{"type_number":"EL51","canonical_url":"https://tubeofile.com/tubes/EL51","manufacturer":"Philips","tube_type_key":"power_pentode","tube_type_display_name":"Power Pentode","base_name":"B8A","description":"AF output power pentode with 45W anode dissipation; indirect heated. European side-contact base (P8A/Ct8) with top-cap anode. Developed by Philips circa 1940; also produced by Tesla and Valvo. Designed for push-pull Class AB output stages.","is_directly_heated":false,"completeness":"high","notes":["The EL51 is an unusually powerful tube (45W Pa) on the P8A side-contact base — significantly more powerful than the common EL50 (18W) but sharing the same base type and top-cap anode arrangement.","No direct equivalents have been identified across any reference source (Brans Vademecum, Philips interchange, RCA, Sams). The Philips interchange guide lists only 'EL51 = EL51' with no alternatives. This makes the EL51 essentially irreplaceable without circuit modification.","Produced by Philips (Eindhoven), Tesla (Czechoslovakia), and Valvo (Germany). The Tesla production continued well into the 1960s, making Czech-made EL51s the most commonly available today.","Part of the same P8A-base wartime family as the EL50 and EF50, developed for military radar and communications equipment where high power output was needed from compact side-contact tubes."],"references":["https://frank.pocnet.net/sheets/183/e/EL51.pdf","https://www.radiomuseum.org/tubes/tube_el51.html"],"source_urls":{"r_type":"http://www.r-type.org/exhib/aai0055.htm","tdsl":"https://tdsl.duncanamps.com/show.php?des=EL51","radiomuseum":"https://www.radiomuseum.org/tubes/tube_el51.html"},"raw_equivalents":[],"equivalents":[],"similar":[{"type_number":"6550","reason":"Similar power class (42W) but Octal beam tetrode — different socket and tube type","has_detail_page":true},{"type_number":"807","reason":"Similar power class (~30W) and era but 5-pin base with top cap — completely different socket","has_detail_page":true},{"type_number":"EL34","reason":"Later Philips power pentode (25W), Octal base — lower Pa (25W vs 45W), different socket","has_detail_page":true},{"type_number":"EL37","reason":"Similar power class (~25W) Mullard pentode but Octal base — not P8A compatible","has_detail_page":true},{"type_number":"EL50","reason":"Same P8A base and pinout but much lower power (18W vs 45W) — not a safe replacement in EL51 circuits","has_detail_page":true},{"type_number":"EL53","reason":"Same P8A base family, 6.3V heater — verify pin assignment and power ratings before substituting","has_detail_page":true},{"type_number":"KT88","reason":"Closest in power class (42W) but Octal beam tetrode — different socket and tube type","has_detail_page":true}],"pins":[{"pin_number":"1","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"2","function":"Heater","function_code":"h","notes":null},{"pin_number":"3","function":"Heater","function_code":"h","notes":null},{"pin_number":"4","function":"Cathode","function_code":"k","notes":null},{"pin_number":"5","function":"Suppressor Grid (g3)","function_code":"g3","notes":null},{"pin_number":"6","function":"Control Grid (g1)","function_code":"g1","notes":null},{"pin_number":"7","function":"Screen Grid (g2)","function_code":"g2","notes":null},{"pin_number":"8","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"top","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":"1.9","numeric_value":"1.9","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Pentode","tube_type_key":"power_pentode","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"750","numeric_value":"750","unit":"V","param_group":"absolute_max"},{"key":"Vs_max","display_name":"Screen Voltage (max)","value":"750","numeric_value":"750","unit":"V","param_group":"absolute_max"},{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"45","numeric_value":"45","unit":"W","param_group":"absolute_max"},{"key":"Ps_max","display_name":"Screen Dissipation (max)","value":"25","numeric_value":"25","unit":"W","param_group":"absolute_max"},{"key":"Ik_max","display_name":"Cathode Current (max)","value":"200","numeric_value":"200","unit":"mA","param_group":"absolute_max"},{"key":"Vhk_max","display_name":"Heater-to-Cathode Voltage (max)","value":"50","numeric_value":"50","unit":"V","param_group":"absolute_max"},{"key":"Rg_max","display_name":"Max Grid Resistor","value":"0.7","numeric_value":"0.7","unit":"MΩ","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"500","numeric_value":"500","unit":"V","param_group":"typical_operating"},{"key":"Vs","display_name":"Screen Voltage (typical)","value":"500","numeric_value":"500","unit":"V","param_group":"typical_operating"},{"key":"Vg","display_name":"Grid 1 Bias Voltage","value":"-22","numeric_value":"-22","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"95","numeric_value":"95","unit":"mA","param_group":"typical_operating"},{"key":"mAs","display_name":"Screen Current","value":"12","numeric_value":"12","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"33000","numeric_value":"33000","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"11","numeric_value":"11","unit":"mA/V","param_group":"typical_operating"},{"key":"Rk","display_name":"Cathode Resistor (self-bias)","value":"100","numeric_value":"100","unit":"Ω","param_group":"typical_operating"},{"key":"Pout","display_name":"Output Power","value":"70","numeric_value":"70","unit":"W","param_group":"output"},{"key":"rl","display_name":"Load Impedance","value":"4800","numeric_value":"4800","unit":"Ω","param_group":"output"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"1.5","numeric_value":"1.5","unit":"pF","param_group":"capacitances"},{"key":null,"display_name":"Output Power (PP Class AB fixed bias, Va=750V)","value":"125","numeric_value":"125","unit":"W","param_group":null},{"key":null,"display_name":"Grid Resistor (max, fixed bias)","value":"0.2","numeric_value":"0.2","unit":"MΩ","param_group":null},{"key":null,"display_name":"Screen Dissipation (max, quiescent)","value":"7","numeric_value":"7","unit":"W","param_group":null}]}],"synced_at":"2026-04-07T12:45:44.632Z"}