US2467474A - Thermionic valve circuits - Google Patents

Description

Aprifil 194-9. I HADFlELD 2,467,474

THERMIONIC VALVE CIRCUITS Filed Oct. 2, 1944 INVENTOR BERTRAM MORTON HADFIELD ATTORNEY Patented Apr. 19, 1949 THERMIONIC VALVE CIRCUITS Bertram Morton Hadfield, Harrow Weald, England, assignor to Automatic Electric Laboratories Inc, Chicago, 111., a corporation of Delaware Application ()ctober 2, 1944, Serial No. 556,878 In Great Britain November 5, 1943 2 Claims.

The present invention concerns improvements in or 1' aating to thermionic valve circuits, and has for its object arrangements for adapting valves from one use to another use and more particularly it applies to valves having an electrode for screening another electrode further on in the path of the electronic current. More specifically the object of the present invention is to provide circuit means whereby the power output capabilities of such a valve may be increased substantially without appreciably aifecting its characteristics for the use to which it is adapted.

According to one feature of the invention an arrangement for adapting such a valve consists of joining the screen electrode to the electrode which is screened by a resistance and making the load connection corresponding to the use to which the valve is adapted to the junction of the screen electrode and resistance.

According to another feature of the invention a pentode or tetrode valve is arranged to function as a triode by connecting the normal anode and screen electrodes together by a resistance and making the load connection coresponding to the valve functioning as a triode to the junction of screen electrode and resistance.

According to a further feature of the invention the pentode or tetrode valve is adapted to function as a diode by joining together the normal anode and screen electrodes through a resistance and making one of the circuit connections corresponding to the valve functioning as a diode to the junction of the screen electrode and the resistance While the other circuit connection is made to the cathode.

The value of the resistance can vary over a considerable range provided that the characteristics of the valve for its adapted use are not appreciably affected. The preferred value of the resistance, which is the maximum usable value, is substantially equal to the anode resistance of the valve, as measured with the screen grid and screened electrode joined together, divided by the ratio of the maximum working cathode current to the cathode current when the grid/ cathode potential is zero.

The invention will be better understood by referring to the accompanying drawings, in which:

Fig. 1 illustrates the invention in its simplest form.

Fig. 2 illustrates the application of the invention to the case where the input is alternating current.

Fig. 3 illustrates the application of the inveng tion to the arrangement where the input current is mainly direct current, and

Fig. 4 illustrates the application of the arrangement to function as a diode.

Referring to Fig. l the valve VI may be a pentode or tetrode, the suppressor grid, if any, not being shown. B is a resistance connected between the normal anode NA and the screen grid SG. A. represents the terminal to which the load connection of the valve is made when it is used as either a triode or diode. G and C are the nor mal control grid and cathode which in the adapted use of the valve constitute the remaining. electrodes of the triode. If Ia, I50 and Ic be the currents flowing in the normal anode, screen and cathode leads then the characteristics of the valve using A, C and G as the terminals for a triode will remain substantially unaltered with increase of R from zero provided that the voltage between the normal anode and cathode remains greater than about one-fifth of the voltage between screen and cathode. This is due to the constant current feature of the anode of a pentode or tetrode valve for changes in the normal anode voltage. To determine the maximum value of R the normal anode/cathode voltage Vac equals one-fifth of the screen grid/cathode voltage Vsc or Ia.R=0.8 Vsc.

Therefore 0.8 Vsc R 70.10 (1) If I0 be the cathode current when the grid/cathode potential is zero, then Vsc:Io.Ra Where Rd is the resistance of the circuit including the valve Ia Where It equals T6 from terminal A to terminal C which is unaltered.

by variations in the value of R within the limits of This. resistance will be considered as obeying In practice It varies from about 0.8 to 0.9 and can only be 1 ultimately, that is when there is nov screen current. By this arrangement a substantial reduction in the normal anode dissipation is obtained or alternatively, for a given anode V dissipation, the power output capabilities of the valve as a whole can be substantially increased provided the screen can withstand the increased dissipation. It will be noted that the determination of the maximum value of R depends upon a maximum working value of Ic in relation to Io, that is in the case of a triode, and since these currents are produced by grid/cathode potentials it is dependent upon the extent to which the working input potential approaches or exceeds a grid/cathode voltage of zero. For instance in the common case where the triode valve is oper ated so that grid current does not flow then the maximum grid cathode potential is zero so that M is 1. If, however, grid current is permitted then the maximum grid cathode potential is greater than zero so that Ic is greater than I and M is greater than 1. Hence the maximum permissible value of R will depend entirely upon the postulated working conditions and is readily determinable once these are stated.

It is evident that under any given operating conditions the resistance R can be increased from zero up to the maximum permitted by these conditions as calculated from Equation 3 and that such increase will result in progressive reduction in the dissipation of the normal anode due to the steady current provided by the supply battery. The degree of reduction will obviously depend on the magnitude of this steady current so that in order to difi'erentiate between this and the working maximum current the symbols Ics and Ias will be given to the steady cathode and anode currents. Hence in general the ratio of the normal anode watts with R to that without R is 1 minus the ratio of the watts in R to the normal anode watts without R, that is to say the normal anode watts with R 1 (I as) R normal anode watts without R Vsc. I as 1 I as.R I 0.Ra

= 1 70.1 cs.R

Obviously this is a minimum when the maximum value of R is used so that the minimum ratio becomes Ics Hence the reduction in the normal anode watts depends entirely on the ratio of Ics to 10, that is the ratio of steady to maximum working currents and is independent of any other feature of the operation of the valve (for instance whether grid ensured so that the reduction in normal anode watts is 120.6. If the valve is to be used as a direct current amplifier and full utilisation is required then Ics may equal 10 and the reduction in normal anode watts is 1:02. Conversely it follows that any given pentode or tetrode valve operated according to the conditions set out above can control 1.67 and 5 times the normal load dissipation for the above respective cases thus enabling correspondingly larger outputs to be obtained for a given anode dissipation provided the screen electrode can withstand the correspondin-gly larger dissipation. In order to obtain a proper screening action so that changes of normal anode potential have little effect on the electrode currents it will always be necessary to employ a fairly substantial screen structure. 0n the other hand the magnitude of the screen current can be reduced considerably by a proper distribution of the screen mesh with respect to the control grid mesh. Hence with modern developments it has already been found that the normal screen wattage is much less than the screen could withstand and as It tends to equal 1 this situation will improve.

The following example is given by way of H- lustration:

A tetrode valve whose anode dissipation was rated at 20 watts had a is value of 0.9, so that the screen rating was 2 watts. It was found possible to raise the screen dissipation to 10 watts without any adverse eflect so that the invention could be tried in the form used for a direct current input. The anode current/anode voltage characteristics were measured as a triode with R zero in Fig. l and at various grid biases up to values giving the rated anode watts. From the zero bias curve the resistance Ra was estimated at 800 ohms. R was increased to '720 ohms and the characteristics re-measured keeping a check on the screen and normal anode dissipations. When this became l0 and 20 watts respectively at zero grid bias the dissipation in R was found to be 75 watts. The gain in normal anode circuit watts was thus 4.75 times whilst the total eil'ective dissipation of the valve was watts compared with the previous value of 22 watts with R zero. The anode characteristics showed no difference from those taken with R zero, up to 22 watts, whilst the extended range up to 105 watts corresponded with what might have been expected with extra-polation. In general the above gain in permissible anode circuit watts refers specifically to the normal anode circuit and in practice the additional watts dissipated in the screen are also available, that is, the total circuit watts of the invention between terminals A and C of Fig. l are (1+k) times those for the normal anode circuit. It should also be noted that the watts dissipated in R both steady and working can be utilised if required in addition to those available in the external circuit in which the valve may be used. The combined emciency of the circuit thus becomes of a high order. Obviously the valve of Fig. 1 may be used in any circuit employing a triode or diode valve.

A typical instance is the cathode follower circuit of which two forms are illustrated in Figs. 2 and 3. In Fig. 2 the grid input e is alternating. The valve V2 is of the type described having a resistance R in accordance with the invention dependent upon the desired ratio of the maximum cathode current to the cathode current when the grid cathode voltage is zero. The normal output is obtained in the load Ll or, if it is desired to eliminate the steady cathode current from the load, the primary winding of a transformer may be substituted for LI which would then be connected to a secondary winding on the transformer.

cated with the usual plus and minus signs. In

Fig. 3 another form of cathode follower circuit is shown in which the predominant grid input is a direct current potential E and a direct current output is desired in load L3. The valve V3 and supply busbars perform similar functions as The supply busbars are indicuit is in very common use as the output stage of electronic voltage stabilisers. If the small input 6! is neglected the output on L3 in Fig. 3 is more stable than the busbar voltage since it only includes a term times the latter (where p is the amplification factor of the triode V3 which remains unaltered for the appropriate value of R. selected). Further the output on L3 also possesses an internal resistance equal to the reciprocal of the mutual conductance of V3 which also remains unaltered by the value of R selected. The voltage E is normally derived from some type of gas discharge tube energised from the supply busbars. By also supplying a small voltage el proportional to any change in the output voltage due to the supply or load variations and of opposite sign generally via another amplifying stage, the output can be made almost independent of such changes. Generally the application of this circuit is limited by the permissible wattage dissipation of the valve V3, especially where a large range of output voltage is desired by varying E, because at low output voltages a very much larger voltage exists across V3. If V3 be as shown, however, and designed in accordance with the invention then because the main input is direct the permissible current flowing through the valve may be raised times, hence the range of output watts may also be raised 5 times and a relatively small and inexpensive valve can be used. In addition of course an equally stabilised output can be obtained from the normal anode circuit of V3 by utilizing part or all of the energy dissipated in resistor R to supply a constant resistance load. Where a diode or rectifying action pure and simple is required of the valve a control grid is no longer necessary or if present may be connected to the cathode via a grid cathode limiting resistance. This is equivalent to saying that VI (Fig. 1) is always operated with the grid cathode potential at zero but apart from this the action of VI and the maximum value of R remain the same. Hence it is possible to construct a power rectifying valve with a screen grid and normal anode and obtain the same gain in permissible cathode current, that is to say at least 5:1, so that the rectified output may be five times as high without any additional heating of the valve. This use of the invention is illustrated by way of example in Fig. 4 which shows a conventional full wave rectifier system comprising a secondary of transformer T2, two rectifying valves V4 and V5 and a load L5 connected between the common cathode and centre tap of the secondary. The primary is of course supplied with alternating current as usual. For this application of the invention to V4 and V5 resistance R may satisfy Equation 3 where M is 1. Other applications where a triode or diode is necessary and it is desired to use a valve with low dissipation will doubtless occur to those skilled in the art without further explanaions.

It will be understood that although one would normally use the maximum value of R as described and in accordance with the proposed use of the valve the invention is not confined to using this value. A lower value may be used if desired with a corresponding reduction in the total permissible dissipation.

The invention is applicable to any valve which incorporates an electrode whose function is to screen a later electrode in the path of the electronic current so that the potential variations of the later electrode do not afiect substantially the flow of current over a wide range.

I claim:

1. In combination, a thermionic valve having at least a cathode, a control electrode, a screen electrode, and an anode, a source of potential and a load serially connected between said screen electrode and cathode, a single resistor connected between said screen electrode and anode to maintain the anode at a potential intermediate to that of the cathode and. screen electrode, a source of control voltage connected between said control electrode and cathode, said resistor having a value greater than zero and less than the anode resistance of said valve, as measured with the screen electrode and anode joined together, divided by the ratio of the maximum working cathode current to the cathode current for zero potential between the control electrode and cathode, whereby more power may be delivered to the load for a given anode dissipation than for the usual triode connection without appreciably afiecting the relationship between load current and control voltage.

2. In combination, a thermionic valve having at least a cathode, an anode, and a screen electrode, a sou-roe of potential and a load serially connected between said screen electrode and cathode, a single resistor connected between said screen electrode and anode to maintain the anode at a potential intermediate to that of the cathode and screen electrode, said resistor having a value greater than zero and less than the anode resistance of said valve, as measured with the screen electrode and anode joined together, and dissipating a substantial portion of the energy which would normally be dissipated by the anode in the absence of the resistor, whereby a higher than normal load current may be carried by the valve for a given anode dissipation.

BERTRAM MORTON HADFIELD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,198,464 Shepard, Jr Apr. 23, 1940 2,245,616 Soller June 17, 1941 2,343,753 Davey Mar. 7, 1944 2,349,881 Peterson May 30, 1944 FOREIGN PATENTS Number Country Date 450,136 Great Britain July 6, 1936

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