Modern Transformers in Sets Operating from B Battery Eliminators

Radio Record, Volume II, Issue 11, 28 September 1928, Page 29

Modern Transformers in Sets Operating from B Battery Eliminators

WHEN some transformers, such as Ferranti, which have high efficiency and high inductance, are used euirrent battery eliminators, trouble may be experienced because of incorrect design of the B eliminator resulting in feedback from the audio transformers. Troubles of this character are indicated by the presence of "burbles" or a series of "put-put’" sounds in the output commonly known as "motor boating." There may also be a residual hum of a pitch corresponding to twice the frequency of the alternating current supply, and this is more to be expected when using audio transformers sets operated from alternating which are efficient over the entire broadcast range, aS any residual due to insufficient filtering is amplified by the transformers in the same way a low note would be. "Motor boating,’ when using a B eliminator, is usually caused by one of two things. First, the filter system may not be correct. Second, the resistances may not be by-passed enough, resulting in feedback through the resistances, For any given frequency there is a relation between the values of inductance and capacity which much be obtained or the filter will not do its work properly. If the filter is not working properly, a small low frequency voltage will be applied to the detector valve and this Wilkebe amplified by the transformers. If audio transformers used in the set not give a high »mplification of the low notes, motor boating may not regult. The.trouble in the filter system is generally due to the inductance. The inductance of the choke coils varies with the amount of direct current flowing. For example, a choke having 20 henrys of inductance with

vo direct current flowing in it may have its effective inductance reduced to 10 or even to 5 henrys with 20 or 30 milliamperes flowing through it. If the resistance across the output of the eliminator is not by-passed with sufficient condenser capacity, there will be an alternating current voltage impressed on the plates of the valves uhead. If this voltage is large enough, regeneration will result, and if the impedance of the transformer is high, the oscillation frequency will be within the audible range. The only way to locate such troubles is by experimenting, and the application of some of the following suggestions will usually eliminate them. Try a larger choke coil or an additional choke coil in the filter system. Increase the value of capacities used in the filter system. It may be necessary to increase both the amount of capacity and inductance used in the filter. Try grounding the B minus of the eliminator, and if a short circuit results due to grounds on the lighting system, make the connection through a condenser of 0.5 mfd. capacity. Try condensers of one of two microfarads capacity connected between the B minus and each tap of the eliminator, although an extreme condition may require as much as four microfarads across the tap connected to the first audio frequency transformer. In some cases choke coils of 12 to 15 henrys connected in series with all leads running from the B eliminator to the set will produce results, although usually such a choke is necessary only in the detector lead. ‘The proper value of the choke coils can only be determined by experimentstion. The by-pass condensers must be connected from the B minus of the eliminator to the lead of the choke that is connected to the radio set.