Radio broadcast .. (1922-30)

504 Radio Broadcast signal strength in order to obtain any volume control at all on the potentiometer. This is because in the case of 2oi-A's, the voltage variation across the potentiometer is from 3 to 5 volts and with igg's from i\ 103 volts, whereas with wo-12's, the variation is only about i volt. Filament Rheostat: If one type of tube is used throughout the set, a single rheostat for all tubes is sufficient. This should be from 6 to 7 ohms for any of the standard tubes. If SOI-A'S are used only in the audio stages, their positive filament leads will have to be brought out independently, when other types of tubes are used up to the audio stages The filament adjustment on the 2OI-A audio tubes may be made by means of an extra rheostat or by means of a small resistance unit placed inside the set and adjusted once. The filament current of the audio tubes is not critical and when once adjusted may be left fixed. If the igg's are to be operated as the first five tubes in the set in conjunction with 2Oi-A's on a 6-volt battery, the rheostat resistance for these five tubes will be from 1 5 to 20 ohms. If WD-I I's or i2's are used for the first five tubes, they should be operated either on a 6-ohm rheostat lead out to a separate A-plus binding post and then to a 2-volt tap on the storage battery or to a separate A 1 * .0005 * A \ ^ - __f > < > 4 r n i/ .0075 ( .000045 ( -6 FIG. 7 How to add an additional stage of radio-frequency amplification to the "super". Another oscillator coupler, tube, socket, rheostat, and variable condenser is necessary for the construction of this separate unit, which should not be attempted except by the radio constructor who is expert at tuning the super-heterodyne because the additional radio stage sharpens the tuning greatly battery. If they are to be operated directly from a storage battery supplying the 2Oi-A's, the rheostat resistance will be about 10 ohms. In both the case of the 199*5 and WD-i2's, run directly from the storage battery, the rheostat used with them should be just barely turned on, as if it is cut all out the full 6 volts will be applied directly to these tubes with disastrous results. Volume Control: The volume of the set may be controlled by the potentiometer, operated in conjunction with the rheostat. The potentiometer might be entirely omitted and the volume controlled by the rheostat only. It will be found that if the full amplification of the set is used on local signals, a slight amount of distortion may be evident. With volume enough to be heard all over a 40 foot square room no distortion will be experienced. In any event it may be controlled by proper rheostat and potentiometer adjustments. It has been found possible to operate 2OI-A tubes with as little as 3! volts on the filaments with perfectly satisfactory results. Location of Rheostat: Tube manufacturers recommend that rheostats be placed in the positive filament lead of the detector tube and in the negative lead of an amplifier. The reason for this change is that in the circuits shown in the tube data sheets an endeavor is made to use the voltage drop across the rheostat for grid biasing purposes. If a separate C battery is used and no endeavor is made to utilize this voltage drop across the rheostat, it is of absolutely no importance which filament lead the rheostat is connected in. It is always advisable, however, to keep it out of the lead which is a common B battery return. The common point in these sets is the negative. For these and other reasons it is shown in the positive filament lead, while the on-off switch is in the negative lead. Plate Voltage: The set will operate satisfactorily with as little as 45 volts on all tubes, but the C batteries will have to be readjusted if this voltage is used. Varying the detector and oscillator plate voltage from 22 to 45 may sometimes improve reception slightly, and decrease consumption a small amount. The current consumption using 2Oi-A's on 90 volts is twenty milliamperes or loss, and in using IQQ'S from 14 to 15 milliamperes. If it is desired to add resistance-coupled amplification to the set instead of transformercoupled audio it may be done by using the amplifier circuit given in Fig. 8. This shows two stages, which will give not quite the