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With just a single output transistor, it needs to run class A, much like a single-ended tube amp, hence the large heatsink. Transistors were expensive back then, but it saved having an unreliable vibrator power supply, and an output transformer. Low power tubes could run off 12V, but not audio outputs. Similarly, in the 60's many makers of consoles put in transistor output stages , even if the rest of the unit still ran on tubes. Saved a lot of iron, and without output tube heaters, they saved on the power transformer too.
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