Radio Broadcast (Nov 1926-Apr 1927)

APRIL, 1927 THE R. B. "LAB" RECEIVER 561 thing is accomplished by using a mid tap on the secondary of the transformer. It is worth noting that a power tube drawing one-half ampere in a four-tube storage batteryoperated receiver represents forty per cent, of the total current drawn from the storage battery, 2-4 mfd -O 00-5000 ohms Loud Speaker 0~ +6 B Device FIG. 3 so that relieving the battery from the final tube current is a distinct advantage from the standpoint of battery economy and care. COIL DATA THOSE who desire to use the correct number of plate turns for various tubes will find the following table useful. It is only necessary to divide the total number of turns in the detector coil by the factor given below to determine the place to insert the bypass condenser. Of course the larger part of the coil is the detector input, "S" in Fie 1. TUBE '99 12 20 1 -A I 12 TURN RATIO i-57 1 75 2.00 3 .00 FACTOR 2.57 For example, if the coil has 75 turns and a 199 type tube is to be used, the plate turns, " P", are 75 -7 2.57 = 29, which leaves 46 for the detector input coil, "S". The matter of tapping the coil properly, however, is not extremely important, and those who desire to improve the sharpness of tuning in the detector circuit will do well to decrease the turns in coil P. This will, however, be at the expense of some voltage gain at the lower radio frequencies. GRID BIAS MANY readers seem interested in securing C bias for the last tube from the device that supplies the plate voltage for the set when such is employed. It must be remembered that in this case the total voltage obtained by adding the C bias to the plate voltage must be delivered by the B power device. For example, the 171 type of tube requires a bias of 27 volts when 135 volts are used on the plate. This means that the total voltage must be 162, which must be maintained when the set is in operation and probably drawing a total plate current of about 20 milliamperes. Inasmuch as there are many devices supplying plate voltages now on the market capable of delivering more than 160 volts at a drain of 20 milliamperes, it is easily possible for them to supply C bias as well, 't is only necessary to force the plate current of the last tube to flow through a resistance on its way back to the negative filament lead — or center tap if a.c. is used on the last tube's filament — and to take the drop across this resistance as C bias. Fig. 3 shows in diagrammatic form the method employed and indicates that the resistance should be variable to get the proper bias. For example, if 135 volts are used on the plate of the 171 tube, the plate current will be about 12 milliamperes. The required voltage drop will then be 0.012 x R = 27, whence R = 27 -f 0.012, or 2250 ohms. It is well to include a bypass condenser as shown as well as a high resistance in the C lead to the transformer. This condenser-resistance filter will keep any hum out of the grid circuit. If there is no output choke or output transformer, a bypass condenser should be placed across the C bias resistance. Readers interested in this method of obtaining C bias should read Laboratory Information Sheet No. 73, in the March, 1927, Radio Broadcast. detectors \ A / HEN a C-battery detector is used, it may V V be found that the regeneration condenser, C4, is not large enough to cause oscillation over the entire band. This is often the case with a plate voltage of 45 and a C bias of negative 4.5 volts, and the solution is to increase the plate voltage to about 67.5, or a little under this figure if possible. For freedom from microphonics, and for somewhat better quality, the oxide-coated 1 12 tubes can be recommended as detectors. They have considerably lower output impedance and, due to their heavier filament, are not so liable to be mechanically noisy. For the best quality and greatest freedom from line noises when using a B power-supply device, a separate dry B battery for the detector is to be highly recommended. It is only necessary to connect its negative terminal to the negative of the power supply device and to connect the detector B wire from the lower side of the first this difficulty, and at the same time make the receiver more stable, more easy to control, and eliminate much unwanted "pick-up" from local stations. Another method of decreasing hand capacity is to use a dial that has no metal connecting to the metal parts of the condenser. Yet Power Device o 5 0 +B +B 48 DeL 90 135 -B B Amp. -0+ B90 6-B -o + B Det. -O-B -O-A Dry B Battery 6+B O-hA -O + C O-C 0~C DETECTOR Shield FIG. 5 audio transformer to the positive terminal of the B battery. This is shown in Fig. 4. With the proper B and C adjustment, the detector should slide into oscillation quietly. With improper adjustments it will go into oscillation with a "whoop," and there will be considerable "hang-over" on the regeneration condenser. HAND CAPACITY THE greatest source of trouble1 with the fourtube receiver is from hand capacity on the antenna or radio-frequency amplifier dial. This is due to the fact that neither rotor nor stator is at ground potential. There are several solutions. In the first place, complete shielding of the radiofrequency amplifier will practically eliminate FIG. 4 another is to use a condenser with a shaft made of insulating material or to use an insulated coupling device between the condenser and the shaft that fits into the metallic dial. USING A LOOP IN OPERATING the four or two-tube "Lab" ' receiver with a loop, several slight alterations must be made so as to allow easy change from antenna to loop connection. The change is made with the aid of a jack and plug. The jack is of the double-circuit type into which a plug, connected to the loop terminals, is inserted. The secondary coil of the antenna stage has its end leads connected to the inner terminals of the jack. The tube circuit, that is, the grid and neutralizing condenser points of the circuit, are connected to the upper and lower outside blades of the jack respectively. When the plug is not in the jack, the two inner blades make contact with the outer blades and connect the coil to the radio-frequency tube. When the plug is inserted, the upper and lower outside blades are sprung away from the inner blades, disconnecting the secondary coil and in its stead connecting the loop to the tube circuit. Since there is a third contact — the center of the loop — provision is made by means of a single pin jack located on the same insulation support in which is mounted the loop jack, so that a flexbile lead from the center point of the loop may make contact with the circuit via the pin jack, which is permanently connected to the center tap on the secondary coil. Reference to Fig. 5 shows the revisions necessary in the circuit to include the loop, while Fig. 6 gives the dimensions of the insulation strip which supports the loop jack and pin jack. The loop has pronounced directional characteristics, and it will be observed that signals are received loudest when the loop is turned with its plane in a line running between the center of the loop and the station received. Where hand capacity effects are noticeable in the operation of the loop, this undesirable condition may be eliminated by turning the loop inside out. This is done by connecting together the two