Radio Broadcast (Nov. 1925-Apr 1926)

684 RADIO BROADCAST APRIL, 1926 Table I Various Types of Tubes and Their Respective Rheostats uv-200 UX-112 NUMBER MAXIMUM NUMBER MAXIMUM OF RESISTANCE OF RESISTANCE TUBES OF RHEOSTAT TUBES OF RHEOSTAT IN PARALLEL IN OHMS IN PARALLEL IN OHMS 1 1.0 1 .5 2 1.0 uv-201 TYPE 199 1 1.0 1 25.0 2 .5 2 12.5 3 .33 3 8.33 4 .25 4 6.25 TYPE 201A TYPE 120 1 4.0 1 12.0 2 2.0 2 6.0 3 1.33 4 1.0 WD -12 NUMEER MAXIMUM OF RESISTANCE TUBES OF RHEOSTAT IN PARALLEL IN OHMS 1 1.6 2 .8 3 .53 4 .4 wound with thinner wire so that the full 20 ohms are accommodated on the same sizeform. The thinner wire is used because per inch it has more resistance than that used on the 4-ohm rheostat. Less current can be carried by small wires than by larger wires. So, by forcing too great a current through a small wire, heat is produced, and in its dissipation sometimes warps or otherwise injures the forms of the rheostat wound with small wire. This is especially true in the case of 20-ohm rheostats where it is intended to use only 4 ohms. This means that one-fifth of the entire wire is used where if a 4-, or 6ohm rheostat were used in the first place, it would be safer because the wire would be larger, insuring ample current carrying capacity and also any desired variation in current would be finer since the change in resistance produced by a movement of the contact arm would be less per unit of change than if the same movement were made on the 20-ohm rheostat. 4 1 1 T! on the rheostat, the required amount of resistance is immediately introduced in the circuit. Then as the battery voltage drops off toward the end of its ampere hour capacity this arm can be advanced over the entire periphery of the rheostat surface to keep the filament voltage constant. Where a 20-ohm rheostat is employed, four-fifths of the rheostat is unused as only the last 4 ohms is required in the circuit. Then as the battery voltage drops, the regulation is over only one fifth the surface as compared to the entire surface of the 4-ohm rheostat. Various diameters and textures of resistance wires have different current carrying properties. In the case just cited, assuming that the 4-ohm, and 20-ohm rheostats are of the same physical dimensions, then the space in which the resistance unit is placed is the same for each. Now if on one, there is to be wound resistance wire totaling 20 ohms, while on the other there is to be only 4 ohms, then the both must be wound with such a diameter of wire as will fill up the whole space. The 20-ohm rheostat will be THE FILAMENT BALLAST 'HE only point that may be raised in objection to the use of filament ballasts is that they do not permit of detector filament regulation for regeneration control as the rheostat does. However, now as ticklers, feedback condensers and variable plate circuit resistors are generally used for oscillation control, that objection is not serious. Examine the curve in Fig. 5, showing the voltage of a battery over a period of discharge hours. The battery practically maintains its voltage until very near the end of its charged life, then rapidly falls. The use of the rheostat or filament ballast is helpful only up to the point where the sudden drop occurs. When past this point the battery must be recharged and rheo FIG. 9 By tapping on to part of the „ . rheostat, it is EG = Grid bias possible to ob tain a voltage drop through the resistance ~r" which can be A effectively used for biasing the grid of a tube with the same result as a C battery. However, this practice is not to be recommended as the difficulties of soldering and unsoldering the lead necessary to obtain the correct grid bias for a tube is great compared to the ease and simplicity of providing a C battery for this purpose stats and filament ballast will not assist for long in maintaining the tube voltage at 5. Look at the curve for a filament ballast in Fig. 7. From 6 volts, the charged state of the battery, to 5.7 volts when the battery is considered discharged, there is only a change of .006 amperes from the rated .25 amperes of filament current, surely not enough to be seriously considered as affecting the proper and efficient operation of the tube in a radio circuit. In the case of dry cell tubes, there is not this constancy of voltage in dry cell batteries as compared with the storage battery unless many batteries are connected in parallel so, therefore, it is essential that a variable resistor be used to maintain the tube filament voltage at a constant level even though the battery does drop in voltage from 4.5 to 3 volts as is the case where 199 type tubes are employed. When the battery voltage falls below 3 then tone quality will probably suffer and it is wise to replace with new batteries. FILAMENT BALLAST CURRENT REGULATION CURVE 0.28 r— 0.27 — 0.26 — 0.25 — 0.24 — 0.23 — 0.22 L5.4 r Discharged : -. 244 Amps/ -Drop in volts from charged to discharged state causes ,006 amperes change in current .25 Amps. 5.5 5.6 5.7 5.8 BATTERY VOLTS 5.9 6.0 6.1 6.2 FIG. 8 Four types of filament ballasts that were tested are shown here. From left to right they are the Elkay manufactured by the Langbien Kaufman Radio Co., the Brach-stat (L. S.Brach Mfg. Co.,) the Amperite (Radiall Co.,) and the Daven ballast, (Daven Radio Corporation.) Several instruments used in the collection and calculation of figures presented in this article are also shown FIG. 7 Filament ballasts are used to regulate the flow of current in a tube circuit. Their purpose is to maintain this current value at a constant point as the battery becomes discharged. From the above it will be seen how successfully this purpose is accomplished. As the battery fell off in voltage, the filament ballast caused a change in the current in the tube circuit from .25 amperes to .244 amperes — .006 amperes, not enough to be seriously considered as affecting the proper operation of the tube at its rated filament characteristic Table II Tube Operating Characteristics TYPE OF TUBE FILAMENT CURRENT IN AMPERES FILAMENT VOLTS BATTERY VOLTS FILAMENT RESISTANCE 201 A .25 5 6. 20 199 .06 3 4.5 50 120 .125 3 4.5 24 112 .5 5 , 6. 10 12 .25 1.1 1.5 4.4 UV-200 1.0 5 6 5 UV-201 1.0 5 6 5