The AA8V 6AG7 Amplifier
by Greg Latta, AA8V
Schematic Diagrams and Circuit Descriptions:

6AG7 Schematic - Small

6AG7 Amplifier Pages:
  6AG7 Amplifier - Main Page and Exterior Photos  Tank Coil Construction Details
 Interior Photos of the Finished Amplifier  Schematic Diagrams and Circuit Descriptions
 Construction Photos  Testing And Preliminary Work
 Typical Operating Conditions  Why Use A 6AG7?


Schematic Diagrams and Circuit Descriptions:
Input and Bias Circuits:
Input Coupling Capacitor
Grid Resistor
Cathode Bias Resistor
Cathode Bypass Capacitor
Key Click Capacitor
Keying Circuit
Output Circuits:
Plate Feed Circuit
Plate Tank Circuit
Antenna RF Choke
6AG7 Tube and Screen Circuit:
6AG7 Vacuum Tube
Screen Dropping Resistor
Screen Bypass Capacitor
Power Supply Circuit:
Primary Circuit
Power Supply Transformer
Power Supply Rectifier
Power Supply Filter
6AG7 Screen Voltage Regulator Circuit:
Voltage Regulator Tubes
Regulator Dropping Resistor
Complete Schematics:
Medium Resolution Schematic
Full Resolution Schematic
Rotated Full Resolution Schematic for Printing


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

Schematic Diagram Tube and Screen Circuit Screen Voltage Regulator Power Supply Output Circuits

Click here for a high resolution schematic.

Click here for a rotated schematic more suitable for printing.

Input and Bias Circuits

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

Input Schematic Key Click Capacitor 6AG7 Tube Keying Circuit Cathode Bypass Capacitor Cathode Bias Resistor Grid Resistor Input Coupling Capacitor

Input Coupling Capacitor:
The input coupling capacitor allows the input signal to pass through to the grid of the tube while preventing the input source from potentially shorting the grid bias to ground. If the grid bias were shorted out, the 6AG7 would overheat and it would be destroyed. The value of this capacitor is not critical at all. As long as the reactance of the capacitor at the lowest operating frequency (in this case 4.5 ohms at 3.5 MHz) is small compared to the value of the grid resistor, the circuit will operate correctly.

Input Coupling Capacitor
Input Coupling Capacitor


Grid Resistor:
The grid resistor allows the grid bias developed by the cathode resistor to pass through to the grid, while providing a load for the input signal. The value of this resistor is not critical. As long as it is high enough to prevent too much loading of the source, while still allowing the grid bias through, the circuit will operate fine.

Grid Resistor
Grid Resistor


Cathode Bias Resistor:
The value of the cathode bias resistor is critical. Cathode current (sum of plate and screen currents) passing through this resistor causes a voltage drop across the resistor. This voltage drop is the bias applied to the grid of the tube.

The bias is critical. In this amplifier the bias is chosen so that the amplifier requires the least amount of drive possible. This means making the bias as small as possible, while preventing the tube from overheating. The various currents and voltages in the tube were measured while the value of this resistor was varied. 180 ohms was finally chosen as this kept the plate dissipation safely under the maximum value of 9 watts.

Cathode Resistor
Cathode Resistor


Cathode Bypass Capacitor:
While the cathode bias resistor generates the tube bias, it also offers opposition to the signal that ultimately flows to the plate (the output signal). This causes negative feedback, which, in this case, is undesirable. The solution is to connect a capacitor from the cathode to ground. This allows the output signal to flow around the cathode resistor, without affecting the bias developed by the resistor. The value of this capacitor is not critical. As long as the reactance of the capacitor at the lowest operating frequency (in this case 4.5 ohms at 3.5 MHz) is small compared to the value of the bias resistor, the circuit will be fine.

Cathode Bypass Capacitor
Cathode Bypass Capacitor


Key Click Capacitor:
When an amplifier is keyed, spurious signals known as key clicks can be generated if the amplifier is turned on and off too rapidly. To prevent this in the 6AG7 amplifier, a 10uf/100V capacitor is connected between the cathode of the tube and ground. This lengthens the rise and fall times of the amplifier when it is keyed and prevents the key clicks. The value of this capacitor is critical. Too little capacitance, and the key clicks occur. Too much, and the keying gets soft and "rings".

Key Click Capacitor
Key Click Capacitor


Keying Circuit:
The amplifier is keyed by opening the cathode return to ground. A bypass capacitor is connected across the key to prevent any stray RF that may enter through the keying jack from affecting the amplifier. The value of the capacitor is not critical.

Keying Circuit
Keying Circuit


6AG7 Tube and Screen Circuit:

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

Tube and Screen Circuit Screen Dropping Resistor Screen Bypass Capacitor 6AG7 Tube

6AG7 Vacuum Tube:
The 6AG7 is a power pentode designed for class A service. It is a metal tube with a maximum plate dissipation of 9W. Is is identical electrically to the miniature 6CL6 tube which was released in May of 1953, but it has a slightly higher plate dissipation than the 6CL6. The higher plate dissipation allows for a slightly higher output than the 6CL6, given the same amount of drive.

You can click here for a copy of the 6AG7 data sheet in .pdf format.

6AG7 Tube
6AG7 Tube


Screen Dropping Resistor:
The screen voltage for the 6AG7 is fairly critical and is obtained from the screen voltage regulator through a dropping resistor. The resistor is chosen so that the screen voltage is the proper value when the tube is drawing rated current. The value of this resistor is fairly critical, since the screen voltage has a big effect on the operation of the amplifier.

Screen Dropping Resistor
Screen Dropping Resistor


Screen Bypass Capacitor:
The screen voltage must be kept fairly constant. To keep any RF from passing through the screen resistor and affecting the screen voltage, the RF is bypassed to ground through a capacitor. The value of this capacitor is not critical.

Screen Bypass Capacitor
Screen Bypass Capacitor



Screen Voltage Regulator Circuit:

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

{short description of image} Voltage Regulator Tube Dropping Resistor

Voltage Regulator Tubes:
The screen voltage in the amplifier is regulated to prevent key clicks from occurring when the amplifier is keyed. Gaseous regulator tubes such as the 0B2 have the property that as long as the current through them is between about 4 mA and 30 mA the voltage across the tube is constant. For the 0B2, the voltage is 108 volts. Tubes may be placed in series to obtain a higher voltage. In this case, two tubes are placed in series for a total of 216 volts.

Voltage Regulator Tube
Voltage Regulator Tube


Regulator Dropping Resistor:
The screen dropping resistor is selected so that the current through the tubes is at least equal to the screen current plus about 4 mA. In this case that corresponds to about 15 mA. Having the idle current higher is OK, as long as it isn't over 30 mA. (Higher idling current does, however, cause more heating in the regulator tubes and power supply.) When the amplifier is keyed, the regulator tubes simply draw less current, automatically keeping the voltage across them constant at 216 volts.

Regulator Dropping Resistor
Regulator Dropping Resistor


Output Circuits:

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

6AG7 Output Schematic Plate Feed Circuit Plate Tank Circuit Antenna RF Choke

Plate Feed Circuit:
In an RF amplifier it is necessary to supply DC plate voltage to the tube (about 350 volts in this case) and at the same time extract the amplified RF that appears at the plate of the tube. In the circuit at right, the 2.5 mH plate RF choke allows the direct current from the plate supply (B+) to pass through it, while preventing the RF on the plate of the tube from flowing back through the plate supply. At the same time, the .0047uf plate coupling capacitor permits the RF on the plate to flow though to the plate tank circuit while blocking the plate voltage. The .01uf plate bypass capacitor short circuits any residual RF that might have gotten through the plate RF choke and prevents it from reaching the B+ plate supply.

Plate Feed Circuit
Plate Feed Circuit


Plate Tank Circuit:
The plate tank circuit is a pi-network that matches the high impedance of the plate to the low impedance of the antenna. At the same time the circuit filters out undesired harmonics from the output signal. The signal from the plate enters through the 0.0047 uf plate coupling capacitor. The 100 pf plate tuning capacitor tunes the plate to resonance. The band switch and tank coil provide the correct inductance for each band, and the 625 pf load capacitor adjusts the network for the best impedance match.

L1 is a plug in coil. Two coils cover 80m-20m. One coil covers the 80m/60m/40m bands and another coil covers the 30m/20m bands.

Pi Network
Plate Tank Circuit


Antenna RF Choke:
The 2.5 mH antenna RF choke performs three important functions:
1. If the plate coupling capacitor should fail and short, the RF choke will short circuit the plate supply, blowing the power supply fuse. This will prevent the plate voltage from appearing on the antenna, a very dangerous situation.
2. The choke prevents any DC voltage from appearing across the load capacitor, lowering the voltage it is required to handle.
3. The choke allows any static buildup in the antenna to drain to ground.

Antenna RF Choke
Antenna RF Choke


Power Supply:

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

Power Supply Schematic Transformer Power Supply Filter Power Supply Rectifier and Transformer Primary Circuit

Primary Circuit:
A three wire grounded cord is used on the amplifier. The hot lead from the cord passes through a one ampere fuse that will blow in case of a catastrophic failure somewhere in the amplifier. A single pole switch switch is placed in the hot lead to act as an On/Off switch. A neon pilot light is connected across the transformer primary to indicate when the unit is powered. (not shown on schematic).

Power Supply Primary
Power Supply Primary Circuit


Power Supply Transformer:
The power supply transformer is a Stancor P-6010. The transformer has three secondaries. One provides 5V AC @ 2A for the 5Y3 rectifier filament, and another (rated at 2A) provides the 6.3V AC @ 0.65A needed by the 6AG7 filament. A third high voltage winding provides 650V center tapped at 40mA for the plate supply.

Power Supply Transformer
Power Supply Transformer Rectifier


Power Supply Rectifier:
The transformer steps up the 117 volts AC in the primary to 650 volts AC in the secondary. When the top connection to the secondary is positive with respect to the bottom connection and center tap, the upper diode of the 5Y3GT rectifier tube (pins 4 and 2/8) conducts, allowing current to pass to the output. During this time the bottom diode (pins 6 and 2/8) is cut off. When the bottom of the secondary is positive with respect to the top and center tap the opposite is true: the upper diode is cut off and the bottom diode conducts. The result is a stream of DC pulses at the output of the rectifier, which is the center tap on the 5V secondary. The output is then fed to the power supply filter.. The 5 volt secondary provides the 5 volts needed to heat the filament/cathode of the rectifier tube.

You can click here for a copy of the 5Y3GT data sheet in .pdf format.

Power Supply Rectifier
Power Supply Rectifier


Power Supply Filter:
The output from the rectifier is direct current (DC) but with a large alternating current (AC) component superimposed. The power supply filter removes the AC component leaving the pure DC. The two 47uf filter capacitors are essentially an open circuit to the DC, but they effectively short circuit the AC component to ground. The 8H filter choke on the other hand allows the DC component to flow through, while offering a very high impedance to the AC component. The result is that little of the AC component reaches the output. The 120k ohm bleeder resistor bleeds off the charge on the filter capacitors when the unit is shut off, preventing the possibility of a dangerous electric shock that could occur even though the unit were turned off and unplugged. The bleeder resistor, along with the screen voltage regulator tubes, also provides a minimum load on the power supply to prevent the output voltage from soaring during standby (key up) periods.

Power Supply Filter
Power Supply Filter


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