Untitled

Radio Record, Volume IV, Issue 12, 3 October 1930, Page 28

ouman ‘The principle of obtaining grid-bias for any valve is to connect the particular grid to a point on the filament wiring that is more negative by the required amount. If objectionable hum is encountered by this method, provide additional smoothing, or bias with a dry battery. Some maing are difficult to smooth and may require a choke in both positive and negative leads. If there is only one choke, it is always in the "live" or unearthed lead. . Particulars given in this article are for a voltage of 230, but will cover the range from 220 to 240 volts if a corresponding adjustment is made in the resistance in circuit. In special cases where a voltagedivider is used across the mains, the total resistance should be not less than 20,000 ohms, that is, considerably higher than is used in the average A.C. eliminator. Needless to say, insulation must be good throughout the receiver, and a fuse of thin copper wire, say, 38 gauge, must be placed in each lead from the mains, along with a_ well-insulated double-pole switch, which will be found handy when making adjustments. No live connections must be allowed outside the panel, so that when the receiver is working, enclosed in its cabinet, there is no liability of a shock being experienced by the operator. Whether shown in the diagrams or not, connection to loudspeaker or *phones should be either through an output transformer or a choke-filter, in which latter a condenser is placed in both leads. When the latter method is used, it must be remembered that the condensers are in series, so that their values are halved, so double the usual capacity must be used for each. At least 500 volts test, and not less than 2 mfds..each. If low notes are missed, increase the capacity to 4 mfds. At this point we shall have to leave the discussion till next week.