In the Wingfoot VFO Exciter the output of the oscillator is fed to a cathode follower stage. The cathode follower isolates the oscillator from later stages in the transmitter. The cathode follower is not keyed so it represents a constant load on the oscillator, improving the keying of the transmitter.
A cathode follower is also known as a grounded plate amplifier, since the plate is kept at ground potential for RF. Input is fed to the grid, and output is taken from the cathode. The cathode follower has negative voltage gain: the output voltage is less than the input voltage. However, it does exhibit current and power gain. The cathode follower has a high input impedance but a low output impedance.
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:
|Plate Bypass Capacitor|
|Output Coupling Capacitor|
The grid resistor serves as a load for the previous stage, and allows the grid bias developed across the cathode resistor to reach the grid of the tube.
The cathode resistor creates cathode bias for the tube, and also serves as a load resistor. The output to the next stage is taken from across this resistor.
A 6J5 was used for the cathode follower in the Wingfoot VFO. The gain of the 6J5 is about the same as the popular 12AU7A miniature twin triode. You can click here for a 6J5 data sheet.
Plate Bypass Capacitor:
In a cathode follower, the plate is kept grounded for RF. The plate bypass capacitor allows RF on the plate of the tube to flow through to ground, but blocks the flow of the DC plate voltage. The value of this capacitor is not critical.
Output Coupling Capacitor:
The output of the cathode follower is taken from across the cathode resistor. The cathode resistor also generates the grid bias for the tube. To prevent the grid bias from flowing through to the next stage, and to prevent the grid bias of the following stage from appearing across the cathode resistor, a coupling capacitor is used. The capacitor allows RF to pass through, while blocking DC.
Cathode Follower Metering Resistor:
One of the unusual features of the Wingfoot VFO Exciter is the use of current metering resistors throughout the transmitter. One of these is included in the plate lead of the cathode follower. By connecting a voltmeter across the resistor and using Ohm's law, the cathode follower current can be determined.
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