Tube Power Amp Reviews

Sidebar 1: Mikey's Praguenosis

When you see how vacuum tubes are made, you immediately understand why the transistor had to be invented: when done correctly, the process is expensive, time-consuming, labor-intensive, and messy. OSHA and the EPA would have big problems with it. I saw how it's done when Dr. Riccardo Kron (footbnote 1) and his wife, Eunice, invited me to the Czech Republic to visit the KR Enterprise facility, in Prague. I'm glad they did—had they just told me about it, I might not have believed them.

Located in a semi-abandoned building that was once part of the sprawling, government-owned Tesla High Vacuum Technology facility, KR Enterprise combines the high-tech knowledge of the Tesla company's former high-vacuum researchers, veteran audiophile Kron's electronic expertise and listening experience, military design engineers, and the Tesla facility's "low-tech/high-tech" high-vacuum equipment.

Though I didn't delve into the Krons' relationship with the Czech government, it's clear that the Swiss-funded company takes advantage of the Czech Republic's pro–private enterprise incentives, which helped it to get the manufacturing space, as well as high-vacuum production equipment once counted on by the Soviet military to supply its tube-driven weaponry.

Walking up the flight of stairs into the drab, dimly lit, curved corridors (must have been expensive to build), I immediately felt as if I'd walked onto the set of Terry Gilliam's Brazil. Anyone accustomed to the sterile modern environment of an American "industrial park" is in for a bit of culture shock.

KR Enterprise began as a tube-making venture in 1994. The amplifiers came a few years later. For a small company, KRE's line is extensive. The key to this is the high-quality, well-finished, "one size fits all" chassis used for all KRE amplifiers. The firm manufactures 300B-based, all-tube, single-ended triode monoblocks, push-pull hybrids like the VT8000 MK reviewed here, single-ended "vacuum transistor" monoblocks, and two stereo integrated amplifiers—all on the same basic chassis. Other designs are in the works, including the Kronzilla, which I watched being assembled (and which was demoed at HI-FI '99), but it was never made clear whether it had been designed as a commercial project or as a technological showcase.

So, yes, KRE makes amps, but the company was founded to build high-tech tubes, and that proved to be the most fascinating aspect of the business part of my visit. Kron and his high-vacuum experts began by setting out to build a better 300B, and by all accounts have done just that. (See Martin Colloms' review of the Cary 300SE LX20 monoblocks, which use KRE's 300BXS, in the August 1999 Stereophile.) The KRE 300B is electrically identical to the original Western Electric version, but outputs more than 18W in class-A. The BXS version outputs 25W.

Other triode output tubes in the KRE line include the 32B, 52B, and 52BX. KRE also developed and builds the KR1 and KR5 input and driver tubes, as well as updated versions of the 2A3 and PX25. All KRE tubes are hand-built (apparently not the case at many other tube facilities) and make use of high vacuums, both of which, the company claims, help ensure the perfect matching that Riccardo Kron claims is essential not just for push-pull designs, but for single-ended stereo pairs as well.

KRE's most noteworthy achievements are the development of the Vacuum Transistor and Vacuum Transducer, extreme–high-vacuum devices that are supposed to handle both high voltages and currents to produce high power outputs, and sound to match. I saw glass cylinders being heated, turned, and shaped into the familiar 300B shape. All of the dies used to stamp out the tiny internal parts are designed and created in-house—I watched one very patient individual hand-stamp hundreds of tiny parts.

In another section of the factory people sat at tables, hand-building the internal assemblies as if from kits. Components are "carbonized" by hand in a messy, complicated, labor-intensive process. The final assemblies are painstakingly inserted into the glass tubes, which have been silk-screened with identifying numbers and the KRE logo. The innards have to be sealed, and then the air evacuated, in a time-consuming process that begins with the individual tubes being opened and connected to glass tubing, and a slow-vacuum device that eventually achieves a 10^–9 Torr. vacuum. The tube is then sealed in yet another long, complex process. Before a tube can be sold, it must be tested for microphonics and meet other specs. Each KRE tube comes with a one-year warranty.

Riccardo Kron was born and raised in Milan, Italy, where he was surrounded by live music. Both his grandfather and father played the violin. Dr. Kron told me his grandfather was good enough to tour professionally, but his grandmother wouldn't let him. His father was a music professor and violin teacher.

Kron considering himself a "businessman, not a technician," and got his first degree in economics, but shortly thereafter earned a degree in electrical engineering as well. While always an audiophile and music lover, he spent most of his career working in Turin for Italy's largest manufacturer of television sets, tubes, components, and other electronics. So I asked him how he ended up in the Czech Republic, owner of a vacuum-tube manufacturing facility?

Riccardo Kron: I met this young Czech engineer, Rlescai Vacec, who was selling old tubes for old radios. These came from Tesla inventory and junked radios, but it was important for collectors from Italy because these tubes coming from the Czech Republic were fantastic for us.

Michael Fremer: Eventually you went into partnership with him?

Kron: Yes. The factory when we started was much smaller. We started it to reproduce the old radio tubes as spare parts...When I was a young guy, my dream was to make a very powerful low-frequency [audio] tube with constant emission, high vacuum, and high dynamics, so with one tube you could drive a single-ended amplifier and any kind of speaker.

Fremer: But first you had to build tubes people wanted, like the 300B.

Kron: Yes. The 300B was, for me, an error. It was much better for me to make a similar, much more powerful tube with a different name, and not be involved in the whole commercial mystique of the 300B.

Fremer: So you set about developing the vacuum transistor, which is kind of a contradiction in terms for most people.

Kron: Of course. Could be I made an error calling it that, but it was the first name in my mind because it was working as a transistor and as a tube...It works in voltage. The 300B is a triode. It has a filament or cathode, a grid, and a plate. Electrons are generated by the filament, then the grid between the filament and the plate accelerates the negatively charged electrons to go to the [positively charged] plate. In a normal tube this [process] is quiet and quick but not so quick, quiet and powerful. Think of it as an engine. It amplifies voltage so a very small signal can become bigger, from microvoltage to voltage. But power is current. Regular tubes can't produce high current. The transistor is the opposite: high current, low voltage.

Fremer: You decided to build a tube that could amplify high current at high voltage. It had never been done?

Kron: No, never been done. The secret is the filament.

Fremer: So while you call it a transistor, it is really a vacuum tube.

Kron: Yes, the high vacuum produces a constant emission, a very low grid current, and a longer life. Why? Because you have no gases inside. But this process needs at least 48 hours to create the vacuum. Then you have absolutely identical tubes, and they stay that way until you lose the vacuum, which could last forever...unless you break the glass!

---

Footnote 1: Dr. Kron was diagnosed with cancer in late 1999 and passed away in 2002.—John Atkinson