General Information
Tubes used are as follows:
6D6 First Radio Frequency Amplifier
6D6 Second Radio Frequency Amplifier
6A7 Mixer and Oscillator (Single conversion)
6D6 First Intermediate Frequency Amplifier
6D6 Second Intermediate Frequency Amplifier
76 Detector
76 First Audio Amplifier
76 Second Audio Amplifier
42 X2 Push-Pull Driver Amplifier
45 X8 Parallel Push-Pull Power Amplifier
79 Electron Relay for Q Circuit
6D6 Shadowgraph Amplifier
85 Automatic Volume Control
6D6 A.V.C. Amplifier
5Z3 X2 Rectifiers for Power Amplifier
5Z3 Rectifier for remainder of receiver
Circuit Description
Radio Frequency Amplifier. This circuit uses a preselector and a tuned R.F.
Stage on the Green band, and a single tuned R.F. stage on the Orange
and Yellow bands. Two tuned R.F. stages are employed on the Red. Due to
the extreme difficulty of obtaining useable R.F. gain on frequencies above 18
megahertz, the antenna is coupled directly to the grid circuit of the Mixer
on the Blue band. Double sheilding is employed on the entire R.F.
section to prevent signal or noise pick-up by the wiring.
Mixer and Oscillator. A 6A7 tube is used as the Mixer and Oscillator.
The input grid of a 6A7 tube has a tendency to become positive
whenever a strong signal is impressed on it, if there is any appreciable
resistance in the grid circuit. The grid return side of the input circuit
is connected to one diode plate of the 85 A.V.C. tube so that if any time
the input grid of the 6A7 should become positive, the diode plate will allow
a low D.C. path to ground, preventing overload distortion.
Intermediate Frequency Amplifier. The circuit employed in the two stages of I.F.
amplification is conventional. The unusual feature of this portion of the receiver
is in the transformers which are so designed that the mechanical coupling ,
and, in turn, the bandwidth or selectivity may be varied continuously without
changing the natural period of either primary or secondary windings. This variation
of selectivity has no effect on the sensitivity of the receiver.
Delayed Automatic Volume Control. A pick-up winding is incorporated in the
third I.F. transformer which feeds the I.F. signal to the control grid of the 6D6 A.V.C.
amplifier. The output of this stage is coupled through an untuned transformer
to a diode plate of the 85 tube. The plate of the 85 is connected directly to
B plus and the control grid to a tap in the diode load resistor. This tube is
biased at approximately 10 volts which places a negative bias on the diode plate
and no A.V.C voltage is developed until a signal is tuned in of a strong enough
value to swing the diode plate positive. At this point A.V.C. voltage is developed,
which in turn makes the grid of the 85 negative and reduces the plate current
which reduces the bias and allows still more A.V.C. voltage to be developed.
This accumulative action allows excellent automatic control of the stronger
signals and eliminates the detrimental effects of A.V.C. on weak signals.
Q.A.V.C. A portion of the resistance load of the 85 diode is incorporated in a
potentiometer on the rear of the upper chassis. The arm of this potentiometer
is connected to the grids of the 79 tube. the two plates of the 79 are connected
in parallel and operate a magnetic relay which short-circuits the grids of the
push-pull 42 audio driver stage. When a signal is tuned in, the grids of the 79
tube become negative stopping the plate current and the relay opens, allowing
the audio system of the receiver to operate. The signal level at which this
occurs is determined by the setting of the potentiometer arm. A switch, operated
by a lever under the band switch knob on the front panel, is in series with
the 79 plate circuit and when opened makes the Q.A.V.C. circuit inoperative. The
Shadowmeter is connected in the plate circuit of a seperate 6D6 whose control
grid is controlled by the A.V.C. voltage. The amplification action of this tube
allows the Shadowmeter to operate on very weak stations.
Preamplifier and Audio Driver. The preamplifier consists of two cascade
resistance-coupled stages using 76 tubes. The value of the grid resistance on the
second stage may be varied by the switch in the upper right hand corner of the
control panel so as to achieve a 400% increase of power sensitivity for use on
Short Wave reception. It is possible to overload the power amplifier when
using this high gain. Two audio transformers of special design couple the
second audio stage to the push-pull driver stage, using two 42 tubes. The smaller
transformer only handles frequencies above 400 Hertz and has a rising high
characteristic. The large transformer handles frequencies below 400 Hertz and
is resonated at 30 Hertz. The voltage output of each of these transformers is
controlled by the Tone control.
The Power Output Stage. This consists of eight 45 tubes connected in parallel
push-pull. A much better balance is preserved in this stage by using several
medium size power tubes rather than a pair of high-power tubes. Also, the voltage
requirements are are greatly reduced. A 99 ohm resistor is incorporated in the
grid circuit of each 45 to prevent parasitic oscillation.
Power supply. There are two rectifying and filtering systems incoroprated in the
lower power amplifier chassis. One uses a single 5Z3 full-wave rectifier and
supplies plate current for the upper chassis and bias voltage for the output
stages only. Special electrolytic capacitors are used on both power supplies.
These capacitors will make a slight frying sound while the tubes are heating,
unlike the more common type of electrolytic capacitors. This is not an indication
of deterioration.
Reproducers. There are three Dynamic Reproducers used. The small one in the
center reproduces the higher register above 4000 Hertz. A filtering system is
used in conjunction with this speaker which prevents the lower frequencies from
being reproduced. This speaker may be made inoperative whenever desired by
means of the switch in the upper left-hand corner of the control panel. The two
large concert type dynamics handle all frequencies below 4000 Hertz. Two are
necessary to handle the 50 watts output of the power stage without distortion.
The leads and connections of all three speakers are color coded so as to insure
correct connections and proper phasing. These connections must not be reversed.
Tuning Ranges.
Green 520 - 1,500 KHZ
Orange 1,450 - 4,200 KHZ
Yellow 3,700 - 10,000 KHZ
Red 8,500 - 23,000 KHZ
Blue 18,000 - 45,000 KHZ
The high efficiency and unexcelled performance of this receiver
has been achieved by the careful selection of high quality of all
components. It is therefore most important that when service is
required only genuine Zenith parts and tubes be used.