Medium output rectifier tubes originally used in early medium power Fender amps with tube rectifiers. This rugged tube is a favorite with high-end guitar amp repair experts and boutique amp gurus because it does not rattle in loud combo amps. 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. Quad of four tubes is matched to exactly the same plate voltage (e.g. 383VDC) in a real guitar amp. If any tubes fail in normal service within 12 months send 'em back to us and we'll replace 'em 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.