screen grid 5 and the other ll) of which ing a screen grid be considerably reduced in is connected between the screen grid 5 and the value from its normal high positive potential, the common cathode point. 45 internal anode-cathode resistance of the valve in- The whole arrangement is such that in normal creases to a high value which, at zero screen grid operation the voltage applied to the grid I ll of the potential, approaches infinity. Accordingly the grid controlled gas filled discharge device 71 is effect of automatically reducing the screen grid sufficient to prevent said device from becoming potential is substantially to reduce the anode curconductive, but if-for 7 example owing to the 5 rent which can flow through the valve. faiure of the high frequency driving source Preferably the automatic reduction Of th the grid current of the screen grid valve drops Screen rid potential s Obt d y Connecting in much below normal, the negative bias voltage on the screen grid circuit a gas filled electric disthe grid of the gas filled discharge device is no charge device of the trigger-action type, or a tube longer maintained at a sufficient value to prevent 55 said device from becoming conductive, and accordingly the said device becomes conductive thereby bringing down the voltage of the screen grid 5 to much below normal value. Accordingly the internal resistance of the valve l rises to an extent amply sufficient to prevent it from damage due to excessive anode current flow.

By suitably choosing the gas filled discharge devicesuch devices giving an anode to filament voltage drop of only about 15 volts when current is being passed, are available on the market at the present timeit is possible to cause the screen grid potential to be reduced almost to that of the cathode, thus reducing the anode feed current of the screen grid valve practically to zero. By virtue of the fact that the resistances I6, I! are high, the high tension current flow through the gas filled discharge device, when the said device is conductive, will be vary low.

The invention is not limited to the use of a gas filled discharge device or other purely electrical relay, since obviously a suitable electromechanical relay could be employed and operated under grid current control, but the illustrated embodiment with its purely electrical action is preferred. Again, as will be obvious, the inven tion can be employed to protect a bank of valves just as well as a single valve.

An important practical advantage of the present invention lies in its extreme simplicity and cheapness of construction.

The following statement of components (this statement is given purely by way of example) relates to a circuit as illustrated and which has been successfully employed in actual experimental practice.

Valve l: Valve as now commercially known under the trade designation Marconi A. C./S. 2.

Anode voltage: 3,500 volts.

Resistance l6: 45,000 ohms.

Resistance I1: 12,000 ohms.

Resistance 4: 12,000 ohms between grid 2 and point 9. 7 1,800 ohms between point 9 an point 3.

Device 1: Gas filled relay as now commercially known under the trade designation G. T. 1.

In this embodiment, under normal oscillatory conditions the anode feed was 250 milli-amperes, and the total anode power was 875 watts with the grid 2 at 200 volts negative, 750 volts (maximum) on grid 5, 26 volts negative on grid l0,

and 750 volts on anode 6, there being, of course, no discharge current through the device I.

On the occurrence of a failure (e. g. a failure of carrier frequency drive to the valve I) the anode feed, the total anode power, the voltages on the grids 2, 5 and I0, all dropped practically to zero and the voltage on anode 6 became approximately 15 volts. Without the safety circuit, a similar fault would have destroyed the valve l by anode dead loss wattage.

What is claimed is:

1. A protective system for a circuit of the type which includes an electron discharge tube for the generation of oscillations, said tube having a cathode, an anode, a control grid and at least one other grid, said protective system comprising a gaseous discharge tube of the trigger-action type, a control grid in the last said tube, means inter-connecting the controlgrids of the two said tubes for causing the gaseous discharge tube to be triggered off in response to a cessation of oscillations in the first said tube, and means acting in dependence upon the conductivity of the gaseous discharge tube for rendering the first said tube substantially non-conductive.

2. A system in accordance with claim 1 and further characterized in that the first said discharge tube possesses a screen grid electrode and said gaseous discharge tube possesses an anode electrode, these two electrodes being inter-connected.

3. A protective system for a multi-grid electron discharge tube on the input circuit of which oscillations are normally impressed, said system comprising a grid-controlled gaseous discharge tube having a space path of relatively low impedance when triggered off, means including a resistive impedance in said input circuit of the first said tube and a connection therefrom to the control grid of said gaseous tube for triggering ofi the latter in response to a marked diminution of amplitude of said oscillations in said input circuit, and means acting in dependence upon a flow of space current in said gaseous tube for rendering the first said tube substantially nonconductive.

4. A system in accordance with claim 3 and having an output circuit for said gaseous tube which connects with a screen grid in said multigrid discharge tube.

DOUGLAS FRANK GEORGE.

Images