US2235414A - Thermionic valve circuits - Google Patents

Description

M ATTORNEY E. L. C. WHITE THERMIONIC VALVE CIRCUITS Filed June 28, 1939 Much 18, 1941.

l m 1 l l 1 INVENTOR ERIC L. C. WHITE BY` Z Patented Mar. 18, 1941 UNITED STATES- PATENT oel-Fics THERMIONIC VALVE CIRCUITS Eric Lawrence VCasling White, Hillingdon, England, assignor lto Electric & Musical Industries Limited, Hayes, Middlesex, England; a' company of Great Britain Y Application June 2s, 1939, serial No. 281,633

In Great Britain June 30, 1938 4 claims.. (o1. 179-171) oscillating due to feedback. The screening tobe effective has to be continued inside .the thermionic valve in addition to the external screening. For this reason the screened grid type of valve has been developed and used. Howevenior var ious reasons, including eheapness of manufacture and ease of operation, itis preferable especially in large power radio-frequency ampliiiers to use Valves of the triode type. These valves have the disadvantage that in the/known circuit arrangements the diiculties of preventing oscillation Without neutralisation are very great, because the anode-to-grid capacity of the valves couples the input and output circuits, and this disadvantage is especially prominent when the amplifier is operated at ultra short Waves.

It is the object of the present invention to provide an improved radio-frequency amplifier in which these difliculties arer substantially overcome.

According to .the invention an ultra-short Wave thermionic valve apparatus is provided comprising three tubular conductors arranged one Within the other, the outer tubular conductor being associated with one electrode of a thermionic valve, the intermediate tubular conductor being associated with a control electrode of said valve and forming a tuned circuit with said outer tubular conductor, the inner tubular conductor being associated with another electrode of said Valve and forming a tuned circuit with said intermediate tubular conductor the arrangement being such tha-t the intermediate tubular conductor screens the inner tuned circuit from the outer tuned circuit.

In a preferred arrangement according to the invention the first mentioned electrode is the anode of a triode thermionic valve, and said inner tubular conductor is associated with the cathode of said valve.

For the purpose of describing the invention more in detail reference will now be ymade to the accompanying drawing which illustrates a particular embodiment according to the invention.

In Ithe figure which illustrates an embodiment according to the invention, a triode thermionic valve, which is preferably of the ring sealed grid type, is used as an amplifier valve in a short Wave transmitter. The metal anode I of the valve is surrounded by a Water jacket I6 for the purpose of cooling the valve. The grid 2 of the valve issituated between the anode I and the cathode y3` and eiectively'shields these electrodes 5 one from the other. The glass insulation of the valve is indicated at 4.

To the grid 3 of the valvefis fastened a cylindrical or rectangular tube 6 ofmetal, for example, copper, the far end having a broad rim which 10 is connected to earth. Concentrically surrounding the tube 6 is another tube' 5 of metal, one end of which is fastened to the anode I of the valve and the other end of which is by-passed to earthbymeans of a condenser. As shown in the 15 diagram the condenser is formed by means ofA a nan-ge on the tube 5 butting against the rimy of the tube 6, a piece of mica 1 serving as an insulator. The outside of the tube 5 is taken to a suitable source of positive potential. Arranged 20 Concentrically Within the tube 5 is an additional metal tube 8, one end of which is connected via blocking condensers 9 to the cathode 3 of the valve, the other end being fastened to the rim of the tube 5 and thus being eiectively earthed. 25 The heater current for the valve is conveyed through the tube 8 by means of a -two wire conductor I0 from a suitable source II (shown as a battery). The bias source I2 for the valve is connected between the source I I and earth.

The input radio frequency oscillations are fed into the amplifier stage by means of a conductor I3 which is suitably .tapped on the cathode circuit. The ouput from the amplier is taken from tappings on the outside of tube 6 and is 35 conveyed by means of lead I4 (which may be a suitable feeder-line) to the load circuit represented by resistance I5.

Since the cathode circuit is inside the tube I and the anode circuit is outside the tube 6, the 40 output and input circuits are effectively shielded one from another. Similarly the grid of the valve shields the anode from .the cathode. Although the circuit has been illustrated with reference to a therrnionic triode valve it will be appreciated that other multi-electrode valves such as tetrodes and pentodes, may be used With appropriate potentials on their electrodes.

In operation it is arranged that the dimensions of the concentric tubes are such that the input and-output circuits are tuned to resonance at the operating frequency. It can be shown that the apparent impedance between the cathode `and the grid is comparatively low and is approximately equal to the reciprocal of the mutual conductance of the valve. The input circuit is thus heavily damped and the circuit is therefore of value when modulated signals covering a 'wide frequency band have to be transmitted, as for example, in television and multiplex systems. It should be noted that, since there is no actual resistance damping, the power absorbed in this impedance is not dissipated, but re-appears in the anode circuit and is finally transferred to the load. Since the input circuit of the amplifier is of comparatively low impedance it will be necessary suitably to adjust .the tapping on the input circuit for matching purposes.

The amplifier may be modulated in known manner by varying the anode potential at modulation frequencies and although cathode modulation can be employed it is not desirable as this causes the damping on the input circuit to vary considerably.

A further advantage of the embodiment shown is that the outside of the tube 5 is effectively at earth potential at radio-frequencies so that there is no harmful capacity or radio-frequency loss introduced by the water jackets or air cooling ns should the latter be used. In, addition, as the only tuning capacity of the output circuit is the anode-to-grid capacity o-f the valve the circuit is particularly useful for television transmission. If desired the positive terminal of the source of supply connected to the tube 5 may be earthed and the cathode of the valve maintained at a suitable negative potential with respect to earth. In this case the grid of the valve is not connected directly to earth but is connected to a suitable biasing arrangement and effectively toI earth by means of a condenser or by an acceptor circuit.

I claim:

1. An ultra-short wave amplifier system comprising anelectron discharge tube having an anode which constitutes a portion of the tube envelope, a cathode and a grid; two resonant circuits constituted by a plurality of concentric tubular conductors and comprising an outer conductor connected to said anode, an intermediate conductor connected to said grid, and an inner conductor coupled to said cathode; a capacitor constituted by metallic flanges connected respectively to the outer and intermediate tubular conductors and having Ian insulation separator interposed therebetween; means including a potential source connected to said cathode for heating the same and also connected to said anode for attracting electrons thereto; means for applying input potentials in circuit between the Y cathode and grid; and a load in circuit between the outer and intermediate tubular conductors; said intermediate tubular conductor constituting shielding means between the two said resonant circuits.

2. A system in accordance with claim 1 and having means including a jacket surrounding the anode and a. fluid therein for cooling the same.

3. A system in accordance with claim l and having atleast one conductor within and shielded by said inner tubular conductor for interconnecting said cathode and said potential source.

4. A system in accordance with claim 1 and having cooling means surrounding said anode, said anode being maintained at ground potential for radio frequencies.

ERIC LAWRENCE CASLING W'HII'E.

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