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MAY, 1928
A D.C POWER AMPLIFIER AND B SUPPLY
19
Radio Broadcast Photograph
TUBES USED IN A TYPICAL A.C PUSH-PULL AMPLIFIER AND B SUPPLY
This photograph is shown for the benefit of those who may complain that there are too many tubes in the d.c unit described in the accompanying article. There are five in all for the d.c. combination, representing an outlay of about $\ 5.50. The tubes in the photograph are necessary for the high-quality a.c. push-pull amplifier and B supply described in the February Radio Broadcast. They represent an investment of more than double that required for the tubes of the d.c. unit. The a.c. unit is, however, capable of
handling considerably more power
available for the plates of the amplifier tubes, thereby reducing the output of the amplifier. Since no current is drawn from the C battery, it will last a very long time. It will be noticed that the positive end of this battery is connected to the center of the resistance, R2, which is connected across the filament circuit. This connection to a center point is used for the same reason that a tap is made at the center of the filament winding in an a.c. power unit, i.e., to eliminate the effect on the grid and plate of any ripple (a.c. component) that exists in the filament voltage. The method is a distinctive feature in that it obviates the necessity of an expensive filter system.
One precaution should be observed when operating the amplifier: Never remove tubes or change them while the amplifier is in operation. To do so will cause excessive filament voltage to appear on the tubes left in the circuit with the result that some or all of them may be blown out
or paralyzed. Always throw the line switch to the "off" position before making any adjustments or alterations in the power amplifier.
The ground binding post on the radio receiver must be connected to the ground binding post on the amplifier. No connection from these posts to ground proper is to be made or a short-circuit will occur.
The plug connecting to the light socket should be reversed if the amplifier fails to operate when the power is turned on. The following parts are necessary for the d.c. amplifier:
Parts List
Ri — Six Ward Leonard Vitrohm Resistors, Type
WX-14.3, in Series R2 — Ward Leonard Vitrohm Resistor, Type
507-53
R3 — Ward Leonard Vitrohm Resistor, Type T25,000
R4 — Ward Leonard Vitrohm Resistor, Type 507-64
Li — Thordarson Filter Inductor, Type R-196 Ti — Thordarson Audio Transformer, Type R-200 T-2 — Thordarson Push-pull Input Transformer, Type 2408
T3 — Thordarson Push-pull Output Reactor,
Type 2420 Ci — Dubilier 4-Mfd. Condenser Type 901 C2, C3 — Dubilier i-Mfd. Condensers, Type 901 C4 — Dubilier 0.005-Mfd. Condenser, Type 6oi Ten Eby Binding Posts Cord and Attachment Plug Two Five-Ampere Fuses with Mountings One Line Switch Five Eby Sockets One ux-201-A (cx-301-A) Four ux-171-A's (cx-371-A's) C Battery, 22.5 Volts (Small type with tap at
16.5 or 18 volts. Five4.5-volt C batteries may
be used.) Ward Leonard Perforated Cage Bakelite or Metallic End Panels for Cage Transite Baseboard, 18" x 7" x \" Four Ward Leonard Mounting Feet Type 507-21
THE D.C UNIT WITH THE PERFORATED CAGE IN PLACE