Radio Broadcast (Nov. 1925-Apr 1926)

APRIL, 1926 THE USE OF THE FILAMENT RESISTANCE 685 FIG. The C battery is shown in its proper place here. This is by far the better ~ way to obtain grid bias for receiving tubes since if it is desired to vary the grid bias voltage, more or less C battery may be placed in the circuit Rheostats are variable resistances with which an accurate control of the filament voltage and current is obtained. The selection of a rheostat for one tube has already been explained and the radio experimenter should not become confused where more than one tube is controlled by a single rheostat. As it has been said, in a radio circuit it is usual to connect several tubes in parallel, the path for the flow of current will become greater hence there will be less resistance to the flow of this current. In tube circuits it is customary to employ tubes all of one nature, that is, all 5 or all 3-voIt tubes. The total resistance then, of a circuit, is equal to the resistance of one tube divided by the number of tubes in the parallel arrangement. See Fig. 4. For instance, 201-A's have a resistance of 20 ohms. Where four are connected in parallel the total circuit resistance is 5 ohms. Therefore, to cut down the battery voltage so that five volts are applied to the filament terminals, 1 ohm of external resistance must be added to the circuit. Before, the method by which the rheostat value was ascertained was to subtract the circuit resistance at 5 volts from the circuit resistance at 6 volts. Another method, also very good, is to employ E — Ei the formula R = — r where R = resistance of rheostat, Ez= filament voltage, E = battery voltage and I = total current of circuit. This last factor, total current, is obtained by multiplying the current rate of one tube by the number of tubes to be controlled by the rheostat. Again, using four tubes, the total current would be 4 x .25 amperes = 1 ampere. Then, applying the formula R = -~ = 1 ohm. WHERE SHOULD THE RESISTANCE GO? INDEPENDENT tube 1 manufacturers as well as the pioneers in tube production advocate the use of the rheostat in the negative side of the A battery lead. The writer became convinced that this procedure was proper only where the rheostat was to be employed to provide a grid bias of a few volts. This usually was supplied by tapping on to a portion of the rheostat so that a voltage drop through that portion of the resistance employed would provide the necessary few volts for grid biasing. In Fig. 9 the circuit shows how this tap is arranged. This manner of obtaining grid bias is unusual inasmuch as it entails first, a calculation as to the necessary amount of resistance wire to be included in the tap-off circuit so as to supply the necessary grid voltage. Secondly, it is sometimes desirable to vary the grid bias and when the connection is soldered to the rheostat a change in voltage is impossible unless the tap-off be unsoldered. The use of a C battery provides the more convenient and simpler way to obtain grid bias and with this change it is possible to place the rheostat in the positive side of the A supply lead so that all the negative leads are at gVound potential as in Fig. 9. The use of the rheostat in the positive side instead of the negative side of the A battery supply makes for more accurate wiring, clearer understanding of circuit diagrams, and the surety that one part of the complete wiring circuit will be at ground potential insuring shorter leads from other units in the circuit which of necessity must be connected to the ground line. Among the rheostats tested because of the difference in design and principle was the Bradleystat, and the Filkostat. Both these devices insure a continuous, even increase in voltage differing from the wire-wound rheostat which provided the increase in steps as more turns of wire was cut out of the circuit. Voltage regulation is secured by the change in resistance of the device as carbon discs are compressed by means of a thumb screw. Table HI Filament Ballasts for Various Tubes TUBE ELKAY RADIALL DAVEN BRACH EQUALIZOR AMPERITE BALLAST BRACHSTAT 1 type 199 No. 50 for 6 No. 6V 199 1 C for 4 volt volt source for 6 volt source No. 4 V 1 QQ for 4 or 4 5 volt source source 1 D for 6 volt source No. 25 No. 120 2 c 1 f vr»p 1 90 No! 25 No 120 2 C 1 type 201-A No'. 4 No. 1-A No. 1 1 B 2 type 201-A's No. 2 No. 112 No. 2 2 B 1 tvnp 1 1 9 A. i.y JJC ! 1 . . No. 2 No.' 112 Use No. 3 for 2 B 3 type-201 A's and No. 4 for 4 type 201A's MU 20 No. O MU 6 No. O WD 11 1 WD 12 WX 12 I C 11 f D 11 1 A c 12 ! CX 12 I If you intend to build a super-heterodyne and control eight tubes with one rheostat then that rheostat should be of the power variety capable of carrying from 2 to 3 amperes of current. This is especially necessary where the new power tubes are to be employed in the audio end since they take more current than the ordinary 201 -A type of tube. Fortunately, filament ballast makers have kept up with the development of the power tubes and today it is possible to purchase special ballasts designed solely for use with these new tubes. The total watts consumed in a filament is 1.5, of this, .25 watts is dissipated in the rheostat or other filament resistance. This means that there is a 16 § per cent, loss of power in the filament resistor and only 83 | per cent, of the power being utilized in the filament directly. Outstanding are these several not-to-be ignored facts: A 4-ohm rheostat is satisfactory for controlling a single 201 a. A variable resistance may be used for detector filament regulation but for other tubes fixed resistances are satisfactory. Filament ballasts may be used successfully in all tube filament circuits except detectors. Dry cell tubes need variable filament control. Where grid biasing is desired, use a C battery. Tone quality suffers when the filament or plate voltage is reduced. PACENT Ammeter Battery FIG. I I Voltmeters and ammeters indicate the amount of voltage and current in a circuit. This is the circuit of the test set actually employed in checking the regulation properties of rheostats and filament ballasts described in this article FILKOSTAT YAXLEY Radio Broadcast Photograph BRADLEYSTAT FIG. 12 Only a few of the many rheostats that were tested are shown here. The Kellog type of resistance unit has its circular form, with the resistance wire thereon as the rotating unit. The same is true of the Cutler-Hammer rheostat. Two carbon-pile type of resistance units are represented in the Filkostat and Bradleystat