The Birmingham age-herald., 1922-10-15
COMMON LEAD OF TUBE CIRCUIT
 IS EXPLAINED
Radio Receiver Connection, Most Perplexing to Fans, Outlined and Principles
 Described

DGoGl ilJEl
 One of the most perplexing things in a receiving circuit to the average radio fan Is the common lead" This is the point in the battery connee- tions where several parts of the cir cult are joined together.
 The puzzling feature of this con nection Is that almost every diagram published shows 1t different, the re- suit being that the radio novice who desires to construct his own set Is left in a bewildering maze of doubt as to just which Is correct and which is wrong.
 It Is my purpose therefore in this story to outline the basic principles which determine the correct method of joining vacuum tube circuits to- gether.
 Three Different Circuits
 In the first place. 1t should be dis tinctly borne in mind that there are three different circuits In a vacuum tube set, no matter where or for what purpose the tube is being used. These are the first the filament circuit, which Of course heats up the fila ment circuit, which of course heats up the filament and causes 1t to emit negative electrons from it's surface. Second, the grid circuit which acts as the control element of the tube, and third, the plate circuit.
 The filament circuit is from the positive of the storage battery through one side of the filament. out of the other side of tho filaInent. back to the negative post of the storage battery. The grid circuit consists of the tuner Cor in the case ol simplifying circuits, the secondary of the transformer) to the grid. and the other side of the tuner or trans- former to one side of the filament. The plate circuit runs from the plate through the telephones Cor primary of transformer through the high voltage, or B battery to the filament In both the case of the grid and the plate circuits, their individual cir cults arc completed inside the vacuum tube from the filament.
 Now let us take first of all the vacuum tube used as an amplifier for radio frequencies. That Is in cases where 1t is used to amplify the weak radio impulses picked up by an aerial before those impulses are passed through the detector for rectifica- tion There are a number of tricky things connected with this form Of qualification, and 1t is necessary tsat the circuit used in connection with it be kept in the utmost state of stability that 1t Is possible to ob- tain, otherwise nothing but squeals and bowling will result.
 Negative Potential Needed
 It is necessary, therefore, that negative potential be maintained on the grid of all the radio frequency amnlifying tubes. In the first stage of radio frequency this can be ac- complished by means of a potentio- meter placed across the storage bat tery and the center contact of the potentiometer being connected to the tuning element in the aerial circuit. The path Is then from the ftlament through the tuner to tho grid. By means of the potentiometer the cor rect amount of negative potentlal can be regulated. and placed on the grid.
 Since a negative potential Is im- perative for this purpose the nega- tive side of the storage battery Will be the common lead' for all radio frequency simplifying tubes. In other words one side of the secondary Of the radio frequency transformer Will be connected to the grid, and the other side of this transformer will be joined to the negative side of the storage battery in every case.
 When we come to the use or the vacuum tube as detector we arc confronted with an entirely different n. a constant accumulation Of nega- tive elections on the grid of the tube during it's operation, and unless there is some means provided this charge will interfere with the proper func- tioning of the tube.
 Grid Leak Required
 The method universally adopted to take care of this situation Is by means of a grid leak. This is noth- a tance, which normally Is in the neighborhood of one megohm--or ex- pressed differently-one million ohms.
 Now In order that the negative charge can leak off from the grid, and permit the tube to operate ef- ficiently as a detector, 1t is necessary leak to the positive side of the stor- age battery. This Is based on the fundamental law that unlikes to at tract each other, and consequently the positive side of the battery draws off the negative charge through the leak, while the grid con- denser permits the high frequency currents to pass through to the grid of the tube without interfering with the action of the grid leak.
 In the case of the vacuum tube be- ing used for the purpose of audio- frequency amplification, the common lead will also be to the positive side of the storage battery. This Is be- cause of the fact that the audio-sec- ton of the receiver Is joined directly to the detector tube.
 :. Such are the basic principles which govern the common lead of the vacuum tube, In the different uses to which 1t may be put There Is very important point that could . be well included in this article In connection with radio frequency am. plification. When this form of am. plification Is used on short waves very serious difficulties are encoun- tered because of the high frequencie. involved It Is imperative under the circumstances that capacity in the circuit be kept down to a minimum.
 Where moro than one stage o1 radio-frequency amplification is used. 1t is Q very unwise plan to use jacks between the stages. Some fans do this for the purpose of facilitat- ing operation In bringing in one O, more stages of amplifica tion, just the same as they do with audio ire quency amplification. The jack however, acts as a condenser in the radio section of the receiver, and should not be used. In fact. the wire connections between the various parts in the radio amplificatfon sec ton of the set should be kept as short as 1t Is physically possible in