V&T News Visits KR

KR Enterprise is a small tube manufacturer in the Czech Republic, which has risen from the remains of the famous Czech company, Tesla, and specializes in directly-heated triodes for purely audio purposes. All KR tubes are new designs that use the latest technology and materials, and offer performance which they believe greatly surpasses that of the classical types. Manufacturers, such as Cary Audio, are taking advantage of these improved tube types in their new amplifier models.

The person behind KR tubes is Dr. Riccardo Kron, who was born in Italy of an Austrian mother and Hungarian father (a professional violinist). Music was an integral part of Riccardo's life from the very beginning. As a child, he had a keen interest in radios, a hobby that would lure him to the business decades later. In school, he worked with high-fidelity and sound installations in cinemas and theaters. Eventually Riccardo decided to pursue a degree in economics, but he never abandoned his first love of music and music reproduction. He worked in managerial positions in Italian electronics companies, so technology has been an integral part of his professional life.

In 1993, Dr. Kron invested some money and shared his engineering and economic expertise with Vaic Valve (VV), a company salvaged from the privatized remains of the former Czechoslovakian multi-branch conglomerate, Tesla. The VV tubes manufactured at that time had severe quality problems, and Dr. Kron completely redesigned several defective types. All production types were fitted with a serial number so that every tube could be traced back to the manufacturing lot, testing procedures, and results. This internal serial number is still a permanent feature of all KR tubes.

In 1994, the company was renamed KR Enterprise to distinguish it from the less glorious past. At that time, Dr. Kron persuaded some Swiss investors into a partnership. This additional funding enabled him to further develop his own tube technology and device types, leaning heavily on the expertise of his Tesla-trained high vacuum and tube specialist team. Three years later, after spending two million dollars on research and development, the gamble began to pay off. KR now has an outstanding set of proprietary tube models, which are all developed in-house, are designed specially for audio use, and meet the company's goal of consistent high quality.

Czech Craftsmanship

Prague has survived the turbulence of history almost damage free. It is one of the most beautiful cities in the world, and contains a lively cultural environment, with live music available every day. For a company that is dedicated to high-quality sound reproduction, this is not an insignificant issue-seasoned concertgoers know how live instruments should sound. Throughout history, Czech people have been known as skilled craftsmen, and technical accuracy and excellence have always been part of Czech national pride.

Of the former Soviet Bloc countries, Czech Republic has been one of the most successful in transforming its system to a market economy. The country has a strong industrial and technological background, dating back hundreds of years. The educational system has provided good general education for the population, and especially strong technical training for engineers.

The KR factory occupies a round building that once housed the tube research department of Tesla. Although it was badly maintained during the Communist regime, it is still eminently suitable for tube production, having gas piping installed throughout the premises, a convenience that is in very short supply in more modern buildings.

Hand-Made Tubes

There are no automatic machines on the factory floor. Instead, a number of in-house designed, hand-operated punches, presses, and cutters are used in the fabrication of the numerous small metal parts employed in the filament, grid, and plate assemblies. These ingenious tools guarantee tight tolerances and faultless assembly results.

Metal parts are cut and bent to tight tolerances, down to 0.01mm (0.4 mils). The plates are made from either pure nickel or titanium, depending on the tube type. Grids are wound with a special winding machine that, despite its age, can maintain tolerances down to a few micrometers.

As a tube's plate stays in high vacuum, the only way to effectively cool it is through radiation. Black surfaces are the most effective heat radiators (physics textbooks aptly use the phrase "blackbody radiation"). There is absolutely no convection, and conduction through the four feedthrough wires to the base pins is negligible.

KR uses a proprietary carbonizing process for the plate parts, giving them a deep matte black surface for maximum thermal radiation. With the heavy-duty construction, large plate area, and good thermal radiation characteristics, the tubes can dissipate as much as 120W of power without turning red or deforming.

The filament, grid, and plate assemblies are designed so that once put together, the filament is automatically and accurately centered inside the grid. With a special tool, the centering can still be adjusted in a 0.05mm range.

The plate assembly is constructed around a glass stem with feedthrough wires for the filament, grid, and plate conductors, and a small-diameter glass tube that is later used for evacuation and sealing. The glass stem, with the metal leads going through the glass, is one of the most critical parts of a tube because it must maintain the extremely tight seal between the two different materials under widely varying temperature conditions. This requires exactly correct materials for the feedthrough parts.

Glass-Blowing Artist at Work

Tube envelope forming begins with laboratory-grade glass pipes, about one inch in diameter. Using an adjustable gas flame, the industrial glass artist first melts the end of the pipe, squeezes the soft edges with large tweezers, and with a twist closes the end of tube. The separated remains in the tweezers fly to the waste bin with one smooth movement, while the other hand already heats the closed end of the tube until it forms into a large "test tube" shape. A few blows into the tube's open end finishes off the shape of the closing hemisphere. All this is done with bare hands, even though the temperature of melted glass, just one foot from the skilled hands, is over 600°C (1100°F)!

Next, the pipes are cut to proper lengths, the excess parts again to be formed into closed tubes, which may be used directly as vessels for KR52s, since they have domed tops. For other types, the blanks are inserted into a special lathe, where the peculiar shapes for other tube types are formed with graphite tools, while a gas flame is heating the slowly rotating tube. For KR302B, the glass top is flattened, and for the others the characteristic "Coke bottle"-shape envelope is created in the glass-turning lathe.

In the final assembly phase, the inner section of the tube, readily assembled around the glass stem, is inserted inside the glass vessel. The glass seal between the stem and the vessel is welded closed. As with everything else, this happens one by one in a small machine that slowly rotates the assembled tube in front of a flame that is slowly heating the outer tube. At the right moment, the worker using the welding machine activates a graphite-ended stick that pinches the softened outer tube against the inner stem and joins both glass parts. At the same time, the rest of the outer tube drops off the machine. A larger hand-held flame finalizes the seam. Now the tube is ready for evacuation.

High Vacuum

The tubes to be evacuated are hand-welded to the glass pipe stubs in the vacuum pump. The craftsman performing this delicate operation uses a rubber hose equipped with a mouthpiece to blow air through the joined parts to keep the inner part of the pipe open. The joint must be hermetic and, at the same time, support the weight of the tube during the evacuation.

The first evacuation step is testing the whole system's sealing. The air pressure inside of the system-the pump and attached vacuum tubes-is reduced by a short pumping sequence. The pressure gauge is then monitored for a few minutes. If the pressure does not remain stable, there is a leak in a welded joint. The leaky joint is located by spraying ethyl alcohol on the seams, which will momentarily stop the leak, a situation easily identified on the gauge. The leaky seam is then rewelded and the test redone until the pumping system is tight.

Evacuating the tubes down to 10^-9 torr (mm of mercury) takes about ten hours. After the evacuation is finished, the tubes containing high vacuum are sealed permanently. This is done by melting the small tube that connects the stem to the evacuation machine by a small gas flame. As the glass softens, the external air pressure pinches the tube shut, sealing it entirely. At the same time the tube is disconnected from the evacuation machine.

At the end of the evacuation process, the getter is heated by a hand-held RF heater. The getter material flashes and condenses on the close-by glass surface as a thin, shiny surface, absorbing any residual gas atoms.

Quality Above All

According to Dr. Kron, the exceptionally high vacuum is one of the key factors in ensuring consistent performance and long life of KR tubes, all of which have a vacuum level of 10^-8 to 10^-9 torr. With this high vacuum, the characteristics of the tube will remain essentially constant over its entire life. The grid current, a good indication of vacuum level, is guaranteed to be below 3mA in all types.

Another special feature is the patented ribbon filament structure that uses quadrangular filament wire. Together with eight filament segments, this improves the emission characteristics of the cathode and ensures constant emission over a wide range of filament supply voltage. Most KR tubes are specified for a wide filament voltage range from 3-5V, yet they show stable characteristics almost unaffected by the filament voltage.

Between the tight tolerances used in the assembly of the tubes, constant emission guaranteed by the special filament construction and consistently high vacuum, KR tubes are exceptionally well matched without any special matching process; the allowed factory tolerance for parameter variations is 3%. The factory gives a one-year warranty for all tubes, provided they are used within specifications.

No Expansion Plans

Tube manufacturing capacity is about 5000 tubes annually. This is a mere fraction of the volume produced by major tube factories in Russia or China, so KR will never be a serious threat to the large tube plants.

Complete lack of automation suggests that the volume could be raised by incorporating new machinery and increasing production personnel, while maintaining the notion of high-quality and fully hand-made tubes. However, the people at KR are not at all enthusiastic about stepping up production numbers. They are convinced this would have an adverse effect on quality, the one thing that the factory absolutely does not wish to jeopardize, due to bitter experience from the past. When queried about expansion plans, Dr. Kron stated, "Frankly, we don't care! It is OK to be a small Rolls Royce of tubes, we do not want to be a Toyota."

Unique Tube Designs

The archetype of an SE amp uses the classic 300B tubes and typically achieves 7W output power. Any directly-heated triode will inevitably be judged by the compatibility with the 300B in this type of amp. Because NOS 300Bs are very expensive, there is considerable demand for devices that are plug-in compatible with the original 300B.

This is the niche market that KR Enterprise started with. But Dr. Kron did not wish to simply re-create a blueprint copy of the classic 300B to meet exact compatibility with tubes that are of ancient design and technological level. Instead, KR300Bs and KR300BXLSs are designed as replacements for the original 300B, but their performance is not limited by the original specs. In computer jargon, these tubes are "downward compatible" with the originals.

All KR tube models are modern designs, using space-age materials such as titanium and proprietary processes. The rugged mechanical structure allows increased plate dissipation. If the circuit allows, the KR versions offer the prospect for increased output power. However, this demands that the bias current of the amplifier must be adjustable. The KR300B is able to produce up to 18W, and the special version KR300BXLS over 25W.

Although mildly criticized as not being true replacements for the 300B-for both electrical and mechanical reasons-KR believe their tubes have a definite edge over other brands because the dynamic range, due to increased power capabilities, greatly exceed the original types, even when the bias current is kept intact.

The other KR types - KR302B, KR32B, and KR52B -have no equivalents, nor are there any traditional amplifier types where they could be used to their full performance potential. All are pin-compatible with the 300B, and may be used as near-equivalent replacements, but their real benefits can be utilized only with circuitry that is specially designed for them. Single-ended power outputs up to 40W are possible with these tubes.

Introducing the Vacuum Transducer

The topology of Vacuum Transducer amplifiers has been painstakingly optimized for best possible performance. The DHT PP amp is a rare bird, but when operated in Class A, it can purportedly offer all the definition, soundstage, and ambience of an SE amp combined with the ample power capabilities of a PP output stage. Dr. Kron keeps a tight lip when discussing the details of these amps, therefore, I could not extract more details of the technical solutions used in them. But he was very generous in auditions. We listened extensively to two types: the top-of-the-line model VT 8000 MK, and the smaller VT 6000 MK.

The 8000 is the lifetime dream of Dr. Kron; it includes everything he regards essential in a high-end amplifier. It incorporates a very wide, forward-reaching and three-dimensional soundstage, and is obviously suitable for large listening rooms only. The smaller 6000 has a less forward soundstage, thus being more suitable for small rooms.

Custom-Designed Tubes

This deal has allowed KR to find an excellent solution for the lack of tailor-fit amplifier applications for its products. Respectively, Cary Audio has established a supply of tubes with unique characteristics and high-quality manufacturing potential for other application-specific tube models in the future.

KR Enterprise was so pleased with this arrangement, that it agreed to discontinue its own previous lines of preamplifiers and single-ended amplifiers, however, KR will continue the unique Vacuum Transducer line of products, as this in no way competes with Cary products.

During the factory tour, I saw some astonishing prototypes of the newbies-multi-deck designs with two triodes on top of each other, the sections parallel connected. Type T-1610, a double-decker over 45cm (3¢) high, has appropriately been nicknamed "Kronzilla". Fortunately, these special tubes are not exclusively for Cary Audio. KR offers them to all purchasers who wish to obtain more power from glass tubes. Traditionally, larger plate dissipations have been available only in ceramic transmitting tubes, normally used in radio transmitters.

All KR tubes are sold through distributors. Welborne Labs is the exclusive distributor for KR tubes in North America. KR Enterprise does not have a website, but Welborne Lab's KR home page (see Sources) is the best source for quick access to KR product range and technical data.

Expect New Amp Designs

KR tubes can certainly be regarded as rare and unusual devices-hand built from carefully selected materials and with advanced production techniques, aiming for the best possible quality. They are expensive, but if properly used, KR claims they should serve the user for at least a decade, maybe more. For many, these features justify the grand title "Rolls Royce of tubes."

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