You may see these listed as BD139-10 or BD139-16. This is similar to the gain grouping denoted by the C suffix on the BC549. The -16 suffix indicates the higher gain grouping. You can use either in this circuit. Alternatives that will definitely work in this circuit are BD137, BD135. This is the NPN transistor in the class B push-pull output driver stage. When active on the positive audio swing, Q5 "pulls" the voltage on the speaker upwards by allowing current to flow from emitter to base in the direction of the arrow.
Navigate Up
Henry's email address:
Recent Edit History
19-APR-2026: canonicalised, direct refs