Click here for a higher resolution (larger) schematic.
Introduction:
The mixer takes signals that have been amplified by the
RF amplifier and converts
them to an intermediate frequency band by combining them with a signal from the
local oscillator.
This process is known as superheterodyning.
With the band switch in the 30m position, signals from 10,100 kHz to 10,150
kHz are converted to 6,954 kHz to 7,004 kHz.
With the bandswitch in the 20m position, signals from 14,000 kHz to 14,350 kHz
are converted to 3,760 kHz to 4,110 kHz.
Mixer
Circuit
Click On Any Part Of The Picture Below For
Information On That Part Of The Circuit
Click On Any Part Of The Picture Above For
Information On That Part Of The Circuit
| B+ Supply: B+ from the power supply comes in on this line and runs to the screen grid and plate of the 6GH8 mixer. B+ is nominally 122V. |
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| Input Network: Signals from the RF amplifier enter the mixer via a 4 turn primary link on L2. A parallel resonant circuit consisting of MIX control C2 and the secondary of L2 pick up the signal and feed it to the grid of the mixer tube. The resonant circuit can be tuned to either 30m or 20m by the MIX control on the front panel. |
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| Local Oscillator Coupling Capacitor: The local oscillator is coupled to the gird of the mixer through a 10pf coupling capacitor. The capacitor allows the RF from the local oscillator to pass through while blocking the DC voltage on the plate of the local oscillator. |
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| Cathode Bias Resistor: The cathode bias resistor is used to bias the mixer. Current flowing through the resistor develops a voltage drop that makes the cathode positive with respect to ground. Since the grid of the tube is connected to ground through the input network, the grid is biased negative with respect to the cathode. |
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| Cathode Bypass Capacitor: The cathode must be kept at ground potential for RF, otherwise the gain of the tube will be reduced due to negative feedback. The cathode bypass capacitor shunts RF around the cathode bias resistor keeping the cathode at ground potential for RF. |
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| 6GH8 Vacuum Tube: The 6GH8 was originally designed for use in the horizontal deflection circuitry of television receivers. Previous experience with it has shown that it is ideal in this application. The triode is used as the local oscillator, and the pentode is used as the mixer. The two sections are internally shielded from each other. Click here for a 6GH8A data sheet . The 6U8 can be used in this application as well. It is a direct plug in replacement. No circuit modifications are needed. The 6GH8 may provide slightly better gain and improved resistance from pulling on strong signals, but the 6U8 should still work fine. Click here for a .6U8A data sheet. |
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| Plate RF Choke: The output of the converter is untuned because the input of the receiver that follows the converter can perform this function. Instead of a tuned circuit at the mixer output, an RF choke allows the plate voltage from the power supply to reach the mixer plate while preventing any RF from getting back to the power supply. The screen/plate bypass capacitor guarantees that the bottom of RFC2 is at RF ground. |
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| Output Coupling Capacitor: The output coupling capacitor permits the RF output of the mixer to pass through to the output of the converter while blocking the DC voltage on the plate. |
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