Our Platinum Grade vacuum tubes are the most thoroughly tested 6N3P-E tubes available. To determine stability, tubes are subjected to multiple test cycles and are matched within 6% for mutual conductance (Gm) and within 5% for tube gain (final testing of these tubes is performed under auto-bias at a fixed plate current ip = 8.8 mA). All matched tubes are auditioned in an amplifier to determine sound quality. FEWER THAN 35% of the 6N3P-E tubes we test meet our high standards for audio use and are chosen for Platinum Grade pairs - we select tubes that show stability during multiple test cycles and exhibit low noise and excellent audio sound during final testing in an audio amplifier. You will not find a more rigorously tested pair of vacuum tubes anywhere. The 6N3P-E tube is the high reliability version ("E" suffix in the tube code) and is a great upgrade for Chinese 6N3 tubes. ***Please Note: We carefully inspect all tubes we use and to the best of our knowledge the 6N3P-E tubes we use in our Platinum Grade sets are NOS tubes. We also guarantee that the tubes will test at NOS levels when received.*** To protect the tubes during shipping and storage, the tubes are supplied in Riverstone Audio's custom "tube cradle" boxes. As a US seller we have confidence in the quality of what we sell. We back these vintage vacuum tubes with a no-nonsense THREE-YEAR money-back warranty - the best warranty in the audio tube business. ***6N3P-E tubes should only be used as a direct replacement for Chinese 6N3 and Russian 6N3P tubes but are generally suitable as a substitute for US/European 5670 / 5670W / 396A / 2C51 tubes (this should be first verified by the amp manufacturer). PLEASE NOTE: AMPLIFIER DAMAGE CAN OCCUR IF YOU SUBSTITUTE THE WRONG TYPE OF TUBE. Before ordering CAREFULLY determine the type of tube used in your amplifier (this information is typically printed or etched on the tube).