US2139716A - Cathode heating circuits - Google Patents

Description

J D. BRAILSFORD CATHODE HEATING CIRCUITS Filed Sept. 11, 1956 Dec. 13, 1938.

INVENTOR JOSEPH D. BRAILSFORD Patented Dec. 13, 1938 UNITED STATES CATHODE HEATING CIRCUITS Joseph Douglas Brailsford, London, England, as.-

signor to RadioCo -porationniiAmcrica, a corporation of Delaware Application September 11, 1936, Serial No. 100,294 In Great Britain October 24; 1935- 2 Claims.

This invention relates to thermionic valve circuits and more particularly to thermionic valveamplifiers. wherein alternating or rippling. cathode heating current is employed for valves of the tetrode, pentode, or other type having two gridsormore. The invention is. applicable both to tubes employing indirectly heated cathodes. and totubes. employing directly heated cathodes;

The object. of the invention is to eliminate. or

substantially reduce hum due to alternating or. rippling cathode heating current.

It is common, particularly in broadcast radio receivers, to use a. pentode or. similar valve in the last stage of a low frequency amplifier in order to obtain large output. power and, in general, the valve in such an output stage is directly heated by alternating current through the filament. It is found in practice that owing to the high magnification of the last stage valve the slight hum voltage introduced due to the alternating Voltage gradient along the filament or cathode is suificient to cause a certain amount of audible hum in the output and although in some cases the amount of hum can be tolerated,

it is very desirable to reduce it as much as possible. A similar effect exists with indirectly heated cathode of output pentodes though the amount of hum is less than with directly heated cathodes. Y

According to this invention hum interference is eliminated or substantially reduced by applying to a control electrode of the valve other than the signal input grid, a balancing alternating current component of voltage derived from 85 an impedance effectively in parallel with the filament or cathode heater. Preferably the filament or cathode heater is shunted by a potentiometer an adjustable tap upon which is connected through a condenser to the electrode to which 40 the alternating current component is to be applied, said electrode being suitably biased through a resistance. a

The invention is illustrated in the accompanying diagrammatic drawing, in which Figures 1 5 and 2 show the characteristic features of the invention applied to a single tube amplifier, and Figure 3 shows the invention in a push-pull amplifier.

Figure 1 shows one way of carrying out this 50 invention as applied to the final stage of an audio frequency amplifier, the tube I being a pentode and having a directly heated filament. Filament 2 of the pentode is shunted by a potentiometer resistance 3 of, for example, 30 ohms 55 and also by the secondary winding 4 of a filamentzheating; transformer 5 whose primary 6 is. energized. from an A. C. source, the mid-point 1 of the secondary winding being grounded through the usual capacity shunted automatic bias resistance: combination 8. An adjustable tap or contact 9-upon the resistance 3'is connected through condenser Ill, known as a decouplingf" condenser, to the. middle grid ll of the pentode, this: middle grid being connected through a decoupling resistance [2 to a suitable 1Q source.of='positive potential at l3. The outer or suppressor grid. I4; of the pentode is connected to the filament, as in the usual way, and the anode [5 receives potential through the load or output impedance device represented at [6. In- 15 put signals to be amplified are applied between the innermost grid ll of the valve and ground.

Figure 2 shows the invention adapted to a final pentode valve with an indirectly heated cathode. Here the cathode 2 is grounded through 20 the usual capacity shunted bias resistance combination 8 and the cathode heater [8 is shunted by potentiometer resistance 3, and by the secondary 4 of heating transformer 5, the secondary having a grounded centre point 1. The rest of 25 the connections are as in the directly heated cathode valve arrangement of Figure 1.

Figure 3 shows an embodiment of the invention in a push-pull circuit comprising a pair of valves 1 and 1' with directly heated cathodes. 30 In this case it is sufiicient to connect only one of the valves (as shown the valve I) in accordance with the invention, the other valve being connected as in the usual way. In view of the similarity between the method of connection of the valve I of Figure 3 and the method of connection of the valve l of Figure 1 it is unnecessaryto describe Figure 3 in detail beyond stating that like references are employed for like parts in Figures 1 and 3 and that in Figure 3 electrodes in, and circuit elements associated with, the valve l are given the same references as the corresponding electrodes and circuit elements of valve I. If the valves of Figure 3 were of the indirectly heated type one of them would be connected after the manner of Figure 2.

The hum balancing E. M. F. applied to the middle grid of the pentode in each case can. be adjusted by adjusting the tap or contact point 9 on the potentiometer, the phase of the applied E. M. F. of course reversing about the electrical center or mid-position of the potentiometer. In practice the impedance of the decoupling condenser should not be too high and preferably it is less than one-third of that of the decoupling 5 to insufficient smoothing of the anode supply.

current and it has been found that this invention can be employed to cancel out or substantially reduce hum arising from this cause.

In some cases the hum balancing voltage component may be derived from a transformer having a winding across the filament or heater instead of from an impedance directly across said filament or heater and the expression effectively in parallel as herein employed in intended to include all cases where the impedance is in effect in parallel with the heater or filament as regards hum components, e. g., where it is in parallel through a transformer.

The invention is applicable in all cases where a very low hum level is required-such as for instance in the case of head telephone Workand by employing the said invention, an extremely low hum level can be obtained in a large power amplifier of relatively low cost.

Having now particularly described and ascertained the nature of my said invention and the manner the same is to be performed the following claims are made:

ring variations in space current resulting from the filament current supply variations comprisinga resistance connected across the terminals of said filament and a condenser of low impedance to said variations connected between said auxiliary electrode and an adjustable contact on said resistance.

2. An amplifier circuit comprising an electron discharge device having a cathode, an anode, an input electrode and an auxiliary electrode, the cathode of said valve being heated by a current with a pulsating component; and means for applying a voltage corresponding to said component to said auxiliary electrode comprising an impedance element in parallel with the heating circuit of said cathode and a condenser of low impedance to said component connected between said auxiliary electrode and a point intermediate the ends of said impedance, said auxiliary electrode being coupled to a biasing potential through a resistance, the impedance of said condenser not exceeding one-third of the impedance of said resistance.

JOSEPH DOUGLAS BRAILSFORD.

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