Manley Laboratories 250 Neo-Classic monoblock power amplifier:
Measurements part 2
The distortion in either mode is heavily second-harmonic (fig.7, tetrode; fig.8, triode), and the increase at low frequencies and at higher currents seems to be due to an increase in the third harmonic (figs.9 and 10).

Fig.7 Manley 250 Neo-Classic, tetrode mode, 1kHz waveform at 16.3W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.8 Manley 250 Neo-Classic, triode mode, 1kHz waveform at 17W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.9 Manley 250 Neo-Classic, tetrode mode, spectrum of 50Hz sinewave, DC-1kHz, at 175W into 4 ohms (linear frequency scale).

Fig.10 Manley 250 Neo-Classic, triode mode, spectrum of 50Hz sinewave, DC-22kHz, at 76W into 4 ohms (linear frequency scale).

The poor ultrasonic linearity implied by the distortion-vs-frequency graphs results in quite high levels of intermodulation distortion, particularly in tetrode mode. Fig.11 shows the spectrum of the amplifier's output while it reproduced an equal mix of 19kHz and 20kHz tones at 33W into 8 ohms. The higher-frequency intermodulation products all lie below -60dB (0.1%), but the 1kHz difference-frequency component lies at -47dB (0.5%), even though the amplifier is nowhere near clipping at this output power. Halving the load impedance doesn't affect the level of the 1kHz component, but does increase the level of the 18kHz and 21kHz products by almost 10dB (not shown).

Fig.11 Manley 250 Neo-Classic, tetrode mode, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 33W into 8 ohms (linear frequency scale).

Figs.12 and 13 show how the Manley's distortion and noise percentage changes as the output power increases in tetrode and triode modes. In triode, the amplifier appears to more than meet its 100W/5 ohms specification at the usual 1% THD+noise clipping point. It actually puts out 132W into 8 ohms (21.1dBW) and 165W into 4 ohms (19.2dBW). In tetrode mode, however, the clipping point has to be relaxed to almost 3% THD+N for the amplifier to put out 250W into 4 ohms (24dBW). Into 8 ohms, 230W was available at the 3% clipping point (23.6dBW), and 218W at 1% THD (23.4dBW). Bearing in mind my earlier comments about the amplifier's low sensitivity in balanced mode, it took 4V or more of input drive to reach these output levels.

Fig.12 Manley 250 Neo-Classic, tetrode mode, distortion (%) vs continuous output power into (from bottom to top at 1W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.

Fig.13 Manley 250 Neo-Classic, triode mode, distortion (%) vs continuous output power into (from bottom to top at 1W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.

With its eight EL34 output tubes, the Manley 250 Neo-Classic is indeed a powerhouse, and it doesn't seem to sacrifice much of that power when those tubes run in the smoother-sounding triode mode. However, I was bothered by the poor linearity at the top of the audioband, which resulted in the higher-than-normal level of intermodulation products with the admittedly demanding mix of 19kHz and 20kHz tones I use to assess this aspect of an amplifier's performance. Nevertheless, it's fair to point put that PB wasn't bothered by subjective problems that could be laid at the feet of this fault.—John Atkinson