{"type_number":"81","canonical_url":"https://tubeofile.com/tubes/81","manufacturer":null,"tube_type_key":"rectifier","tube_type_display_name":"Rectifier","base_name":"UX4","description":"Half-wave high-vacuum rectifier with directly heated (coated filament) cathode. Uses UX4 base (4B basing designation). Designed for use with either choke or condenser input filter. Can be used singly for half-wave rectification or in pairs for full-wave rectification. 7.5V/1.25A filament.","is_directly_heated":true,"completeness":"low","notes":["Tung-Sol datasheet (1939) confirms: half-wave HIGH VACUUM rectifier, 7.5V/1.25A coated filament, 4B base (medium 4-pin bayonet base).","NOTE: MIL-HDBK-213A incorrectly classifies the 81 as a mercury vapor rectifier. The Tung-Sol datasheet, GE Essential Characteristics, and radiomuseum all confirm it is a high-vacuum type.","Raytheon data confirms: 7.5V/1.25A filament, 4D-SM4B basing, half-wave rectifier.","GE Essential Characteristics lists: Half-Wave High-Vacuum Rectifier, 4B base, 7.5V/1.25A filament.","MIL-HDBK-213A lists other_number as 2935A.","Sams substitution data lists UV216 and UX216B as substitutes for the 81.","Half-wave single tube: AC plate voltage 700V RMS max, DC output current 85 mA max.","Full-wave two tubes: AC plate voltage 700V RMS max, DC output current 170 mA max, Vdrop 91V at 170 mA.","The 4B base is similar to UX4 but uses a bayonet locking mechanism rather than the push-in arrangement of the 4C base.","Primarily used in transmitter power supplies and higher-voltage receiver applications of the late 1920s and 1930s."],"references":["https://frank.pocnet.net/sheets/127/8/81.pdf"],"source_urls":{"r_type":null,"tdsl":"http://tdsl.duncanamps.com/show.php?des=81","radiomuseum":"https://www.radiomuseum.org/tubes/tube_81.html"},"raw_equivalents":["CV2546","UX281","VT-81","2935A"],"equivalents":[],"similar":[{"type_number":"80","reason":"Same UX4 base, full-wave high-vacuum rectifier — different filament voltage (5V vs 7.5V), higher current per tube (125 mA vs 85 mA) but requires only one tube","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Anode (Plate)","function_code":"a","notes":null},{"pin_number":"2","function":"Filament","function_code":"h","notes":null},{"pin_number":"3","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"4","function":"Filament","function_code":"h","notes":null}],"specs":[{"key":"Vh","display_name":"Heater Voltage","value":"7.5","numeric_value":"7.5","unit":"V","param_group":"heater"},{"key":"Ah","display_name":"Heater Current","value":"1.25","numeric_value":"1.25","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Rectifier (half-wave, single tube)","tube_type_key":"rectifier","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"700","numeric_value":"700","unit":"V","param_group":"absolute_max"},{"key":"Vdrop","display_name":"Voltage Drop (typical)","value":"91","numeric_value":"91","unit":"V","param_group":"rectifier"},{"key":"Idc_max","display_name":"Max DC Output Current (total, not per-anode)","value":"85","numeric_value":"85","unit":"mA","param_group":"rectifier"}]}],"synced_at":"2026-04-08T07:53:43.306Z"}