Radio Broadcast (May 1928-Apr 1929)

A and B Power from the D» C. Lines B. DALL WHERE 1 10 volts d.c. is the house-lighting current supplied by the local power company, there is opportunity for supplanting storage or dry batteries for both filament and plate circuits in a radio set by means of comparatively simple and inexpensive devices. These have been described in the columns of radio periodicals and are fairly well understood by those who "roll their own" in d.c. districts. There are a few pitfalls in this sort of apparatus which have been touched on in the form of warnings, but practicable solutions do not seem 3amp fuse 2 ampchoke || 2 Mfd ■ Socket where (connection to light bulbs is plugged in Ri vs/vy/wv^ 30 Ohms 2 Mfd. .1 Mfd. ■WWVWWWV* <WV\A/wvv> 20,000 Ohms 500.000 Ohms B+45 B+100 FIG. I to be generally known. The purpose of this article is to indicate means for polishing off some of these rough edges. First let us state briefly the sort of A battery substitute used. In simplest form it is a bank of electric light bulbs, wired in parallel, in series with the filament circuit of the radio set and the light socket. The total wattage of the bulbs is adjusted to the current consumption of the set, the basis being about 30 watts per quarter-ampere tube. This is a little in excess of actual needs, but the excess may be bypassed around the set through a 30 or 50-ohm rheostat, R, in Fig. 1, the varying of which offers a form of filament control to compensate line voltage variations. This resistance is across the current source, and consequently the current into the set increases as the resistance is increased. The bank of bulbs may be one or several reading lamps which in the evening would be lighted anyway, and, as a result, the set current costs the user nothing additional except for daytime operation. HOW TO REDUCE COMMUTATOR HUM A CHOKE coil, capable of carrying about 2 amperes, and capacities across the line, although not always necessary, are frequently used as a filter to curtail commutator hum which is present in the d.c. supply voltage. See Fig. 1. The writer has found that various types of 5-voIt tubes operate with complete satisfaction at 4.5 volts. His ear at least can detect no difference between a set operating at and one operating at 5 volts. An inexpensive and compact dry battery of 45 volts is everywhere available in the form of a C battery. If the voltage at the set is adjusted to 4^, and a 45-volt C battery is placed across the A substitute, the plus to plus and the minus to minus, the hum, if there be any in the speaker, will decrease to a negligible amount. In normal operation there will be no current drawn from the battery and its life will consequently be long. This little battery has another important function if used as indicated, which cannot be performed by high-capacity condensers. It will absorb what otherwise might be a considerable voltage rise and so protect the tubes in the set against burn-out. The danger in the use of the d.c. A battery substitute described, lies in the fact that if a tube be removed from the set while the current is on, or if a tube filament should fail or a filament prong become open-circuited, the voltage at the A plus and A minus posts will at once increase considerably. Excess current will consequently be forced on the remaining tubes in the set. If that shock blows a second tube, another rise occurs and the remaining tubes are sure to go. With the handy little 45-volt battery across the set, one tube may be taken out of a 4-tube set and the voltage on the remaining three will rise only about to of a volt (varying a bit with the internal resistance of the battery), instead of jumping about if volts as would be the case without the dry battery shunt. Consequently a tube filament may fail and the other tubes in the set will be perfectly safe. On the same principle this battery also absorbs voltage surges. Were the dry battery to be left in this circuit after the house current was turned off it would drain itself in a few moments in a vain endeavor to supply current to the set. To make disconnection of the battery automatic, a relay comes in handy wired as indicated in Fig. 2. Any of the types used for cutting out chargers and cutting in B power units will serve. In such a relay the two contacts on the plug by which connection is usually made to the light socket may be connected to A plus and to A minus, and the leads from the B power unit socket on the relay connected to the C battery. The actuating coil of the relay goes in series with the A plus lead. It is essential that this coil lie between the point at which + C is connected and the line, and not between the C battery connection and the set. In the latter position the relay will not function properly and there would be a considerable drain on the C battery as the latter would still be feeding the set through the relay. In the correct position, the relay action is instantaneous. One C battery lead may be broken, or two, as many commercial relays are equipped with a double set of contacts. WAYS FOR ECONOMIZING ON CURRENT OPERATING multi-tube sets on no-volt d.c: for filament supply is not economical unless the current-limiting resistances (in the case described, electric light bulbs) are used at the same time for lighting the room. A sixtube set using quarter-ampere tubes would require the use of the equivalent of 180 watts of such bulbs, which, on an average use of four hours per day, would add 22 kilowatt-hours to the electric light bill each month. To cut this current consumption the kink is to use the 199type tubes up to, but not including, the last audio stage. If five tubes be so changed, we have .06 x 5 = 0.3 ampere, instead of .25 x 5 = 1.25 ampere for 20iA-type tubes, a saving of nearly one ampere of filament current. The saving is nearly 120 watts, or two thirds of the original consumption. A 15-ohm rheostat in series with the A plus lead to the 199-type tubes is sufficient 34 to cut the 45 volts across the A-battery substitute to 3 volts for the set of five 199's. If the reading lamp used in connection with this A-battery substitute is to be lighted at times when the set's operation is not desired, a snap switch, indicated in dotted lines in Fig. 2, may be connected across the A plus and A minus just back of the relay. When that switch is open, the lamp will light and the set operate; when that switch is closed, the lamp will light but the set will be off. It is understood, of course, that the switch on the set itself is always left in the "on " position, the whole apparatus being controlled from the light socket at which the current originates. A B-POWER UNIT FOR D.C. OPERATION THE B-power unit side of a 1 10 d.c. outfit is simple. One 30 to 60-henry choke, of d.c. resistance not exceeding 600 ohms (preferably lower), and two 4-microfarad condensers (good by-pass condensers will do, as the voltage does not exceed 110) comprise an adequate filter. Allowing for a voltage drop through the filter, an output of 90 to 100 volts is obtained. One variable high resistance gives the 45-volt tap for detector. Fig. 1 shows the hook-up. With a push-pull last stage audio for four power tubes, as described in Radio Broadcast for May, 1928 (page 18-19), tne B plus is taken directly from the light-socket connection around the filter, since thereby the full 1 10 to 115 volts is obtained for the power tube plates and any hum is balanced out by the push-pull arrangement. It is customary to equip these power devices with 3-ampere fuses. This is of course protection to the house fuses, but not to the radio apparatus. At 3 amperes, a short-circuit could play considerable havoc with good radio equipment. To obviate this danger, the writer has inserted an old 199 tube in series with the B plus 1 10-volt plate supply lead. This limits the current through this lead to 60 milliamperes. The drop across the tube at a drain of 40 mils is only 2 volts, and all tubes and associated apparatus are well protected. The 199 tube used may be an old one, whose decreased filament emission renders it unfit for radio set use. The tapped choke output for push-pul! stage is a satisfactory substitute in the use of two 30-henry chokes, the two outside connections going to the plates and speaker, and the inside common connection to both chokes going to the B plus. This kink permits the use of chokes which the builder may have on hand. A* Switch -A -Relay rJlMlll AH VCBat A+ (Radio Set) FIG. 2 -A