VTL 225W Deluxe monoblock power amplifier KT90 Measurements

Looking next at the 225's behavior with the KT90 output tubes (a different amplifier sample), there were a few differences compared to the EL34 version. I'll note these differences when they are significant.

Output impedance with the KT90s was virtually identical, measuring 0.95 ohms at 20Hz and 1kHz, decreasing to 0.87 ohms at 20kHz. Input impedance, though still fairly high, was much lower than the EL34'd 225's input impedance of 129k ohms at 74.5k ohms. Input sensitivity was 1.63V, about half a volt more being required than the EL34 version to be driven to clipping. Since the two amplifiers have nearly the same power output, this means the KT90's gain is lower, requiring a higher input voltage to drive it to clipping. This was indeed the case, the voltage gain into an 8 ohm load measuring 27.4dB—a full 3dB lower than the EL34 version. 27dB of voltage gain is typical of most power amplifiers.

The KT90'd 225 is polarity-correct (non-inverting), and unweighted S/N ratio was an excellent 74.7dB referenced to 1W into 8 ohms, more than 6dB higher than the EL34'd 225.



Fig.8 plots the KT90'd 225's distortion vs power output into 8 ohms (lower curve), 4 ohms (middle curve), and 2 ohms. Again, 3% THD was chosen as the clipping point due to the amplifier's high intrinsic distortion below clipping when driving low impedances. Maximum output power was nearly identical to the EL34 version, clipping at 185W into 8 ohms (22.7dBW), 234W into 4 ohms (20.7dBW), and 251W into 2 ohms (18dBW). THD+noise vs frequency is shown in fig.9. The curves represent, from bottom to top, 1W into 8 ohms, 2W into 4 ohms, and 4W into 2 ohms. The KT90's distortion levels are about the same as the EL34's through the midband, but have a greater rise in the high frequencies. At 10kHz, the KT90's distortion was about double that of the EL34-equipped 225 into any impedance.


The KT90's distortion products when reproducing a 1kHz sinewave at 2W into 4 ohms are shown in the lower trace of fig.10. The distortion is primarily second-harmonic, in contrast to the nearly pure third-harmonic product produced by the EL34'd 225. Both types of distortion are relatively benign, however, since they both occur naturally in music.



Fig.11 shows a low-level 10kHz squarewave. The overshoot is more damped, looking substantially better than the EL34's squarewave (fig.5). Frequency responses at 1W into 8 ohms and 2W into 4 ohms, shown in fig.12, have fairly steep rolloffs above 20kHz compared with the EL34s, which had a rising curve above 20kHz.


Looking at fig.13, a spectral analysis of the 225's harmonic distortion products when reproducing a 50Hz sinewave at 72W into 4 ohms, we can see a fairly high level of distortion. Comparing fig.13 with fig.6 (the EL34's distortion spectrum), the KT90 can be seen to have about the same level of third-harmonic as the EL34, but also a much higher level of second-harmonic distortion at 100Hz. In addition, the KT90 has many more clearly identifiable upper-order harmonics than the EL34. The fourth harmonic is higher in level and the fifth is about the same, but the KT90 has more upper-order harmonic products, seen as spikes rising from the FFT-analysis noise floor.


Similarly, the KT90's intermodulation products when reproducing a mix of 19kHz and 20kHz at 44V p–p (31W RMS) were higher than the EL34's (fig.14). The 1kHz component is a full 7dB better suppressed in the EL34'd 225, the KT90 version's lying 47dB down (about 0.5%) and the 18kHz and 21kHz products are about 6dB lower with the EL34-equipped amplifier.

Overall, these measurements are typical of tube amplifiers: moderately high output impedance, current-limited into low impedances, and fairly high distortion levels. While the distortion was higher than solid-state amplifiers, the products were mostly benign low-order harmonics. The measurements indicate that an audition with one's loudspeakers is recommended—a conclusion reinforced by my listening impressions over the very different Magnepan 2.6 and Hales System Two Signature loudspeakers.

The EL34 version had generally lower levels of harmonic and intermodulation distortion than the KT90. This is, however, no indication of how the two amplifiers reproduce music—both performed well in the listening room.—Robert Harley

Vacuum Tube Logic
4774 Murrietta, Suites 9&10
Chino, CA 91710
(909) 627-5944