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DECEMBER, 1928
PHONOGRAPH-RADIO AMPLIFIERS
89
R7 One Electrad Truvolt resistor, 50-ohm, type V-50;
Rs One Electrad Truvolt resistor, 10-ohm,
type V-10; R9 One Ward Leonard resistance bank,
type 507-6; Rio One Centralab power potentiometer,
175-ohm, type PF-175; Ti, T2, Two Sangamo audio transformers,
type A;
T3 One National filament-lighting transformer;
T4 One General Radio power transformer,
type 565-B; T5 One General Radio loud-speaker filter,
587-B;
Three Benjamin sockets, ux-type; One Benjamin socket, uv-type; Belden Hook-up wire; Eight Eby binding posts; One Platter orthophonic cabinet, type PRThe following tubes are required: One power tube, 250-type; Two rectifier tubes, 281 -type; One tube, 227-type.
The input to the amplifier is through Nos. 1 and 2 which form a complete c
^frequency characteristic of not
.using filtering ingTid and plate . . supply to tubes .
5000 10,000
CYCLES PER SECOND
FIG. 4. RESPONSE CURVE OF THE AMPLIFI DESCRIBED IN THIS ARTICLE
ER
type
918.
leads ircuit
for the audio-frequency currents, these currents being kept out of the B-supply unit by the resistor Ri. Filter systems, consisting of resistors and by-pass condensers, are used in the grid and plate circuits of the 227-type tube, and also in the grid circuit of the 250-type tube, for the purpose of keeping all of the audio-frequency currents out of the B supply. The filter in the grid circuit of the 227-type tube consists of R2, C2, the plate-circuit filter is R4, C3 and the grid-circuit filter of the 250-type tube is R6, d. The output of the amplifier feeds into the chokecondenser unit T5.
The B-supply unit for the amplifier is conventional, consisting of two 281type tubes in a full-wave rectifier system. In series with the output of the rectifier is placed a small 5-volt flashlight bulb, L. If a short circuit occurs in the filter system, or at any other point in the circuit, the current through this lamp will increase sufficiently to burn it out, thereby protecting all the apparatus from damage.
The excellent frequency characteristics of this amplifier are indicated by the solid curve in Fig. 4. To give an idea of the importance of the various filter circuits, mentioned in a previous paragraph, we have also shown in dotted lines the frequency-characteristic curve of this amplifier without the filters. The importance of such filtering is obvious.
The home experimenter who likes to construct his own gear may desire to build such an amplifier as we have described, but the professional set builder who has or hopes to get some orders for the construction of a phonograph-radio combination can do the job more quickly by buying a kit of parts or a completely wired amplifier.
FIG. 5. A GROUP OF AMPLIFIERS SUITABLE FOR USE IN PHONOGRAPH RADIO COMBINATIONS