{"type_number":"866A","canonical_url":"https://tubeofile.com/tubes/866A","manufacturer":"RCA","tube_type_key":"rectifier","tube_type_display_name":"Rectifier","base_name":"UX4","description":"Half-wave mercury-vapor rectifier with directly heated (filament) cathode. Designed for high-voltage power supply service in radio transmitters. Uses a medium-shell small 4-pin base with bayonet. The mercury vapor produces a characteristic blue-violet glow during operation.","is_directly_heated":true,"completeness":"high","notes":["The filament must be heated for at least 15 seconds before applying anode voltage to allow the mercury to vaporize and reach operating temperature. Failure to observe this warm-up time will damage the cathode coating.","Condensed-mercury temperature must be maintained between 20-80 degrees C for reliable operation. Below 20 C, the mercury vapor pressure is too low; above 80 C, arc-back risk increases. Forced-air cooling may be required.","Peak inverse anode voltage ratings vary with condensed-mercury temperature: 2500V max (20-80 C), 5000V max (20-70 C), 10000V max (20-60 C).","Average anode current: 0.25A max (20-70 C), 0.5A max at restricted temperature range. Peak current 2A max (20-80 C).","Filament: 2.5V at 5A. Peak tube voltage drop approximately 15V.","Shields and RF filter circuits should be provided if the 866A is subjected to extraneous high-frequency fields. Mercury vapor is susceptible to RF-induced breakdown."],"references":["https://frank.pocnet.net/sheets/049/8/866A.pdf","https://tdsl.duncanamps.com/show.php?des=866A"],"source_urls":{"r_type":null,"tdsl":"https://tdsl.duncanamps.com/show.php?des=866A","radiomuseum":"https://www.radiomuseum.org/tubes/tube_866a.html"},"raw_equivalents":["AG866A","EE-866A","RK866A","CV32","DCG4/1000G","Z-225"],"equivalents":[],"similar":[{"type_number":"836","reason":"Similar mercury-vapor rectifier on UX4 base but rated for lower inverse voltage — not a drop-in at full ratings","has_detail_page":true},{"type_number":"872A","reason":"Similar mercury-vapor rectifier but higher current rating (1.25A avg vs 0.25A avg) — different size class","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Filament","function_code":"f","notes":null},{"pin_number":"2","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"3","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"4","function":"Filament, Cathode Shield","function_code":"f","notes":null},{"pin_number":"TC","function":"Anode (top cap)","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":"5.0","numeric_value":"5","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Mercury-Vapor Rectifier","tube_type_key":"rectifier","specs":[{"key":"PIV","display_name":"Peak Inverse Voltage","value":"10000","numeric_value":"10000","unit":"V","param_group":"rectifier"},{"key":"Vdrop","display_name":"Voltage Drop (typical)","value":"15","numeric_value":"15","unit":"V","param_group":"rectifier"},{"key":"Ia_peak","display_name":"Peak Anode Current (surge)","value":"2000","numeric_value":"2000","unit":"mA","param_group":"rectifier"},{"key":"Idc_max","display_name":"Max DC Output Current (total, not per-anode)","value":"250","numeric_value":"250","unit":"mA","param_group":"rectifier"}]}],"synced_at":"2026-04-08T07:54:09.876Z"}