{"type_number":"40","canonical_url":"https://tubeofile.com/tubes/40","manufacturer":"RCA","tube_type_key":"small_signal_triode","tube_type_display_name":"Small Signal Triode","base_name":"UX4","description":"Directly heated high-mu triode for detector and voltage amplifier service. 5V/0.25A filament, same heater requirements as the 01A. Intended as a high-gain first-stage amplifier or grid-leak detector in AC-powered receivers of the late 1920s-30s. UX4 base (4-pin), glass bulb.","is_directly_heated":true,"completeness":"medium","notes":["DHT tube: the filament serves as the cathode; there is no heater-cathode isolation.","Very high plate resistance (150000 Ohm) and low plate current (0.2mA) — requires careful circuit design and high-impedance plate loads.","GE Essential Characteristics: Va=180V, Vg=-3V, Ia=0.2mA, ra=150000 Ohm, gm=0.2 mA/V, mu=30.","MIL-HDBK-213A lists this tube as VT-40.","No Pa_max or Rg_max is specified on known datasheets. Va_max=180V per GE Essential Characteristics."],"references":["https://frank.pocnet.net/sheets/121/4/40.pdf"],"source_urls":{"r_type":null,"tdsl":"https://tdsl.duncanamps.com/show.php?des=40","radiomuseum":"https://www.radiomuseum.org/tubes/tube_40.html"},"raw_equivalents":["240","340","440","VT-40","CV2501"],"equivalents":[],"similar":[{"type_number":"01A","reason":"Same UX4 base and identical 5V/0.25A filament but much lower mu (8 vs 30) and lower ra (10000 vs 150000 Ohm); general-purpose triode vs high-mu detector","has_detail_page":true},{"type_number":"30","reason":"DHT detector/amplifier triode with same UX4 base but 2V battery filament and lower mu (~9.3 vs 30)","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Filament (f-)","function_code":"h","notes":null},{"pin_number":"2","function":"Plate (Anode)","function_code":"a","notes":null},{"pin_number":"3","function":"Grid","function_code":"g1","notes":null},{"pin_number":"4","function":"Filament (f+)","function_code":"h","notes":null}],"specs":[{"key":"Vh","display_name":"Heater Voltage","value":"5","numeric_value":"5","unit":"V","param_group":"heater"},{"key":"Ah","display_name":"Heater Current","value":"0.25","numeric_value":"0.25","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":"180","numeric_value":"180","unit":"V","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"180","numeric_value":"180","unit":"V","param_group":"typical_operating"},{"key":"Vg","display_name":"Grid 1 Bias Voltage","value":"-3","numeric_value":"-3","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"0.2","numeric_value":"0.2","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"150000","numeric_value":"150000","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"0.2","numeric_value":"0.2","unit":"mA/V","param_group":"typical_operating"},{"key":"mu","display_name":"Amplification Factor (μ)","value":"30","numeric_value":"30","unit":"—","param_group":"typical_operating"}]}],"synced_at":"2026-04-07T12:09:35.844Z"}