As can be seen from the circuit diagram we are really using five transistors as the BD679 and BD680 are "Darlington" pairs of transistors. The led must be a green led for correct operation. (The voltage across a green led is higher than the voltage across a red led.) When using a 12v supply the voltage at T₁ collector should be (about) 5v and at T₂ base (about) 7v. This amplifier will be unstable (it will oscillate) without the 4k7 variable resistor connected to the input.