Note 1) Open Filaments means, the filaments and the grid are longer than the Anode, and stick out on both ends for about 5...8mm. The not fully warm ends of the filaments do not have a very good function, and cause distortion. This tube doesn't use this part of the filaments. Emitted electrons from the "cold" ends, will bypass the anode, and land on the glass. At maximum Power this may cause fluorescent, blue glow on the tube glass inside. Mesh tubes may develop a fluorcescent blue glow om the Plate. See also our Tech Brief on Blue Glow in Tubes.
Note 2) Hard-metal filaments are operated at much higher temperature than normal tubes. This tube will glow bright orange, instead of the dark orange color of nickel filament tubes.
Note 3) Individual Test data, such as: Matching Data, Grid Current, Vacuum, Filament Current, etc, are on the Certificate that comes with each individual Tube. Each tube is numbered, inside the glass with a metal Tag
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