Matched pair of vintage 6K4P-EV (6К4П-ЕВ) tubes from manufacturer NEVZ, Novosibirsk (RU). Tested and measured with the L3-3 tube tester and matched in pairs. Long life pentodes with enhanced mechanical strength (not less than 5000 operating hours, when running under prescribed conditions). These tubes are equivalents of the EF93, 6BA6, 6F31 and also the chinese 6K4. If you are an owner of the FX-AUDIO TUBE-03 or similar preamplifiers using the chinese 6K4 tubes, definitely do not hesitate to try them out. The 6K4P-EV are improving the sound outcome really remarkably. New old stock, directly from the factory packing. Year of production: 1977.