The W8EXI Wingfoot VFO Exciter
Final Amplifier Schematic Diagram and Circuit Description:

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General Information:
The final amplifier in the Wingfoot VFO Exciter uses a 2E26 beam power tube in Class C service. The amplifier operates straight through on all bands and is grid-block keyed. Grid leak bias is used for operating bias and a voltage regulator circuit is used to control the screen voltage on the tube. This allows the output to be smoothly varied from about 3 watts to 27 watts. Separate meters are used to simultaneously read the grid, screen, and plate currents.

Final Amplifier
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:

Final Amplifier Keying Input From CW Key and Keying Tube Screen Voltage From 12AU7 Screen Voltage Regulator Bias Output To 12AU7 Clamper Tube Grid RF Choke Plate Current Metering Resistor Screen Current Metering Resistor Grid Current Metering Resistor Plate Coupling Capacitor Plate Bypass Capacitor Plate RF Choke Plate Parasitic Suppressor 2E26 Tube Screen Bypass Capacitor Filament Bypass Capacitor Grid Leak Resistor Grid Parasitic Suppressor Input Capacitor

Or click on one of the links below:

Final Amplifier
 Input Coupling Capacitor
 Grid Parasitic Suppressor
 Grid RF Choke
 Grid Leak Resistor
 Filament Bypass Capacitor
 2E26 Tube
 Screen Bypass Capacitor
 Plate Parasitic Suppressor
 Plate Coupling Capacitor
 Plate RF Choke
 Plate Bypass Capacitor
 Grid. Screen, and Plate Metering Resistors
 Bias Output To Clamper Tube
 Screen Voltage Input From Screen Voltage Regulator
 Keying Input From CW Key

Final Amplifier:
Input Coupling Capacitor:
The input coupling capacitor allows the signal from the output of the 2nd buffer/multiplier to pass through to the input of the final amplifier, while preventing the DC bias on the tube from flowing backwards into the 2nd buffer/multiplier output. The value of this capacitor is not critical. 100pf is a commonly used value.

Input Capacitor

Grid Parasitic Suppressor:
The 2E26 tube is capable of operating up to 125MHz. To help suppress undesired parasitic oscillations the grid is loaded with a parasitic suppressor. The suppressor consists of 7 turns of #16 wire wound on a 22W resistor.

Grid Parasitic Suppressor

Grid RF Choke:
During key-down periods the grid RF choke allows the DC grid current to flow through without letting any RF through.

During key-up periods the grid block keying voltage flows through the RF choke to cut off the final amplifier tube.

Grid RF Choke

Grid Leak Resistor:
The input signal from the 2nd buffer/multiplier is strong enough to drive the grid of the final amplifier tube positive on the positive peaks of the signal. This causes the grid to rectify some of the driving signal. The grid current developed flows through the grid RF choke and the grid leak resistor, developing a voltage across the resistor. This voltage is used to provide the operating bias for the tube.

This method of biasing the tube is called grid leak bias and is an excellent way to bias a class C amplifier (but not a linear amplifier). As the input signal gets stronger, the grid current increases and the bias also increases, lowering the gain of the tube. Grid leak bias thus provides some measure of automatic gain control.

The down side of grid leak bias is that in the absence of a driving signal there is no bias on the tube and the plate current will rise to a very high value, damaging the tube. In the Wingfoot VFO Exciter a large negative voltage from the keying circuit is applied to the grid when no signal is present, cutting off the plate current and protecting the tube. A clamper tube is also used to limit the plate current in the unlikely event that drive is lost while the key is down.

Grid Leak Resistor

Filament Bypass Capacitor:
RF always tends to get into places that it shouldn't in a transmitter. To prevent any RF in the final amplifier from getting into the filament supply circuit, a capacitor is connected between the hot side of the filament and ground to short circuit any undesired RF to ground. The 60Hz AC filament voltage is far too low in frequency to pass through the capacitor and is blocked by the capacitor.

Filament Bypass Capacitor

2E26 Tube:
The 2E26 is a beam power tube like the 6L6, 807, and 6146. It is sometimes referred to as a "baby" 6146. However, since the 2E26 was released at least 4 years before the 6146, perhaps the 2E26 should be referred to as an "older brother" of the 6146.

The 2E26 is capable of 27 watts of output in class C at a frequency up to 125MHz. Like its bigger brother the 6146, the 2E26 is a very rugged tube.

You can click here for a 2E26 data sheet.

2E26 Tube

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.

Screen Bypass Capacitor

Plate Parasitic Suppressor:
The 2E26 tube is capable of operating up to 125MHz. To help suppress undesired parasitic oscillations the plate is loaded with a parasitic suppressor. The suppressor consists of 8 turns of #16 wire wound on a 100W resistor.

Plate Parasistic Suppressor

Plate Coupling Capacitor:
The plate coupling capacitor allows the RF on the plate of the tube to pass through to the final tank circuit while preventing the DC plate voltage from reaching the tank circuit. The value of this capacitor is not critical, but it must be able to withstand at least twice the DC plate voltage on the final amplifier tube. A value between 500pf and 1000pf is typically used.

Plate Coupling Capacitor

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 RF Choke

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.

Plate Bypass Capacitor

Grid, Screen, and Plate Metering Resistors:
One of the unusual features of the Wingfoot VFO Exciter is the use of current metering resistors throughout the transmitter. One each of these is included in the grid, screen, and plate leads of the final amplifier.

For most of the resistors, you must connect a voltmeter across the resistor and use Ohm's law to determine the current. This could be done for the final amplifier grid, screen, and plate currents. However, since these currents should always be monitored, permanent meters are connected across these resistors and mounted on a separate chassis that plugs into the main chassis. These meters have such a low resistance compared to the metering resistors the metering resistors have no effect when the meters are connected. However, if the external meters are disconnected, the current will flow through the metering resistors instead and the transmitter will still operate normally.

Metering Resistor

Bias Output To 12AU7 Clamper Tube:
The final amplifier in the Wingfoot VFO Exciter uses grid leak bias, and some method must be used to limit the plate current and protect the tube when there is no driving signal present. In normal operation, blocking bias is applied to the final amplifier tube during key-up periods to cut it off. However, in the unlikely event that the blocking bias is lost during key-up or that the drive is lost during key-down, there will be no bias on the tube and the plate current will rise to a dangerously high value.

To protect the final amplifier tube in these unlikely events, a clamper tube is used. The connection shown here feeds the grid bias (if present) on the final amplifier to the grid of the clamper tube, cutting off the clamper tube. The plate of the clamper tube is connected to the final amplifier screen grid regulator. When the clamper tube is cut off, it has essentially no effect. But if no bias is present on the final amplifier tube (and thus the clamper tube) the clamper tube turns on and lowers the voltage on the final amplifier screen grid, limiting the plate current to a safe value.

Feed To Clamper Tube Grid

Screen Voltage Input From 12AU7 Screen Voltage Regulator:
Screen grid voltage from an adjustable voltage regulator circuit is fed to the 2E26 at this point. By varying the screen voltage, the output of the amplifier can be smoothly varied from about 3 watts to 27 watts.

When operating at an output of 27 watts, the screen voltage varies less than 11% between key-up and key-down. This prevents keying spikes from occurring in the output stage as it is keyed, and improves the overall keying of the transmitter.

Screen Voltage Input

Keying Input From CW Key and Keying Tube:
The final amplifier is grid-block keyed. During key up, about -140V is applied to the grid of the amplifier tube from this connection, cutting off the tube. When the key is closed, this -140V is shorted to ground and the grid-blocking voltage is removed, turning on the final amplifier.

Keying Input

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