{"type_number":"7581","canonical_url":"https://tubeofile.com/tubes/7581","manufacturer":"Tung-Sol","tube_type_key":"beam_tetrode","tube_type_display_name":"Beam Tetrode","base_name":"Octal","description":"Beam-power pentode designed for audio-frequency power amplifier applications. Direct replacement for the 6L6GC with additional controlled zero-bias characteristics and a low-loss base. Manufactured by Tung-Sol.","is_directly_heated":false,"completeness":"high","notes":["The 7581 is a direct replacement for the 6L6GC with same pinout (JEDEC 7AC). Brans Vademecum cross-lists it in the 6L6/5932 family.","Maximum ratings (pentode): Va=500V, Vs=450V (Vs_max = 500V with screen tied to plate in push-pull), Pa=30W, Ps=5W.","Triode-strapped characteristics at Va=Vg2=250V, Vg1=-20V: mu=8, gm=4700 µmhos, ra=1700 Ω, Ia=40mA. Max Pa (triode) not specified separately.","Maximum ratings (triode connection): Va=450V.","Pentode Class A1 at Va=250V, Vs=250V: gm=6000 µmhos, ra=22500 Ω, Ia=72mA, Pout=6.5W into 2500 Ω (same as 6L6).","Pentode Class A1 at Va=350V, Vs=250V: gm=5200 µmhos, ra=33000 Ω, Ia=54mA, Pout=10.8W into 4200 Ω.","Rg_max: 0.1 MΩ (fixed bias), 0.5 MΩ (cathode bias) — applies to both pentode and triode connections.","Basing is 6-pin Octal with short medium shell, low-loss material (JEDEC 7AC, outline drawing 17-19). Pins 1 and 6 have no physical pin."],"references":["https://frank.pocnet.net/sheets/127/7/7581.pdf"],"source_urls":{"r_type":null,"tdsl":"https://tdsl.duncanamps.com/show.php?des=7581","radiomuseum":"https://www.radiomuseum.org/tubes/tube_7581.html"},"raw_equivalents":["7581A"],"equivalents":[],"similar":[{"type_number":"5881","reason":"6L6 family member with same pinout — ruggedised military variant with lower ratings than 7581","has_detail_page":true},{"type_number":"6L6GC","reason":"Direct replacement — 7581 is a drop-in for 6L6GC with same pinout and improved zero-bias characteristics","has_detail_page":true},{"type_number":"KT66","reason":"Sams lists KT66 as substitute — GEC beam tetrode with similar application and ratings","has_detail_page":true},{"type_number":"KT77","reason":"Sams lists KT77 as substitute — GEC beam tetrode designed as 6L6GC/EL34 replacement","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":"Plate","function_code":"a","notes":null},{"pin_number":"4","function":"Screen Grid","function_code":"g2","notes":null},{"pin_number":"5","function":"Control Grid","function_code":"g1","notes":null},{"pin_number":"6","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"7","function":"Heater","function_code":"h","notes":null},{"pin_number":"8","function":"Cathode","function_code":"k","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":"500","numeric_value":"500","unit":"V","param_group":"absolute_max"},{"key":"Vs_max","display_name":"Screen Voltage (max)","value":"450","numeric_value":"450","unit":"V","param_group":"absolute_max"},{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"30","numeric_value":"30","unit":"W","param_group":"absolute_max"},{"key":"Ps_max","display_name":"Screen Dissipation (max)","value":"5","numeric_value":"5","unit":"W","param_group":"absolute_max"},{"key":"Vhk_max","display_name":"Heater-to-Cathode Voltage (max)","value":"200","numeric_value":"200","unit":"V","param_group":"absolute_max"},{"key":"Rg_max","display_name":"Max Grid Resistor","value":"0.1","numeric_value":"0.1","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":"5.0","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":"Pout","display_name":"Output Power","value":"10.8","numeric_value":"10.8","unit":"W","param_group":"output"},{"key":"rl","display_name":"Load Impedance","value":"4200","numeric_value":"4200","unit":"Ω","param_group":"output"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"0.6","numeric_value":"0.6","unit":"pF","param_group":"capacitances"},{"key":"Cin","display_name":"Input Capacitance","value":"10","numeric_value":"10","unit":"pF","param_group":"capacitances"},{"key":"Cout","display_name":"Output Capacitance","value":"6.5","numeric_value":"6.5","unit":"pF","param_group":"capacitances"}]},{"section_number":2,"section_label":"Triode Strapped","tube_type_key":"power_triode","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"450","numeric_value":"450","unit":"V","param_group":"absolute_max"},{"key":"Rg_max","display_name":"Max Grid Resistor","value":"0.1","numeric_value":"0.1","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":"Vg","display_name":"Grid 1 Bias Voltage","value":"-20","numeric_value":"-20","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"40","numeric_value":"40","unit":"mA","param_group":"typical_operating"},{"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"},{"key":"Pout","display_name":"Output Power","value":"1.4","numeric_value":"1.4","unit":"W","param_group":"output"},{"key":"rl","display_name":"Load Impedance","value":"5000","numeric_value":"5000","unit":"Ω","param_group":"output"}]}],"synced_at":"2026-04-07T12:31:13.352Z"}