Medium output rectifier tubes originally used in early medium power Fender amps with tube rectifiers. Tubes imprinted exactly as depicted. Manufactured in Russia 2016. Audibly and operationally tested by Angela Instruments. We want the tubes to work in your amp so we test 'em in ours. Not just once, not just twice but THREE TIMES. Pair of two tubes is matched to exactly the same plate voltage (e.g. 383VDC) in a real guitar amp. If a tube fails in normal service within 12 months send it back to us and we'll replace it once. Why so cheap? We bought a big bunch of these from overstocked/downsizing Fender Groove Tubes 'bulk packed' in crates of 100 tubes. Each tube you purchase will be expertly boxed by us for safe shipment. Since 1977 Angela Instruments has shipped many thousands of rare and fragile items worldwide.