{"type_number":"437A","canonical_url":"https://tubeofile.com/tubes/437A","manufacturer":"Western Electric","tube_type_key":"small_signal_triode","tube_type_display_name":"Small Signal Triode","base_name":"Magnoval","description":"High figure of merit triode with indirectly heated cathode, designed primarily for the output amplifier of the Bell System L-3 tandem two stage amplifier. Features high transconductance (46 mA/V) and low plate resistance (900 ohms) in a Magnoval envelope.","is_directly_heated":false,"completeness":"high","notes":["Developed by Bell Telephone Laboratories for the AT&T L-3 coaxial carrier system. First issued October 1955 (Bell System Practices AB46.437A, Issue 1).","Uses Magnoval (B9D) 9-pin base with T9 bulb envelope. Pins 3, 7, and 9 are no connection.","The datasheet specifies three typical operating conditions: Eb=140V (Ia=29 mA), Eb=180V (Ia=25 mA), and Eb=160V with cathode bias (Ia=36.5 mA, Rk=262 ohms, Ec1=+7.5V supply). The third condition is the primary L-3 amplifier design point.","Operation with control grid positive with respect to cathode is not recommended (Note 1).","Grid circuit resistance is very low: 0.05 Mohm for fixed bias, 0.10 Mohm for cathode bias — typical of high-gm types.","Two cathode pins (6 and 8) provided for reduced lead inductance."],"references":["https://western-electric.squarespace.com/s/437A.pdf","http://www.the-planet.org/gkaufman/pdf/WE437A.pdf"],"source_urls":{"r_type":null,"tdsl":null,"radiomuseum":"https://www.radiomuseum.org/tubes/tube_437a.html"},"raw_equivalents":[],"equivalents":[],"similar":[{"type_number":"3A/167M","reason":"STC equivalent — electrically identical per STC datasheet (same gm, ra, ratings), but uses Loctal (B8G) 8-pin base instead of Magnoval (B9D) 9-pin. Not pin-compatible.","has_detail_page":true},{"type_number":"417A","reason":"Similar WE high-gm triode but uses B9A (Noval) base — not pin-compatible. Lower dissipation (2.5W vs 7W).","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Plate","function_code":"a","notes":null},{"pin_number":"2","function":"Heater","function_code":"h","notes":null},{"pin_number":"3","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"4","function":"Grid","function_code":"g1","notes":null},{"pin_number":"5","function":"Heater","function_code":"h","notes":null},{"pin_number":"6","function":"Cathode","function_code":"k","notes":null},{"pin_number":"7","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"8","function":"Cathode","function_code":"k","notes":null},{"pin_number":"9","function":"No Connection","function_code":"nc","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":"0.45","numeric_value":"0.45","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Triode","tube_type_key":"small_signal_triode","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"250","numeric_value":"250","unit":"V","param_group":"absolute_max"},{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"7.0","numeric_value":"7","unit":"W","param_group":"absolute_max"},{"key":"Ik_max","display_name":"Cathode Current (max)","value":"45","numeric_value":"45","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":"0.10","numeric_value":"0.1","unit":"MΩ","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"160","numeric_value":"160","unit":"V","param_group":"typical_operating"},{"key":"Vg","display_name":"Grid 1 Bias Voltage","value":"+7.5","numeric_value":"7.5","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"36.5","numeric_value":"36.5","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"900","numeric_value":"900","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"46","numeric_value":"46","unit":"mA/V","param_group":"typical_operating"},{"key":"mu","display_name":"Amplification Factor (μ)","value":"41","numeric_value":"41","unit":"—","param_group":"typical_operating"},{"key":"Rk","display_name":"Cathode Resistor (self-bias)","value":"262","numeric_value":"262","unit":"Ω","param_group":"typical_operating"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"3.8","numeric_value":"3.8","unit":"pF","param_group":"capacitances"},{"key":"Cin","display_name":"Input Capacitance","value":"11.1","numeric_value":"11.1","unit":"pF","param_group":"capacitances"},{"key":"Cout","display_name":"Output Capacitance","value":"1.0","numeric_value":"1","unit":"pF","param_group":"capacitances"}]}],"synced_at":"2026-04-07T12:09:55.228Z"}