Class A2 single-ended parallel amplifier circuit

The A2 class single-ended parallel amplifier is shown in the figure. In the A2 class amplification state, the cathode resistor is still used to provide a self-supplied gate negative voltage. In order to directly couple with the previous power driver, a 2kΩ resistor is connected in series between the output stage direct-heated filament circuit and the reference ground to increase the filament potential to 131V. In order to prevent the feedback formed by the inter-electrode capacitance in the RF range from generating RF parasitic oscillation, a small inductor is connected to the anode lead of 841.

Since Class A2 amplifier is an amplifier with gate current, the requirements for the driver stage output tube and gate circuit are the same as those for Class B amplifier.

In order to make the output tube grid have lower impedance, the front stage adopts cathode output circuit, that is, the pentode power tube 6F6 is connected into a triode as cathode output power amplifier as the power driving stage, and the front stage voltage amplifier uses pentode 6SJ7 for voltage amplification in order to obtain higher gain.

The balance of the static anode current of 841 can be adjusted by a 100Ω potentiometer connected in parallel between the two tube filaments. When the amplifier has no feedback, the output power is 9W and the distortion is 5.3%.