{"type_number":"394A","canonical_url":"https://tubeofile.com/tubes/394A","manufacturer":"Western Electric","tube_type_key":"thyratron","tube_type_display_name":"Thyratron","base_name":"IO","description":"Western Electric mercury-vapour thyratron designed for use as a controlled rectifier. Directly heated filament cathode with an octal (IO) base. The anode is a flat plate at the top of the envelope; inside the screen is the helical filament and control electrode. A mesh mercury reservoir is located near the pinch. Developed by Western Electric for industrial control and power supply applications.","is_directly_heated":true,"completeness":"high","notes":["Mercury-vapour thyratron: once the grid loses control and the tube fires, the grid cannot regain control until the anode voltage is removed or reversed.","Must be operated within the specified ambient temperature range to ensure correct mercury vapour pressure.","Allow adequate filament warm-up time (typically 30 seconds minimum) before applying anode voltage.","The Valve Museum describes the classic envelope as 45 mm in diameter and 128 mm tall (excluding base pins).","Anode connection is via the top cap (tc), not through the octal base."],"references":["https://tdsl.duncanamps.com/show.php?des=394A","https://www.radiomuseum.org/tubes/?tube=394A"],"source_urls":{"r_type":"https://www.r-type.org/exhib/abh0311.htm","tdsl":"https://tdsl.duncanamps.com/show.php?des=394A","radiomuseum":"https://www.radiomuseum.org/tubes/tube_394a.html"},"raw_equivalents":["CV2639"],"equivalents":[],"similar":[],"pins":[{"pin_number":"1","function":"No Connection","function_code":"nc","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":"Grid","function_code":"g1","notes":null},{"pin_number":"5","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"6","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"7","function":"Filament","function_code":"h","notes":null},{"pin_number":"8","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"tc","function":"Anode (Plate)","function_code":"a","notes":null}],"specs":[{"key":"Vh","display_name":"Heater Voltage","value":"2.5","numeric_value":"2.5","unit":"V","param_group":"heater"},{"key":"Ah","display_name":"Heater Current","value":"3.25","numeric_value":"3.25","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Thyratron","tube_type_key":"thyratron","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"1250","numeric_value":"1250","unit":"V","param_group":"absolute_max"}]}],"synced_at":"2026-04-07T12:09:19.804Z"}