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142
RADIO BROADCAST
JULY, 1928
of this type unless all of the apparatus used in conjunction with it is also of the best. The output of the amplifier should be fed into a good cone or dynamic type loud speaker such as the Magnavox or Jensen. In operating the unit care must be taken that it is so located to provide sufficient ventilation for the tubes. The 250 type amplifier, especially, becomes very warm in operation. If the unit is placed inside of a cabinet it will be a good idea to tack a piece of asbestos above the amplifier to protect the cabinet from the heat.
Take care in constructing this amplifier. Remember there are many kinds of accidents. One is the useful that leads to discoveries and inventions. Another kind, somewhat different, and sometimes associated with the construction of high-voltage amplifiers, leads to burnt finger tips or perhaps to severe shocks. The voltages used in this and other types of amplifiers using 210's and 250's are positively dangerous and every precaution must be taken in wiring and handling the amplifier so that there is no possibility of shock. Do not make any changes in the amplifier while the power is on.
From Rectifier
From Rectifier
<B> FIG. I
Sketch A illustrates a typical filter circuit such as is incorporated in the usual B-power unit. Sketch B shows a modification of this circuit, condenser Q being omitted, which greatly reduces the peak value of current which the rectifier tube is called upon to deliver. This improvement has been incorporated in the power amplifier described in this article
LIST OF PARTS
Ti Dongan Transformer No. 7568 . $ 13.50
T2 Dongan Output Transformer No.
1 177 12.00
T3 Dongan Type H Audio Transformer Ratio 35-1 .... 4 50
Li Dongan Double Choke Unit No.
6551 15.00
Ci Dongan Condenser Unit No. D
600 23 . 00
Q> Dongan Condenser Unit No. D
3°7 • • 10.00
Ri Ward Leonard Resistor No. 507-6 6 75
R2 Ward Leonard Resistor No. 507
55 1 . 00
2 ux-281 or Cunningham cx-381 . 15 .00 1 ux-250 or Cunningham cx-350 . 12. 00
3 Benjamin Sockets 2.25
1 Porcelain Socket for Fuse . . .25
1 3 Amp. Clearsite Fuse ... .15
8 Eby Binding Posts .... 1.20
B Flashlight Bulb and Socket . .50 1 25-ft. length Belden colorubber
hookup wire .50
Baseboard, Wood Screws, Bake
lite Strips, etc. Approx. . . 1.00
Total $1 16.25
The following additional items are required in order to make the device operative.
2 cx-381 (ux-281) Rectifier Tubes
1 cx-350 (ux-250) Power Amplifier Tube
Input
FIG. 2
OB+135 V
OB+90V.
OB+45V
OB
Output lnPut