{"type_number":"5B/254M","canonical_url":"https://tubeofile.com/tubes/5B%2F254M","manufacturer":"STC (Standard Telephones and Cables)","tube_type_key":"beam_tetrode","tube_type_display_name":"Beam Tetrode","base_name":"Loctal","description":"Beam power tetrode designed for RF and AF power amplifier service. Electrically equivalent to the 807 but packaged in a compact Loctal (B8G) envelope with top-cap anode connection. Rated for operation up to 125 MHz. Developed by STC as a reliable replacement for the earlier 5B/251M.","is_directly_heated":false,"completeness":"high","notes":["The anode connects via a top cap, not through the base pins. Top cap size is 1/4 inch (6 mm).","Cathode bias is not recommended per the manufacturer datasheet; fixed bias is required.","The 5B/255M is electrically identical. The 5B/257M is a 12V heater variant (12.6V, 0.47A). The 5B/258M and CV2347 are similar but with multiple differences.","Grid stoppers at the socket and RF chokes in the anode circuit may be required to suppress oscillation tendencies.","Pins 5 and 6 are both control grid connections (double grid pins), and pins 2, 4, and 7 are all cathode/beam plate connections.","Triode-strapped data sourced from Tom Schlangen, 'Overview of pentodes / BPTs strapped as triodes' (2009), not from the manufacturer datasheet."],"references":["https://www.valvecollector.uk/5b254m.htm","https://www.r-type.org/exhib/aab0149.htm","https://www.radiomuseum.org/tubes/tube_5b254m.html","https://diyaudio.com/forums/tubes-valves/161255-cv428-5b254m-useful.html"],"source_urls":{"r_type":"http://www.r-type.org/exhib/aab0149.htm","tdsl":"https://tdsl.duncanamps.com/show.php?des=5B254M","radiomuseum":"https://www.radiomuseum.org/tubes/tube_5b254m.html"},"raw_equivalents":["CV428","5B/251M","5B/255M","5B/254G","CV2347","5B/253M"],"equivalents":[{"type_number":"5B/255M","equivalence_type":"direct","notes":null}],"similar":[{"type_number":"5B/256M","reason":"19V/0.3A heater variant (CV499) — not pin-compatible due to different heater ratings","has_detail_page":true},{"type_number":"5B/257M","reason":"12.6V/0.47A heater variant — not pin-compatible due to different heater ratings","has_detail_page":true},{"type_number":"5B/258M","reason":"19V/0.3A heater variant — 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":"Cathode / Beam Plates","function_code":"k","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":"3","numeric_value":"3","unit":"W","param_group":"absolute_max"},{"key":"Ik_max","display_name":"Cathode Current (max)","value":"120","numeric_value":"120","unit":"mA","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"300","numeric_value":"300","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":"-45","numeric_value":"-45","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"72","numeric_value":"72","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"1500","numeric_value":"1500","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":"mu","display_name":"Amplification Factor (μ)","value":"9","numeric_value":"9","unit":"—","param_group":"typical_operating"},{"key":"Pout","display_name":"Output Power","value":"80","numeric_value":"80","unit":"W","param_group":"output"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"0.1","numeric_value":"0.1","unit":"pF","param_group":"capacitances"},{"key":"Cin","display_name":"Input Capacitance","value":"13","numeric_value":"13","unit":"pF","param_group":"capacitances"},{"key":"Cout","display_name":"Output Capacitance","value":"7","numeric_value":"7","unit":"pF","param_group":"capacitances"},{"key":"freq","display_name":"Max Operating Frequency","value":"125","numeric_value":"125","unit":"MHz","param_group":"noise_rf"}]},{"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","numeric_value":"8","unit":"—","param_group":"typical_operating"}]}],"synced_at":"2026-04-07T12:12:01.287Z"}