A pair (2 tubes) of closely matched military-grade GE JAN 5670W vacuum tubes. The GE 5670W provides a premium replacement for 2C51 and other 5670 tubes. The 5670 tube can also generally be used as an upgrade for 6N3 tubes used in headphone amps and other audio preamps and amplifiers (note that the glass envelope of the GE 5670W tube is approximtely 5 mm shorter than 6N3 tubes). The "W" designation in the tube number refers to ruggedized, military grade, tubes; better quality control and internal design changes to enhance reliability. Each tube is paired for mutual conductance using a Hickok 752 tube tester. Other quality checks include shorts/leakage and gas tests. All tubes test at NOS (new old stock) levels of 5000 micro-mho or higher. 5670 tubes have two plates; the mutual conductance readings between the two plates will differ by 500 micro-mho or less, and by 500 micro-mho or less between the two tubes in a matched set. Boxes are labeled with measured mutual conductance readings. These vintage, mint condition tubes, which have a 1986 manufacture date, have never been used. The tubes came from sealed/unopened military surplus boxes. We reject any tubes that do not meet our standards. Tubes that show even the slightest abnormality in the quality checks, or have lower Gm values, are simply rejected. Only the best are selected for these matched pairs. All tubes are carefully packaged for shipping.