Replacment tubes for headphone and audio amplifiers utilizing 6N3 or 5670 vacuum tubes. The 6N3 tubes in this listing have been carefully tested and matched. Each tube is paired for mutual conductance using a Hickok 752 tube tester. Other quality checks include plate shorts/leakage and gas. Boxes are labeled with mutual conductance reading. All tubes test at NOS levels. 6N3 tubes have two internal plates, both of which are tested. The tubes are selected and matched in pairs with mutual conductance readings differing by no more than 250 micro-mho between plates and matched pairs. 6N3 vacuum tubes, as well as many other tube types, have been out of production for many years. The tubes in this listing are vintage, unused tubes. After testing many vintage 6N3 vacuum tubes from various production years in an assortment of headphone amps and amplifiers, we selected this batch of 6N3 tubes for their ability to improve sound quality. However, as with all tube based equipment, overall sound quality is ultimately determined by the circuit design and quality of the components used (transformers, chokes, resistors, etc) as well as the quality of other tubes, such as power tubes and rectifiers, that are utilized. Please note that the tubes will be in excellent unused condition; however, after decades of storage, some of the cardboard tube boxes will have faded or discolored - but are still in good to fair condition. All tubes are carefully packed for shipping.