depending on which model you get, the half power switch does different things. On the 100 watt, it disables two of the 4 power tubes but operates in the same power mode. In the 50 watt, it switches from pentode (50) to triode (25). This changes the tone and feel more than the volume. Pentode = big, bold, punchy. Triode = smoother, less bottom and top end. Bluesy stuff in triode with alnico speaker.... ah, nice!