The THD+noise vs frequency curves are shown in figs.4 and 5. The latter compares the unbalanced triode and tetrode configurations, which are relatively similar. The changes in the distortion curves from the unbalanced to the balanced configurations are insignificant—smaller than the differences in fig.5—and are not shown here. Fig.6 shows the distortion waveform into 4 ohms; it is primarily second harmonic. The waveform in the tetrode configuration, very similar, is not shown. The same is true with 8 ohm and 2 ohm loads (also not shown), though the latter shows clearer signs of higher harmonics.
VTL MB-750, triode mode, THD+noise vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms; 2W into 4 ohms; 2.83V into simulated speaker load; and 1W into 8 ohms.
VTL MB-75 THD+noise vs frequency at 2W into 4 ohms (from top to bottom at 10kHz): triode mode, tetrode mode.
VTL MB-750, triode mode, 1kHz waveform at 2W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).