Radio broadcast .. (1922-30)

Measuring Tube Characteristics A Paper Delivered Before the Radio Club of America Discussing the Several Bridge and D. C. Systems for Use in Obtaining Tube Characteristics r T IS neither the desire nor purpose of the writer to burden this article with an eulogy • on the vacuum tube. Nearly every article that has appeared on the tube in the popular radio press has done that, pointing out that it is a most wonderful device, the modern Aladdin's Lamp, and a number of other superlatives that fill up space. There can be no doubt that the tube is important. Witness our present broad- casting structure, our long-distance telephone service, our communication by telephone across the Atlantic, and by high frequencies and ex- ceedingly low powers to all parts of the world. All of these things depend upon the tube. No study of the tube can be complete without a knowledge of its varied services. For example, it is possible with a tube to convert direct current from a set of batteries to alternating current of all frequencies from as near zero as one likes up to 60,000 or more kilocycles. It is then possible, with another tube exactly similar to the first, to convert these extremely high frequencies back to direct current. It is also possible to amplify both direct and alternating currents, and hence amplify power. It is also possible to separate what is placed on the input of the tube into direct and alternating currents of practically any ratio desired. All of these varied functions are carried out without any moving parts, with- out noise, with practically no loss of power, and with so little fuss that tubes now exist that are capable of giving service for 20,000 hours, a life greater than that of the circuit in which they are used. There can be no doubt about the import- ance of the tube in the field of electrical engineer- ing. In addition, tubes and their associated circuits are now being used to measure the rate of growth of plants, to measure extremely small differences of thickness, the strength and rapidity of a man's pulse, and from this latter, to de- termine whether he is a lover, a thief, a liar. The tube has been harnessed and trained to do a vast number of interesting tricks. Now this little assembly of glass and metal performs its multitudinous functions with the aid of three elements. The first and most im- portant of these elements is the filament. This filament has undergone rather remarkable changes since tubes first came into existence. The first ones were made of tungsten which operated with a high temperature, then going to low temperature oxide-coated filaments manu- factured by a complicated and difficult process, thence to our most recent filament, the thoriated wire. The measure of efficiency of the filament is its emission per watt expended in heating it, and the newest thoriated wire is exceedingly efficient. Pure tungsten filaments operate at a very high temperature. Oxide filaments consume considerable current at low voltage and at a much lower temperature. Thoriated filaments are somewhere between. The other two elements are the grid and plate, and because these elements can be changed in By KEITH HENNEY Director, Radio Broadcast Laboratory size and relative position, tubes differ in charac- teristics. There must, then, be some means by which engineers can compare tubes just as they rate generators and motors or other electrical apparatus. TUBE CONSTANTS IN TUBE engineering, there are two very important factors which, when known, define the tube in exactly the same manner that we used to say in school that the United States is bounded on the north by Canada, on the east by the Atlantic Ocean, etc. The two constants— which really are not constants at all—are the amplification factor and the plate impedance, and every function that the tube performs and its efficiency in doing so may be discovered by a knowledge of these factors and the constants of the circuits into which the tube works. Another factor is the mutual conductance which, contrary to popular opinion, is not so important as it may seem. The term is somewhat difficult to picture physically. It has the dimen- sions of a conductance, i.e., a current divided by a voltage, but the current exists in one circuit and the voltage in another, with the tube as the connecting link. It is due to Professor Hazeltine. These constants, or factors, are variable within rather wide limits. For example, the amplification factor may range from 3 to 30, while the impedance varies, as someone has said, from Hell to Peru. The amplification factor is pretty well determined when the tube is sealed and pumped; that is, it depends to within very narrow limits upon the geometry of the tube. The mesh of the grid and its spacing with re- spect to the other elements are the governing factors. At low grid voltages the amplification factor falls off somewhat, rising to a maximum at zero grid voltage, and remaining constant there- after, or falling gently in some tubes. The plate impedance depends upon a lot of things, the filament efficiency, the amplification constant, and the grid and plate voltages. No one curve or graph can show how it varies. To properly represent it would require a three- dimensional model, such as has been constructed by Doctor Chaffee and others. Some photographs of very beautiful models of this nature may be seen in the Proceedings of the I. R. E. After the tube is sealed and placed in operation! the impedance changes with each change in in-', stantaneous plate or grid voltage—all of which makes the theory of the tube more or less com- ' plicated. These three elements, the filament, the grid. ' and the plate, cause any current flowing in the plate circuit to change, making it go through very wide fluctuations. The plate current is de- fined by the equation: IP = f (E P + jAEg) By maintaining constant any one of the three variables in this equation and varying the other two, we arrive at the relation between the plate current and the voltage on the grid or plate that we usually know as characteristic curves, and it is by means of these curves that the important tube factors are defined. For example, both grid and plate potential have some effect on the grid voltage, but the grid is relatively more important than the plate. In Fig. i it may be seen that at zero grid bias, changing the plate voltage from 90 to 135 changes the plate current by 5.2 milli- amperes, while changing the grid bias by 5 volts will do the same thing. The amplification factor is then defined as the ratio of the change in plate potential to the change in grid potential which produces the same effect in plate current. In this case the ampli- fication factor is 9: .. _ AEp l35-oo««4i; H- — T-p— = = = Q AEg 5-0 5 The other factor of importance, the plate i impedance, is the ratio between the change in plate voltage to the resultant change in plate current. In this case it is 45 volts divided by 0.0052 amperes, or roughly 8700 ohms: Rp = AE P= _ J1 5-oo_ ._i5_ =8 Alp 0.0116-0.0064 0.0052 Now, as has been indicated by the Greek letter A in the definitions, these factors are de- • fined by changes, and for accuracy the changes must be small. The mutual conductance, defined as the ratio between a change in piate current and the change in grid voltage that produced it, is also the ratio between the amplification factor and the plate impedance, as can be seen from the mathematics below. For comparing tubes under exactly the same conditions, this factor is somewhat import-