Coincident Speaker Technology Frankenstein II Tube Monoblock Amplifier

It’s Alive!

Equipment report
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Tubed power amplifiers
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Coincident Speaker Technology Frankenstein II
Coincident Speaker Technology Frankenstein II Tube Monoblock Amplifier

In 1935, at about the same time Western Electric released the 300A power triode, RCA was putting the finishing touches on a new device dubbed the beam power tube. In a real sense, the 6L6 and its progeny became the 300B’s commercial nemesis. And even though the 300B stayed in continuous production for 50 years, it had little impact on the hi-fi scene through tube audio’s golden decades of the 50s and 60s. The problem then, as it is now, is that most audio gear is designed by engineers who are first and foremost concerned with specifications. A beam power tube push-pull output stage with sufficient global feedback looks really good on paper, at least into a resistive load, in output power, efficiency, and THD. Add distributed loading in the form of an Ultra-Linear connection and the resultant linearity exceeds even that offered by a push-pull triode output stage. In those days power amplifiers competed on the basis of specifications; most notably, power delivery and THD were assumed to be the final words in sound quality.

Even in 1960, the single-ended triode amplifier (SET) seemed like an anachronistic relic of the past. It was left to Japanese audiophiles in the 1970s to re-ignite interest in the SET and high- efficiency speakers. The SET phenomenon spread in the 1980s from East to West, and the past 20 years have seen a multitude of various SET designs. Their claim to fame is the first watt. That is the notion that the first watt sets the stage for a high-efficiency speaker and that a single-ended amplifier, the king of Class A, delivers the best-sounding first watt. Surely, the next 99 watts hardly matter if that first watt is compromised.

AT&T, the parent company of Western Electric, ceased production of the 300B in the fall of 1988. Since much of the SET resurgence centered around the 300B, production of 300B tube-types has continued, presently ongoing in China, Russia, and the Slovak Republic, and much of it in boutique fashion. The Frankenstein II's stock 300B tube is the Shuguang Treasure Black Bottle 300B-Z. In addition to a Western Electric 300B reissue, which is a faithful copy of the original, I was also fortunate to have on hand samples of the Sophia Electric Princess mesh-plate and carbon-plate 300B tubes. As you might have suspected, not all 300B types sound alike. Differences in grid and plate structure typically give rise to perceptible sonic characters. My objective was to sample a cross-section of the available 300B population with the end goal of identifying types that work best in the Frankenstein. Choice of operating point and output transformer primary impedance greatly influence sonic performance. Therefore, note the caveat that just because a particular tube sounds best in this context does not imply universal superiority. It would be foolhardy to generalize on the basis of a single output stage.

The name of these amps is either a case of inspired marketing or simply represents a moment of geek. I presumed that it referred metaphorically to the act of bringing the music back to life. But when I asked Coincident’s designer, Israel Blume, about it, this is what he had to say: “When I set out to design and create the Frankenstein, I thoroughly analyzed and studied every existing 300B amplifier typology as well as other DHTs [direct- heated triodes] like 2A3 and 45. In building countless prototypes and auditioning existing units, it became apparent that no one design contained all the necessary requirements for building the finest SET amplifier ever made. There were specific elements of one particular design that were desirable and other elements of other circuits that fit the mold. The end result became what I termed analogous to Dr. Frankenstein’s creation.”

Mr. Blume’s design goal was nothing less than “creating the purest, most transparent amplifier with the least electronic sound of its own, yet still capable of adequately driving sensibly designed loudspeakers.” Toward that end, he says, “the circuit had to be a model of simplicity but not too simple; only the finest state-of-the-art components could be used; circuit layout and build-quality had to be based on not only hardwiring but as much lead-to-lead construction as possible; and the circuit had to consist of an input/driver tube, 300B output, and 5U4 rectifier.” The input stage is built around a 6EM7 dual triode, which once upon a time was used in TV sets as a vertical-deflection oscillator. It combines high-gain and low-gain triodes in a single glass envelope. It has over the past few years been deployed as a stand-alone two-watt headphone amplifier because of its high-current-drive capability. In the Frankenstein, the high-gain triode is used as a voltage amplifier, while the low-gain/high- current triode is configured as a cathode follower and drives an interstage transformer (IT), a double-C-core type using 6N copper windings. Of course, the IT represents added cost, but Blume is a firm believer in eliminating coupling capacitors. Unusually, there are no capacitors in the signal path—only a single resistor. The IT drives the 300B in single-ended mode. The output transformer is also a double-C-core type, 6N-copper- wound. Both the IT and output transformers are wound in Japan to Coincident’s specs. The 5U4 rectifier is followed by a choke input filter. No electrolytics here! All power-supply storage caps are polypropylene types. The power transformer is said to be operating at 30% capacity, which accounts for why these amps run cool to the touch and the absence of mechanical noise. Every detail, says Blume, no matter how seemingly small, had to be addressed in keeping with the philosophy that everything matters. For example, all internal wiring is 6N copper with air and Teflon dielectrics. Even the binding posts use solid quarter-inch posts of 6N copper.

The Frankenstein is operated quite conservatively with a 300B plate dissipation of about 25 watts. That’s only about 63% of the maximum plate dissipation. The cooler a tube runs the greater its longevity, and with the current cost of primo 300B tubes that’s a good thing. The original Western Electric 300B was rated for an average life expectancy of 40,000 hours when operated within published specifications. It’s not clear what life expectancy you’ll get from the various electrically-equivalent types on the market. According to Blume, “The reliability record of the Frankensteins is a testament to the build-quality and overall design. Since we have switched to the Black Bottle 300B Treasure tube, we have not experienced a single failure in over two years.”