OTL power amplifier circuit in transistor radio a

Shown is the OTL power amplifier circuit commonly used in transistor radios . VT5 and the bias resistor form the excitation stage. VT6 and VT7 are a pair of complementary symmetrical tubes. VT6 is a germanium material NPN transistor, and VT7 is a germanium material PNP transistor. VT6, VT7 and bias resistors form a typical OTL amplifier circuit.     To illustrate the operation of the circuit, the bias resistor and excitation resistor are omitted from the circle.