Audio by Van Alstine Ultravalve Power Amplifier & Salk Sound SongTower QWT Loudspeaker (TAS 204)

A Marriage Made in Audio Heaven

This turned out to be a twofer review—two for the price of one—shortly after I accepted the Ultravalve assignment and considered compatible loudspeakers. Speakers are a critical decision that can make or break an amplifier review. It seems to me that many reviewers simply drop an amp into their existing system and allow it to sink or swim. That’s equivalent to playing Russian roulette with the outcome. A more elegant approach is to investigate load compatibility by using several loudspeakers, giving the amp under test several chances to shine. In fact, the DALI Helicon 400 Mk2 ($7000/pr.) and the JAS Audio Oscar ($3450/pr.) turned out to be perfectly happy with the Ultravalve. However, a secondary criterion for speaker selection in this case was cost. It would only make sense to evaluate a budget amp with speakers that were not too far apart cost-wise, as that would represent the most likely real-world scenario. The ideal solution, it appeared, was to duplicate the coupling that our esteemed Editor-in-Chief, Robert Harley, was mightily impressed with during the 2009 Rocky Mountain Audio Fest. And that’s how Salk’s SongTower entered the picture. I’m glad it did, as this turned out to be a hugely synergistic coupling, a marriage made in audio heaven.

According to Frank Van Alstine, the Ultravalve’s lineage is traceable to the Dynaco Stereo 70, which he denotes as its “great grandmother.” The ST-70 has got to be the most successful basic power amplifier of all time with estimated sales in excess of 300,000 units. This is the amp that in the 70s steered me clear of solid-state designs and cemented my lifelong love affair with tube amplification. In fact, I still own a beautiful Will Vincent ST-70 rebuild. Frank started taking a close engineering look at the ST-70 in the late 70s, the result being an upgrade kit that achieved reduced distortion and improved stability. Power-supply design improvements followed later, including a separate high-voltage regulated power supply for each plate of each small-signal tube, and these were incorporated into a new board design. The end result was the Ultimate 70 amp-rebuild kit, which is said to be the “mother” of the Ultravalve amplifier.

In essence, the Ultravalve represents the distillation of over 30 years of design experience. It is intended as a thoroughly modern and rationally priced vacuum tube amplifier. A new polished-steel chassis is sourced from Dynakitparts, while the double-anodized, bead-blast-finished gold faceplate is sourced from a local vendor. The basic Dyna internal layout has been maintained, as it was felt that it works very well. However, all active electronics are located internally. Coupling and feedback circuitry use a new double-sided ground plane PC board. The power bandwidth is said to have been widened to -3dB at 50kHz and 5Hz while still maintaining the goal of no feedback-related overload under any condition of use. The basic signal path and tube choices remain unchanged from the original. A single 6GH8A pentode-triode tube is used per channel. It is similar in performance (but not as far as pinout) to the 7199 used by Dynaco prior to the ST-70 Series II. The switch was necessitated when the supply of 7199s dried up. The pentode section provides plenty of voltage gain while the triode is deployed as a cathodyne phase-inverter. The power supply is tube-rectified via a single 5AR4. The push-pull output stage uses a pair of 6CA7/EL34s in ultralinear connection. My sample was outfitted with Electro Harmonix 6CA7EH beam power tetrodes. Of course, EL34 power pentodes may be used as well. The whole thing, says Frank, except for some active and passive electronic parts, is made in the U.S.A.

Individual bias pots are provided for each channel to set the bias. You’ll need an inexpensive digital multimeter in order to measure and tweak the voltage to the specified 1.6VDC, which corresponds to a combined 100mA quiescent current flow for each pair of tubes. The amp is plug ’n’ play right out of the box; tubes are already installed and the bias is preset at the factory. The bias was right on after a few minutes of warmup. However, the bias does drift a bit after about 20 hours of break-in and should be rechecked at that time.

The SongTower is advertised as a mass-loaded quarter-wave transmission line. I will concede that the internal cabinet volume is stretched to form a tall column, but its length is not nearly sufficient to approach that required by a classic transmission line, possessing a lowest quarter-wave pipe-resonance of only 80Hz. Its bass performance appears to be equivalent to that of a conventional bass-reflex design with a nominal box-tuning frequency of about 37Hz. Current production uses a pair of SEAS ER-15RLY five-inch cone mid/woofers featuring edge-coated-reed/paper-pulp cones, a large magnet system, and a long copper-clad aluminum voice coil. These woofers recently replaced the CA-15RLY and are a bit more expensive, but are said to measure a tad better. The tweeter is the Danish Hiquphon OWII ¾" soft dome, which features multiple coatings, excellent frequency linearity, and exponential horn loading to improve phase response. By the way, the O and W in the tweeter model name are the initials of Hiquphon’s owner: Oskar Wroending who is personally responsible for all research and development.