Radio Broadcast (May 1928-Apr 1929)

AUGUST, 1928 RADIO BROADCAST 205 o o Radio Broadcast's Home Study Sheets Testing V acuum Tubes August, 1921 E \ FIG. 'T'HERE are very few radio receivers nowadays that do not employ one or more vacuum tubes. Many receivers use as many as ten of them; the average in the United States is probably five per receiver. Experiments on such tubes engage part of the time of every serious student of radio because they are the heart of all of our modern circuits and equipment utilizing those circuits. To understand something about the characteristics of such tubes we shall need the following: LIST OF APPARATUS 1. Source of current, dry cells or storage battery. 2. A rheostat, such as the Frost 20-ohm type 720. 3. A voltmeter, preferably a double range instrument, reading up to 10 volts on its low scale, and up to 100 or more on its second scale. A good one is the Weston Model 506 which reads 7.5 and 150 volts, or the Jewell Pattern 77 which has similar ranges. 4. A milliammeter reading about 10 milliamperes full scale. The one used in the Laboratory to perform this experiment was a Weston Model 301. It lists at $8 and is very useful. 5. A source of plate voltage that is variable in steps of 10 or 20 volts up to about 90 volts. 6. A receiving tube, such as a CX-301A. 7. A baseboard made from 5-ply or -J-inch stuff and about 6" x 16" in size. 8. Fahnestock clips, wire, such asCelatsite or Belden Colorubber. PROCEDURE Screw the socket to the baseboard; place the clips in a row along one margin of the board, and mount the rheostat on a small piece of brass strip. If a General Radio or Pacent rheostat is used it can be screwed to the baseboard, and the mounting strip avoided. The connections are shown in Fig. 4, and the experimenter is encouraged to follow out this arrangement of parts since the set-up will be used many times in the laboratory experiments. An hour's time making everything fast and ship-shape according to this layout may save much annoyance later, and possibly prevent a good meter from going back to its manufacturer for repairs. Fig. 3 is a photograph of the set-up used in the laboratory. Twist a pair of leads, preferably of different colors and about a foot long, and attach one end of the twisted pair to the low voltage voltmeter terminals. Attach the other ends of the pair to the filament terminals on the socket. Make the following connections: 1. Milliammeter between clips 6 and 7. 2. A plus to clip 1. 3. A minus to clip 2. 4. Clip 5 to clip 4. 5. B minus to clip 2. 6. B plus, about 22.5 volts at the start, to clip 8. A schematic diagram of the above is shown in Fig. 2. Turn on the rheostat slowly and watch both filament voltmeter and plate milliammeter. If either reads backwards, reverse the connections to the meter. Note down as in table 1 the plate current as the filament voltage is changed; then turn off the rheostat, increase the plate voltage to approximately 45 and repeat. Repeat for higher values of plate voltage or until the plate milliammeter needle reaches its full deflection. Plot this data as shown in Fig. 1. Remove the voltmeter from the filament terminals and measure the plate voltages applied to the tube in the above experiment by attaching the twisted pair to the high voltage posts of the meter and the other end of the pair of wires to the B minus and the several B plus posts which were used in the experiment. DISCUSSION A good description of what happens within the glass wall of the vacuum RADIO BR0ADCA LABORATORY CX 301 -A No 20 E^O 3T -so EP=iS tube may be found in pages 450 to 470 of the Signal Corps book, Principles Underlying Radio Communication. The experimenter is encouraged to read these pages, and to put in his notebook his own digest of the information contained there. When the filament is heated by the current passing through it, a current begins to flow across the evacuated space between the filament and the positive plate. This current is very small, of the order of thousandths of amperes, or milliamperes. It is carried on the negatively charged electrons emitted from the filament. An ampere is the current carried by 6.28 x 10'8 electrons per second. The greater the temperature of the filament the greater is the electron stream, and the greater the plate current. Therefore the plate current increases with increase in filament temperature — up to a certain point; then the plate current remains fixed in value regardless of how much the temperature of the filament is increased. This is shown in the curves on Fig. 1. This is known as the saturation effect. The only way to increase the plate current further is to increase the plate voltage, which increases the attraction for the negative electrons and increases the number that arrive per second. This experiment demonstrates 1. The effect of filament voltage on plate current. 2. The saturation effect at low plate voltages. 3. The fact that under no conditions is there any need or benefit in increasing the filament voltage beyond 5 volts. The experimenter should explain each of these points in his notebook. These explanations, and the answers to the problems given below, may be sent to the Laboratory, where they will be criticized and returned. PROBLEMS 1. The d. c. resistance of the tube, that is, the resistance offered to the flow of electrons through the space inside the tube, is the ratio between the plate voltage applied and the plate current (in amperes) flowing; calculate the resistance of the tube for each value of filament and plate voltage applied, and plot against filament voltage. 2. If 6.28 x 1018 electrons per second constitute a current flow of one ampere, how many electrons per second make up a current of one milliampere? 3. If power in watts is the product of amperes times volts, calculate the power used up in the plate circuit at each value of plate and filament voltage used, and plot against filament voltage. 4. Where does this power come from? Where does it go? 5. If the average B battery has a life of 5000 milliampere hours, how long should it last furnishing plate current for the tube used in this experiment if the filament voltage is 5 and the plate voltage is 45? If a receiver uses four of these tubes at 90 volts on the plate and one tube at 45 an the plate, all at zero grid bias, how long will the batteries last? TABLE I CX 301 -A TUBE NO. 20 Ef EP = 22 EP = 42 Ep = 67 EP = 90 2.0 .15 2 .2 .2 3.0 .25 1 2 2.25 2.45 4.0 .30 1 4 3.4 5.0 .32 1 45 3.6 6.0 .35 1 47 3.65 FIG. 2 Ef = filament volts Ep = plate volts Ip = plate current in milliamperes o+l| +o 0+ B-Bat. +° 0 FIG. 3