{"type_number":"5B/255M","canonical_url":"https://tubeofile.com/tubes/5B%2F255M","manufacturer":"STC","tube_type_key":"beam_tetrode","tube_type_display_name":"Beam Tetrode","base_name":"Loctal","description":"Indirectly heated beam-power tetrode designed for RF transmitter and audio amplifier service, electrically similar to the 807. Developed by STC for British military and telecom use. Loctal (B8G) base with double-ended construction (anode brought out to top cap). Rated to 60 MHz at full ratings, usable to 125 MHz at reduced anode voltage.","is_directly_heated":false,"completeness":"high","notes":["Characteristics identical to 5B/254M per STC datasheet. The 5B/254G has flexible leads instead of a base for direct wiring.","The 5B/256M (CV499), 5B/257M (CV2220), and 5B/258M (CV2347) are the same valve with 12V/0.47A, 12V/0.3A, and 19V/0.3A heaters respectively — not pin-compatible substitutes due to different heater ratings.","Anode is brought out to a top cap as well as to pin 2 — double-ended construction for improved HF insulation. For high-altitude or anode-modulated operation, the double-ended versions (5B/254G, 5B/254M, 5B/258M) are preferred due to base insulation.","DC grid circuit resistance must not exceed 30 kOhm at maximum ratings. At less than maximum ratings the value may be increased to 100 kOhm.","Cathode bias is not recommended per STC datasheet; fixed bias source or a combination of methods should be used.","Bulb temperature must not exceed 250 °C under any operating condition. Adequate ventilation must be provided.","Prototype designation: LS650."],"references":["https://frank.pocnet.net/sheets/130/5/5B254G.pdf","https://frank.pocnet.net/sheets/155/5/5B-254M.pdf","https://frank.pocnet.net/other/ServiceTypes/cv3/CV391.pdf"],"source_urls":{"r_type":"http://www.r-type.org/exhib/aaa0035.htm","tdsl":"http://tdsl.duncanamps.com/show.php?des=5B255M","radiomuseum":"https://www.radiomuseum.org/tubes/tube_5b255m.html"},"raw_equivalents":["CV391","5B/254M","5B/254G","5B/252M","CV8028","5B/251M"],"equivalents":[{"type_number":"5B/254M","equivalence_type":"direct","notes":null}],"similar":[{"type_number":"5B/256M","reason":"12V/0.47A heater variant (CV499) — not pin-compatible due to different heater ratings","has_detail_page":true},{"type_number":"5B/257M","reason":"12V/0.3A heater variant (CV2220) — not pin-compatible due to different heater ratings","has_detail_page":true},{"type_number":"5B/258M","reason":"19V/0.3A heater variant (CV2347) — not pin-compatible due to different heater ratings","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Heater","function_code":"h","notes":null},{"pin_number":"2","function":"Anode","function_code":"a","notes":null},{"pin_number":"3","function":"Screen Grid","function_code":"g2","notes":null},{"pin_number":"4","function":"Cathode & Beam Plates","function_code":"k","notes":null},{"pin_number":"5","function":"Control Grid","function_code":"g1","notes":null},{"pin_number":"6","function":"Control Grid","function_code":"g1","notes":null},{"pin_number":"7","function":"Cathode & Beam Plates","function_code":"k","notes":null},{"pin_number":"8","function":"Heater","function_code":"h","notes":null},{"pin_number":"TC","function":"Anode (Top Cap)","function_code":"a","notes":null}],"specs":[{"key":"Vh","display_name":"Heater Voltage","value":"6.3","numeric_value":"6.3","unit":"V","param_group":"heater"},{"key":"Ah","display_name":"Heater Current","value":"0.9","numeric_value":"0.9","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Beam Tetrode","tube_type_key":"beam_tetrode","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"600","numeric_value":"600","unit":"V","param_group":"absolute_max"},{"key":"Vs_max","display_name":"Screen Voltage (max)","value":"300","numeric_value":"300","unit":"V","param_group":"absolute_max"},{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"25","numeric_value":"25","unit":"W","param_group":"absolute_max"},{"key":"Ps_max","display_name":"Screen Dissipation (max)","value":"4.5","numeric_value":"4.5","unit":"W","param_group":"absolute_max"},{"key":"Ik_max","display_name":"Cathode Current (max)","value":"125","numeric_value":"125","unit":"mA","param_group":"absolute_max"},{"key":"Vhk_max","display_name":"Heater-to-Cathode Voltage (max)","value":"135","numeric_value":"135","unit":"V","param_group":"absolute_max"},{"key":"Rg_max","display_name":"Max Grid Resistor","value":"0.03","numeric_value":"0.03","unit":"MΩ","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"250","numeric_value":"250","unit":"V","param_group":"typical_operating"},{"key":"Vs","display_name":"Screen Voltage (typical)","value":"250","numeric_value":"250","unit":"V","param_group":"typical_operating"},{"key":"Vg","display_name":"Grid 1 Bias Voltage","value":"-14","numeric_value":"-14","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"72","numeric_value":"72","unit":"mA","param_group":"typical_operating"},{"key":"mAs","display_name":"Screen Current","value":"5","numeric_value":"5","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"22500","numeric_value":"22500","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"6.0","numeric_value":"6","unit":"mA/V","param_group":"typical_operating"},{"key":"Rk","display_name":"Cathode Resistor (self-bias)","value":"170","numeric_value":"170","unit":"Ω","param_group":"typical_operating"},{"key":"Pout","display_name":"Output Power","value":"6.5","numeric_value":"6.5","unit":"W","param_group":"output"},{"key":"rl","display_name":"Load Impedance","value":"2500","numeric_value":"2500","unit":"Ω","param_group":"output"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"0.13","numeric_value":"0.13","unit":"pF","param_group":"capacitances"},{"key":"Cin","display_name":"Input Capacitance","value":"12","numeric_value":"12","unit":"pF","param_group":"capacitances"},{"key":"Cout","display_name":"Output Capacitance","value":"8.5","numeric_value":"8.5","unit":"pF","param_group":"capacitances"},{"key":"freq","display_name":"Max Operating Frequency","value":"60","numeric_value":"60","unit":"MHz","param_group":"noise_rf"},{"key":null,"display_name":"Screen Grid Amplification Factor","value":"9","numeric_value":"9","unit":"—","param_group":null}]},{"section_number":2,"section_label":"Triode Strapped","tube_type_key":"power_triode","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"300","numeric_value":"300","unit":"V","param_group":"absolute_max"},{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"25","numeric_value":"25","unit":"W","param_group":"absolute_max"},{"key":"Ik_max","display_name":"Cathode Current (max)","value":"125","numeric_value":"125","unit":"mA","param_group":"absolute_max"},{"key":"ra","display_name":"Anode Resistance","value":"1700","numeric_value":"1700","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"4.7","numeric_value":"4.7","unit":"mA/V","param_group":"typical_operating"},{"key":"mu","display_name":"Amplification Factor (μ)","value":"8.0","numeric_value":"8","unit":"—","param_group":"typical_operating"}]}],"synced_at":"2026-04-07T12:12:08.422Z"}