Radio Broadcast (Nov 1926-Apr 1927)

478 RADIO BROADCAST MARCH, T927 VOLTAGE REGULATION TH E usefulness of a power device is determined by the range of current drains it supplies at usable voltages. This is termed its voltage regulation. If the unit is of insufficient capacity, it will deliver less to the tubes of the set than the required voltage at heavy current drains. Voltage regulation is modified by the filter element of the device, so that the only way to be thoroughly sure of its capacity is to measure its voltage output over the entire range of current drains encountered with normal receiving sets. The accompanying diagram shows the voltage outputs of several devices tested at the Radio Broadcast Laboratory. Before considering the significance of these curves, however, we will examine the filter element, an essential part of the circuit. The filter is essentially a condenser "tank" fed through high impedances which absorb low frequency current variations. The rectifier pumps out a pulsating current, just as a giant water pump, filling a reservoir. The chokes act like a breakwater, seeking to meet the impact of the pump strokes, so as to make the reservoir beyond as smooth as possible. At the far end of the condenser reservoir, smooth, ripple-free current is'drawn. Obviously, the larger the impedance of the chokes and the larger the capacity of the condenser the smoother the output wave form. No amount of inspection will reveal the ruggedness of the condensers or the impedance of the chokes. The size of the choke is not a guide to its effectiveness because a large choke with a cheap iron core may not be as good as a small one using high quality, selected and aged iron. avoiding condenser breakdowns THE purchaser of a B power-supply device can reduce the chances of condenser breakdown to almost nil if he obeys {the instructions of his dealer and those in the booklet accompanying the device as to the order in which receiving set tubes 285 I 1 1 1 1 1 1 r and B unit should be turned on and off. Some manufacturers are quite precise in their instructions; others neglect to mention this important point. The mystifying failure of a power device which has given good service throughout the previous evening is probably the penalty of turning on the set incorrectly. In going over the instruction booklets of a number of manufacturers, we do not find any uniformity in their recommendations. In the absence of advice from the manufacturer, the best practice is to turn on the receiving set first and then the power unit. This places an immediate drain on the device so that excessive voltages will not be encountered. The strain on the smoothing condensers is thereby greatly reduced. When turning off the set, first turn off the B supply unit, thus dissipating the charge which would otherwise remain on the -smoothing condenser. Then turn off the receiver tubes. If this practice is invariably followed, the chances are that little or no difficulty will be encountered due to condenser breakdowns. These instructions had best be followed unless the manufacturer definitely states they are not necessary. Many devices are so constructed that difficulties of this nature do not arise. The series of curves in the accompanying diagram were prepared by Radio Broadcast Laboratory, showing the voltage output of numerous makes of B supply units throughout the normal range of current drains. All of these devices show that the voltage falls as the current drain increases. If the tubes of the receiver are not lighted, the current drain is zero and the instantaneous voltage rises in some cases to dangerously high values. These curves also show the great variety in the output of different devices. Some, when subject to heavy load, strain and weaken because the rectifier tube is not capable of supplying the demand. Others hold up well. Those giving the most uniform output are of two types, (1) chemical rectifier devices and (2) those equipped with ballast or voltage regulator tubes. The chemical devices obtain their good voltage regulation because of their low internal resistance, while the regulator tube types accomplish the result because they automatically vary the rectifier tube's load so that the output is maintained at a specific voltage. The chemical types do not, in a general way, cost more than the usual run of B power-supply devices but those equipped with regulator tubes are more expensive than the average, but they give good service with almost any kind of set. The family of voltage regulation curves shows a tremendous variation in the performance of different devices. It is a well known fact that a B power-supply unit may give satisfaction with one make of set and fail with another seemingly quite similar. This may be due to faults in receiver design, but more than likely it is due to the power devices' inability to furnish the plate voltage required by the set at its particular current drain. select a b-supply unit that meets your set's requirements SINCE the voltage output of most B-supply units depends upon current drain, selection is a matter of knowing a receiver's plate current requirements and finding a device which delivers sufficient voltage for each of the tubes in the set at that particular drain. If the plate voltage supplied to a radio-frequency amplifier is somewhat too high, the average set is almost certain to oscillate. To give leeway in plate voltage output, most B devices are equipped with variable resistances. These and other resistances in the output circuit must be suitably bypassed. A resistance not properly bypassed may cause coupling between two or more stages of the amplifier and thus cause a squeal. Howling and whistling is usually due to excessive plate voltage supply to radio-frequency amplifier or detector tubes. The lower the current drain the higher the voltage output of most B-supply devices regardless of the terminal markings on the binding posts. Whistling is usually curable by readjustment of voltage supply to the set. A few poorly designed units with inadequate bypassing condensers, cannot be cured of their whistling tendency by adjustment of voltages supplied to the set. DETERMINING THE CURRENT DRAIN T F YOU know the exact drain of your set and ' the voltage required by each tube, it is not difficult to determine, from the voltage regulation curve, whether a certain B-supply device will give it the required plate voltages at the required current drain. As an aid to those who dc not possess an accurate milliammeter for measuring current drain, the accompanying table gives the plate current of standard tubes at grid potentials and plate voltages customarily used. Individual tubes vary somewhat from the figures given, and a small percentage deviation in filament current or plate voltage modifies the figures somewhat. But the table is better than no guide at all; with its aid, it is possible to compute the total current drain of almost any receiver. The i-J-volt positive grid bias figures should be used as the basis for determining the plate current of detector tubes in circuits where the grid return is made to the positive filament lead. INTERPRETING THE VOLTAGE REGULATION CURVE THE curves in the diagram on this page show the maximum voltage output of various typical B power-supply devices at various current drains. Device No. 1, for example, gives 128 volt* at 18 milliamperes and 115 volts, at 30 milli 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 LOAD CURRENT. MILLIAMPERES CURRENT-VOLTAGE CURVES These curves are of a dozen typical B power-supply units and were made in the Radio Broadcast Laboratory. They indicate clearly how the output voltage rapidly drops as the current drain is increased