Using Darlington amplifier

The circuit principle is shown in Figure 2-3. ov, rlr and VT2 form a two-tube direct-coupled input amplifier stage.
Local feedback is used from the emitter of VT2 to the base of VTr to stabilize its DC operating point.
    VT3 is the emitter output device. Used as impedance transformation to achieve matching with the push stage and provide larger
operating current to the push stage.
    VT4 is the output driving stage, which uses high-power tube 3DD15 to output larger driving power.
    VTs and VT6 are Darlington complementary power output stages.
    Rf (22kfl) and RF (1.2kfl) form a feedback loop and determine the closed-loop gain of the circuit. Av=22/1.2=
1. 03, which is 25dBo