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:
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. |
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. |
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. |
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 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. |
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. |
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. |
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. |
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. |
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