Balanced Audio Technology VK-5 preamplifier & VK-60/75 power amplifiers The BAT VK-75 and VK-75SE power amplifiers, July 2001

Sidebar 5: The BAT VK-75 and VK-75SE power amplifiers, July 2001 (Vol.24 No.7)

The original VK-60 was already an excellent power amplifier when it was introduced in 1995, and BAT has continued to improve it in a variety of ways technical and sonic (see my Follow-Up in the April 2000 issue). The VK-75 continues on this evolutionary path, with a re-engineering of the power supply, power-supply transformer, and circuit design. It's still a fully balanced, all-triode, bridged single-ended amplifier, now with power increased from 60Wpc to 75Wpc. And the price is up, from $4950 (unchanged since the VK-60's introduction) to $6000.

The VK-75SE's main claim to fame is its incorporation of the 6H30 SuperTube, also used in the SE versions of the VK-50 preamplifier and VK-D5 CD player. According to BAT's Victor Khomenko, the 6H30 provides the closest approximation to the "dream" amplification device, with technical specifications unsurpassed by any similar tube. Made in Russia by Sovtek, the 6H30 was until 1998 restricted to Russian military use, and export was banned. With the easing of East-West tensions, the 6H30 became available for export, and BAT obtained the entire stock of 11,000 tubes (footnote 1).

The 6H30 is used in the first stage of the VK-75SE, which has a front-end circuit driven by a dedicated power supply. Both the VK-75 and the VK-75SE sport new, thicker black faceplates, the VK-75SE having a stylish machined top plate covering the front part of the chassis, with openings for the tubes. The VK-75SE costs $8500.

I'm a great believer in making equipment comparisons by starting with the familiar and changing one component at a time. Review logistics don't always allow this, but in evaluating the new BAT amplifiers I was fortunate in still having on hand the VK-60 sample that I'd evaluated for a Follow-Up review. My first comparison was between the VK-60 and the VK-75; I followed this by comparing the VK-75 to the VK-75SE (footnote 2). I first did this using the digital front-end (PS Audio Lambda II transport, Perpetual Technologies P-1A/P-3A) and preamplifier (Convergent Audio Technology SL-1 Ultimate) that I was very familiar with, and then, one by one, adding the BAT VK-40 and the VK-D5SE to the mix, periodically switching back to the more familiar components for reference. The BAT amps have a robust, battleship-like construction that suggests immunity to the sonic effects of vibration; however, the VK-60, VK-75, and VK-75SE all benefited from being placed on Aurios MIB component supports; when they were, the sound became more airy and open.

My initial impression of the VK-75 was that any differences between it and the VK-60 were subtle at best. But, of course, the VK-60 had been well broken-in, whereas the VK-75 was brand new. As I continued to listen, and as the VK-75 was breaking in over a period of a few weeks, some differences began to emerge, but nothing that I would describe as dramatic. The VK-75 seemed to have a little less midbass warmth, and was thus more neutral throughout this part of the range. It also had slightly lower levels of noise, both mechanical and electrical, the latter difference highlighted by my +100dB-sensitive Avantgarde Unos. In other respects, the amplifiers sounded very similar, both possessing that smooth, easy-on-the-ears quality that characterizes the best tube amplifiers.

The differences in the sounds of the VK-75 and the VK-75SE were more marked, the VK-75SE reaching a substantially greater level of neutrality and musicality. The midrange and mid-treble picked up additional smoothness and liquidity, and there were improvements in dynamics and bass definition.

Although the VK-75SE costs a non-trivial $2500 more than the VK-75, I encourage anyone considering the VK-75 to try to stretch their amplifier budget to the VK-75SE level. Later-series VK-60 amplifiers can be factory-modified to SE status ($2500), incorporating the 6H30 tube. (The VK-60 cannot be updated to the VK-75.) Based on the differences I heard between the VK-75 and the VK-75SE, I would think this provides a very worthwhile upgrade path for VK-60 owners. The VK-75/75SE are also available as bridged monoblocks with double the power, and designated the VK-150/150SE. With these new amps, BAT is clearly staying at the top of the tube amplifier game.—Robert Deutsch

Footnote 1: The 6H30 is now in production again, although, according to Khomenko, the current production tubes are similar, not identical, to the old. Audio Research is in the process of revamping their line to incorporate the 6H30, and I've talked to at least one other well-known designer of tube amplifiers who is evaluating this tube.—Robert Deutsch

Footnote 2: My time with the VK-75/75SE was not entirely trouble-free. When I first installed the tubes, it was apparent that one of the big 6C33C-B output tubes had suffered some internal damage in shipping: there was a chip of glass rattling around inside, and, when I installed the tube in the VK-75, the LED bias light corresponding to that tube did not come on. Fortunately, BAT had sent along a matched pair of spares, so I used these instead. Then, after I'd finished all my review-related listening to the amps, as I was just listening to music—I do that once in a while—there was a loud thump through the right speaker, followed by loud hum in the same channel, and one of the bias LEDs went out. (The amp was the VK-75SE.) I switched tubes between left and right, but the same LED stayed dark, and the tube from the "bad" channel worked fine in the other channel, so it seemed that the problem was not with the tube.

I shipped the amplifier back to BAT; they checked it out and reported that the fuse for that output tube had blown (there are fuses for each tube, not just each channel), presumably due to a fault in the tube. As to why the tube worked fine afterwards, Victor Khomenko suggested that there may have been a weak spot in the tube's construction, which produced a spark that led to the fuse blowing, and that same spark "healed" the tube.—Robert Deutsch