quencies. The value of this condenser is selected so that it will not affect the audiofrequencies
and so the audio will remain and is passed on to the volume control.
The volume control is a fixed resistor with a sliding tap which allows any desired portion of the
signal to be selected. The tap connects to the grid of the 12AV6 through a condenser which
keeps the negative voltage present at the control away from the tube. Audio amplification takes
place in the triode section of the 12AVO and the boosted signal is fed to the grid of the 12A6
beam power amplifier through an isolating capacitor. The condenser keeps the high voltage
present at the plate of the 12AV6 from the grid of the 12AG but passes the audio signal. This
method of energy transfer is known as resistance-capacity coupling, or, just resistance cou-
pling.
A small amount of voltage at the grid of the124'6controls a substantial amountof current in the
tube. When the audio voltage becomes positive, the current increases, and the current de-
creases when thevoltagebecomes negative. These currentvariations pass through the primary
of the output transformer, wldch steps the high voltage down and couples it to the loudspeaker .
The speaker is an electrical-mechanical device. When a voltage is presented to its voice coil,
the paper cone moves. A permanent magnet is installed in the speaker. The voice coil is an
electromagnet. Any voltage applied will cause the magnets to attract or repel each other, de-
pending on the polarity of the voltage. Since the electromagnet coil is connected directly to the
paper cone, it will move proportionally to the voltage applied. Motion of the cone causes air to
move in a similar manner. Air movement of this type is detected by the ear as sound.
Power for the receiveris obtained from the power supply, which involves the powertransform-
er, the 5Y3 tube, and the electrolytic filter condenser. The transformer supplies filament
voltage for all tubes, and high AC voltage to the plates of the rectifier. The rectifier action is
exactly the same as that taking place in the diode section of the 12AV6, described above. The
output voltage is a series of positive pulses, one for each half of the 60 cycle line waveform.
The voltage applied to the tubes must be free of these pulses, or ripple, or only a load buzz will
be heard in the speaker. It is for this reason that the high capacity filter condenser is used.
The first section of the condenser charges to the voltage from the 5Y3 tube. When the tube is
not conducting, the condenser starts to discharge through the load presented by the other tubes
in the receiver. However, the next positive charge takes place before the condenser has time
to discharge fully, and so the voltage is smoothed out somewhat. Final filtering action takes
place in the second section of the filter condenser. This section is isolated from the first by
the 4?00O resistorwhich tends to help smooth the voltage because it resists current variations
in the filter circuit. The second section of the condenser smooths out any variations passed by
the first part of the condenser, and the voltage output is "pure" direct current (DC).
An automatic volume control (AVC) circuit is employed in the BR-2 receiver. Fading, blasting
and other undesirable effects are eliminated or substantially reduced by the action of this cir-
cuit. AVC operates on the principle of reduced tube gain when negative voltage or "bias" is
applied to the control grid. In this case, the negative voltage is obtained from the second de-
tector stage of the receiver. As previously mentioned, the diodes of the 12AV6 tube draw cur-
rent when a station is tuned in. The stronger the signal strength of the station, the more cur-
rent the diodes will conduct. This current develops a negative voltage across the volume con-
trol which is proportional to the current. An isolating network consisting of a 3.3 megohm re-
sistor and a.05 pfd condenser is used to filter out the audio voltage, and one end of the network
is connected to the 'rhottt side of the volume control. The grids of the 12BEO and 128.{6 RF and
IF tubes connect to the opposite side of the network. When a strong station is tuned, the nega-
tive voltage is relatively high, and so the gain of the RF and IF tubes will be low. If a weak
station is tuned, the voltage is lower, and the gain of the tubes higher. Therefore, strong and
weak stations will tend to maintain approximately the same output level. AIso, station fading is
reduced, due to the reduction in bias when the signal becomes weaker, etc.
The 12AV6 and 1246 audio amplifier stages operate at a "fixed" bias level. "Contact bias" is
used in the 12AV6. A very high value of resistance in the grid circuit of a tube will cause the
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