{"type_number":"KT66","canonical_url":"https://tubeofile.com/tubes/KT66","manufacturer":"GEC","tube_type_key":"beam_tetrode","tube_type_display_name":"Beam Tetrode","base_name":"Octal","description":"Beam tetrode for audio output stages with 25 W design maximum anode dissipation (30 W absolute). Manufactured by M-O Valve Co. (GEC) from 1937. Commercial version of CV1075. Widely used in high-fidelity audio amplifiers and the original Marshall JTM45.","is_directly_heated":false,"completeness":"medium","notes":["Pa_max has two ratings: absolute maximum 30 W, design maximum 25 W. Ps_max: absolute 4.5 W, design 3.5 W. Combined Pa+g2 absolute max is 32 W (design 27 W).","The KT66 is a commercial version of the CV1075. CV321 and VR47 are additional British military designations (Brans Vademecum 1963).","Triode-strapped characteristics at Va = Vg2 = 250 V, Vg1 = -15 V: gm = 7.3 mA/V, ra = 1.3 kΩ. Schlangen reference gives mu = 8, gm = 4.7 mA/V, ra = 1.7 kΩ at a different operating point.","Pin 1 is an internal shield connection. Pin 6 has no physical pin on the Octal base.","Rg_max for cathode bias: 1.0 MΩ (Pa+g2 ≤ 27 W), 500 kΩ (Pa+g2 > 27 W). For fixed bias: 250 kΩ (Pa+g2 ≤ 27 W), 100 kΩ (Pa+g2 > 27 W).","In push-pull operation it is essential to use two separate cathode bias resistors — one per tube.","Average life expectancy at absolute maximum ratings is at least 8000 hours. At Pa+g2 = 21 W, at least 10,000 hours."],"references":["https://frank.pocnet.net/sheets/086/k/KT66.pdf"],"source_urls":{"r_type":"http://r-type.org/exhib/aaa0101.htm","tdsl":"https://tdsl.duncanamps.com/show.php?des=KT66","radiomuseum":"https://www.radiomuseum.org/tubes/tube_kt66.html"},"raw_equivalents":["CV1075","CV321","VR47"],"equivalents":[],"similar":[{"type_number":"5881","reason":"Sams lists as substitute — ruggedised 6L6 variant with similar ratings","has_detail_page":true},{"type_number":"6L6GC","reason":"Sams lists as substitute — RCA beam tetrode with same pinout and similar characteristics but different internal construction","has_detail_page":true},{"type_number":"7581","reason":"Sams lists KT66 as substitute for 7581 — improved 6L6-family beam tetrode with higher ratings","has_detail_page":true},{"type_number":"EL37","reason":"Philips interchange lists EL37 as replacement — Mullard Octal power pentode with similar application but different internal design","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Internal Shield / Beam Plates","function_code":"g3","notes":null},{"pin_number":"2","function":"Heater","function_code":"h","notes":null},{"pin_number":"3","function":"Anode","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 (no physical pin)","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":"1.3","numeric_value":"1.3","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":"500","numeric_value":"500","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":"3.5","numeric_value":"3.5","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":"150","numeric_value":"150","unit":"V","param_group":"absolute_max"},{"key":"Rg_max","display_name":"Max Grid Resistor","value":"1.0","numeric_value":"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":"-15","numeric_value":"-15","unit":"V","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":"7","numeric_value":"7","unit":"mA/V","param_group":"typical_operating"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"1.1","numeric_value":"1.1","unit":"pF","param_group":"capacitances"},{"key":"Cin","display_name":"Input Capacitance","value":"14.5","numeric_value":"14.5","unit":"pF","param_group":"capacitances"},{"key":"Cout","display_name":"Output Capacitance","value":"10.0","numeric_value":"10","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":"500","numeric_value":"500","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":"200","numeric_value":"200","unit":"mA","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":"-15","numeric_value":"-15","unit":"V","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"1300","numeric_value":"1300","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"7.3","numeric_value":"7.3","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":"2026-04-07T12:49:11.676Z"}