Radio Broadcast (May 1928-Apr 1929)

270 RADIO BROADCAST SEPTEMBER, 1928 TABLE 2 CURRENT DRAINS FOR TUBE COMBINATIONS Combination of Tubes Primary Current Rating (amperes) 2, 3 or 4 226 type tubes 1 227 type tube 1 171 type tube 280 or Raytheon Rectifier 0. 25 3 or 4 226 type tubes 1 227 type tube 2 171 type tubes (push-pull) 280 or Raytheon rectifier 0.45 3 or 4 226 type tubes 1 227 type tube 1 210 type tube 281 rectifier 0. 50 3 or 4 226 type tubes 1 227 type tube 2 210 type tubes (push-pull) 2 281 rectifiers 0.75 2, 3 or 4 226 type tubes 1 227 type tube 1 t^ou type iudc 2 281 rectifiers 1.00 2, 3 or 4 226 type tubes 1 227 type tube 2 250 type tubes (push-pull) 2 281 rectifiers 1.25 ■ 1 «■ ■ .luiciipnmff ceivers, a number of line voltages above 1 10, and the value of resistors needed to reduce excess incoming voltage to 1 10 volts — a safe figure for use with any commercial a.c. set. Take the case of a receiver with a primary current rating of 0.50 amperes and a known line voltage of 1 19 volts. Checking on the table we find that a resistor of 22 ohms will reduce the excessive line voltage to a safe figure. Another common current drain, usual with modern receivers using 210 or 250 type power tubes, is 1.25 amperes. Suppose the line voltage reads 120.2. Checking on the tabulation in Table 1 it is readily seen that a 10-ohm resistor will do the work of stepping the excess voltage down to 1 10, the safe figure. If the set is designed for a normal line voltage of 1 1 5 volts, instead of 110, subtract five volts from the actual measured line voltage, and use the result as the line voltage in using the table. Power resistors with Edison basts suitable for screwing into ordinary light sockets and floor receptacles are now on the market, and are particularly handy for this work. Type EB Ward-Leonard Vitrohm resistors and type 997 Aerovox Pyrohm resistors are of this type, and may be obtained in a wide range of resistances. Having selected a resistor of the correct rating by reference to Table 1, the work of utilizing it is a matter of ten or fifteen minutes at the FIG. 2 The power rheostat is connected in series with the receiver power transformer by tapping one of the supply leads. The rheostat may he mounted in any convenient place, provided it is not near any combustible material, as the rheostat is required to dissipate considerable heat FIG. 1 At the left is an ordinary plug-in connection to an a.c. line. At the right is the method of installing a power resistor of the Edison base type in series with the receiver supply transformer, by means of a series tap. An ordinary power resistor of the correct value may be used, hut does not make as neat a job as that made with an Edison base type of resistor most. Fig. 1 at the left shows the connection to the receiver from the wall outlet before the installat'on of the resistor. At the right are the revised connections, using a resistor of the Edison base type. A series tap, such as the Hubbell No. 7029, provides a convenient means of mounting the resistor, and has an opening at the side for the cord running to the radio receiver. First remove the leads from the plug. Connect the leads to the series tap. Screw the resistor into the series tap. CURRENT DRAIN OF KIT SETS \ A / HERE the receiver is built at home from v " a kit of parts, there is no quick way of getting the primary current rating as in the case of the manufactured set. Table 2 gives the current values computed for tube combinations used in most kit sets, and will prove of value in helping to determine the correct value of resistor for use with sets built at home. The combination of tubes is indicated at the left and the primary current rating at the right. Knowing the primary current rating of the home built set and the value of line voltage, it is an easy matter to select the correct value resistor from Table 1 and apply it as described before. REGULATION WITH POWER RHEOSTATS THE information given above takes care of those cases in which the line voltage is excessive but constant. But what about those cases in which the line voltage is sometimes normal and at other times excessive? In such instances we cannot use a fixed resistor to take up the excessive voltage, but must use a variable resistor with a maximum value of resistance equal to or somewhat more than the resistance required to reduce the line voltage to 1 10 volts from the maximum value of line voltage encountered during the day. For example, suppose that with the a.c. voltmeter we measure the line voltage at various times during the day and find that it reaches its highest value at 1 1 :oo p.m., at which time it is 125 volts. We must then use a variable resistor with a maximum value that will reduce the voltage for the set to 1 10 volts when the line voltage is 125 volts. If the receiver draws 1.0 ampere from the line, then, from Table 1, the required resistance is 22 ohms. Companies that manufacture power rheostats that can be used are mentioned in Table 3. [There are other methods of line voltage control, but they require more elaborate and expensive apparatus. These appliances do not fall within the scope of this article, but will be treated in a later one. In "Strays from the Laboratory," on page 259, will be found some information in regard to them. — The Editor.] Fig. 2 shows how to connect the rheostat to provide adjustment of incoming line voltage. First determine the primary current rating of the receiver and choose the power rheostat most nearly suited to your requirements from the data in Tables 1 and 3. Mount the rheostat at a convenient point, away from combustible material. Break one lead of the parallel cord running from the plug to the set, and attach the rheostat in the lead as shown in Figure 2. Finally, adjust the rheostat until the voltage across the tube filaments is the minimum satisfactory voltage for tube operation. TABLE 3 POWER RHEOSTATS FOR LINE VOLTAGE CONTROL Name of Manufacturer Catalog Number of Rheostat Remarks Ward-Leonard Electric Company Vitrohm Rheostat No. 507-63 No. 507-59 No. 507-83 0-50 ohms for currents up to 0.9 amperes 0-20 ohms for currents from 1.0 to 1.75 amperes 0-12.5 ohms for currents from 2.0 to 2.5 amperes Central Radio Laboratories Centralab Power Rheostat Available in suitable sizes to cover all cases. Clarostat Manufacturing Co. Power Clarostat Available in three sizes: 0 to 10 ohms, 25 to 500 ohms and 200 to 100,000 ohms. The first two sizes are most useful for line voltage regulation. Carter Radio Company Hi-Watt Rheostat Type SW Available in 19 different sizes from 1 ohm up to 500 ohms Herbert H. Frost, Inc. De Luxe Rheostat Series, 1800 Available in sizes suitable for line voltage control, from 2 ohms to 20 ohms