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Before he toured me through the KR Audio factory's tube-manufacturing facility, chief engineer Marek Gencev provided a brief lesson on the history of the audio tube and an explanation of how KR creates its own tubes.

"The vacuum tube is simple," Gencev began. He explained that Thomas Alva Edison invented it in 1879, using a glass-enclosed bamboo filament heated by an electric current. The filament heated up and produced light -- the first light bulb. However, it was John Ambrose Fleming who developed the vacuum tube further by inventing the diode in 1904, capturing the electrons and photons emitted by the filament on a plate (anode) that rectified AC current to DC, thus making amplification possible. Finally, in 1906 Lee de Forest invented the Audion tube, the first triode, adding a grid between the filament (the cathode) and anode, moderating the amount of current that reached the anode plate. These created the basis of all tubes that came later.

Lee de ForestLee de Forest with Audion tubes

"We are only following," Gencev said in his clipped yet charming Czech manner. "What is new? Only materials." In place of the classic metals, KR uses mica-oxide supports, pure-nickel cathodes and anodes, gold-plated molybdenum for the grids, and oxide-coated nickel filaments (except for the M-O replica R valve, which uses tungsten).

845 tube

At KR there are no modern machines -- they work with artisanal skills and a lot of tube-making experience. "Only knowledge and old machines produce these tubes," Gencev said, handing me an 845 triode. I felt its serious heft.

"No one is teaching vacuum tube," Gencev explained. KR learns from studying old designs, first testing and measuring old-stock tubes for their electrical characteristics and dynamic parameters. It's also important to find out their design geometry, noting the physical dimensions of each tube under scrutiny, its spacings, size of grid, etc. When data sheets and a working tube aren't available, Gencev cuts into an old tube, opening it up and analyzing it for reverse engineering. "It's basic science," Gencev pointed out. "We make calculations to determine gain qualities."

tubes and torch

At KR there are roughly ten steps to the tube-manufacturing process: 1) mechanical parts manufacture and assembly, wherein various metal parts are fabricated and then assembled together into the cathode, anode, and grid of each tube (making up the electrode); 2) glasswork, wherein tube envelopes are cut, heated, and shaped into their characteristic bottle profiles and the glass stems made with their lead-in wires; 3) tube assembly, where the glass stems are connected to the inner electrode system and then put together with their appropriate glass envelopes; 4) tube exhaustion, where the air inside each tube is evacuated so a vacuum forms within the tube; 5) inductive heating, when the anodes of each tube are heated via a coil that surrounds the bottles, burning off impurities; 6) inductive heating of the stem area to draw ion particles permanently onto the glass -- this creates the characteristic silvery "flashing" on the tubes; 7) cathode activation, when the barium compound that coats the filament is super-heated so the cathode can produce active electrons; 8) closing the tube via melting the thin, glass pipe connector to each tube; 9) gluing each tube to a base and burning it in; and 10) testing, matching pairs, and storage for final shipping. These steps take place in five different rooms of the factory: a mechanical room, a glassworks room, a vacuum room, a chemical room, and a testing/packaging room. The entire process takes about 120 hours to make a single tube out of roughly 128 separate pieces.

Garrett Hongo
Contributor, The SoundStage! Network