The oscillator signal is somewhat reduced after passing through the cathode follower, so it must be amplified. The frequency must also be multiplied for use on the higher bands. The first buffer/multiplier uses a 6AG7 with cathode bias. The bias is set so that the amplifier does not operate as a linear amplifier. Instead, the amplifier distorts the signal, creating higher harmonics in addition to the fundamental frequency. The desired harmonic (or the fundamental) can then be selected with a tuned circuit and sent on to the next stage. The first buffer/multiplier is keyed along with the oscillator, second buffer/multiplier, and final amplifier.
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:
|Cathode Bias Resistor|
|Cathode Bypass Capacitor|
|Screen Bypass Capacitor|
|Plate RF Choke|
|Plate Bypass Capacitor|
|Plate Coupling Capacitor|
|Plate Tank Coils|
|80m Padding Capacitor|
The grid resistor serves three purposes:
1. It provides a load for the previous stage.
2. It allows the cathode bias to reach the grid of the tube when the key is down.
3. It allows blocking bias to reach the grid of the tube when the key is up.
Cathode Bias Resistor:
As current flows through the cathode bias resistor it develops a voltage drop which is used to provide bias for the tube. This resistor is chosen so that when the tube is driven it is sometimes completely cutoff. This "chops" the waveform and generates the harmonics that are needed for multiplication. The voltage across the resistor is smoothed out by the cathode bypass capacitor.
Cathode Bypass Capacitor:
The voltage developed across the cathode bias resistor will fluctuate with the output signal, giving negative feedback, unless it is smoothed out by the cathode bypass capacitor. Another way to look at it is that the cathode bypass capacitor shunts any RF around the cathode bias resistor while preventing the flow of any direct current through the capacitor.
Since the 6AG7 used in the oscillator also makes a great amplifier, it was used again in the 1st buffer/multiplier. Using the same tube in several places is a good idea since it makes maintenance easier. The best way to check a tube is to replace it with a known good tube. If the same tube is used in several circuits, only one new tube need be kept in stock for testing purposes.
You can click here for a 6AG7 data sheet.
Screen Bypass Capacitor:
It is important that no RF be present on the screen grid of the tube. The screen bypass capacitor shunts any RF on the screen to ground while preventing the DC voltage on the screen from being shorted to ground.
Plate RF Choke:
The RF output is taken from the plate of the tube. The plate RF choke prevents the RF on the plate from flowing back to the plate supply, while allowing the DC voltage from the plate supply to reach the tube. The value of this choke is not critical. 2.5mH is a commonly available value, often used in this application.
Plate Bypass Capacitor:
The plate bypass capacitor affords an extra measure of protection by shunting any RF that might have leaked through the plate RF choke to ground. The combination of the plate bypass capacitor and the plate RF choke insures that the RF doesn't get someplace it doesn't belong.
1st Buffer 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 1st buffer/multiplier. By connecting a voltmeter across the resistor and using Ohm's law, the 1st buffer/multiplier current can be determined.
Plate Coupling Capacitor:
The plate coupling capacitor allows the RF on the plate of the tube to pass through to the plate tank circuit while preventing the DC plate voltage from reaching the tank circuit. The value of this capacitor is not critical. Any value of a few hundred pf is fine, as long as the capacitor can handle the DC plate voltage on the tube.
The bandswitch selects the plate tank coil to be used in the output of the 1st buffer/multiplier. Note that the same coil is used on the 40m and 15m bands and the 20m and 10m bands.
Plate Tank Coils:
The plate tank coil selected by the bandswitch resonates with the output capacitance of the tube at the desired frequency, except on the 80m band, where extra capacitance is needed. The plate tank coils are wound on high quality ceramic coil forms and are adjustable.
The plate tank resonates on the following frequencies:
1. 80m - 3.5MHz. - Straight Through
2. 40m and 15m - 7.0MHz - Doubler
3. 30m - 10.1 MHz - Tripler
4. 20m and 10m - 14.0MHz - Quadrupler
Click here for a complete listing of the internal frequency arrangement of the Wingfoot VFO Exciter.
80m Padding Capacitor:
The output capacitance of the tube alone is not sufficient on the 80m band unless an excessively large number of turns are used on the coil. To keep the coil size reasonable, an an extra capacitor is connected across the coil to achieve resonance.
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