A plot showing the output spectrum resulting from a 50Hz input is shown in fig.6 (the power level was 99.4W into 4 ohms). The artifacts are relatively high (-47.4dB or about 0.4% at 100Hz, and -36.7dB or about 1.5% at 150Hz). A similar spectral plot in fig.7 shows the IM at the output resulting from a 19+20kHz input at 70.3W into 4 ohms (the highest output possible with this signal prior to visible clipping). The 1kHz IM distortion here is relatively high (46.2dB or 0.5% at 1kHz, and -36dB or about 1.5% at 18kHz). The corresponding spectral responses in the triode mode, and also into our simulated real load, differed slightly in detail from those shown, but were essentially very similar.

Fig.6 VTL MB 175, tetrode mode, spectrum of 50Hz sinewave, DC-1kHz, at 99.4W into 4 ohms (linear frequency scale).

Fig.7 VTL MB 175, tetrode mode, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 70.3W into 4 ohms (linear frequency scale).

The THD+noise vs output power sweeps for the MB 175 (at 1kHz) are shown in figs.8 and 9. The discrete clipping levels are given in Table 1. Note that these are given for 3% THD+noise at 1kHz, instead of our standard 1%. While the amplifier just exceeds its power spec in triode mode, it is about 0.6dBW short in tetrode mode.

Fig.8 VTL MB 175, tetrode mode, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

Fig.9 VTL MB 175, triode mode, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

	Table 1 VTL MB 175 Discrete Clipping levels (3% THD+noise at 1kHz)
	Tetrode W (dBW)	Triode W (dBW)
8	108.8 (20.4)	88.6 (19.5)
(line)	123V	118V
4	149.8 (18.8)	91 (16.6)
(line)	123V	118V
2	57 (11.6)	68 (12.3)
(line)	123V	118V

While the S/N measurements of the VTL MB 175 are a little high, its other measurements are about average for a tube amplifier. Not remarkable, perhaps, but respectable.—Thomas J. Norton