{"type_number":"26","canonical_url":"https://tubeofile.com/tubes/26","manufacturer":"RCA","tube_type_key":"small_signal_triode","tube_type_display_name":"Small Signal Triode","base_name":"UX4","description":"Amplifier tube containing a filament designed for operation on alternating current. For use as an RF or AF amplifier in equipment designed for its characteristics. Not ordinarily suitable for use as a detector or power output tube.","is_directly_heated":true,"completeness":"medium","notes":["Directly heated triode — filament is the cathode. No separate heater-cathode insulation; Vhk_max is not applicable.","Grid voltage is measured from the mid-point of the AC-operated filament.","The coated filament should be operated at exactly 1.5V via a step-down transformer. Designed specifically for AC filament operation (unlike the earlier UX-201A which used thoriated tungsten).","Original 1920s production used globe-shaped envelopes (UX-226); later production (1930s onward) used ST-14 bulbs. Both are electrically identical.","High plate resistance (7.3 kOhm) limits its usefulness as a driver for power tubes — a step-down interstage transformer is recommended for low output impedance.","No maximum plate dissipation rating is specified in the original RCA datasheet. At the maximum rated operating point (180V, 6.2mA), plate dissipation is approximately 1.1W."],"references":["https://frank.pocnet.net/sheets/021/2/26.pdf","https://tdsl.duncanamps.com/show.php?des=26"],"source_urls":{"r_type":"https://www.r-type.org/exhib/aaa1036.htm","tdsl":"https://tdsl.duncanamps.com/show.php?des=26","radiomuseum":"https://www.radiomuseum.org/tubes/tube_26.html"},"raw_equivalents":["226","UX226","CX326","CV943","REN326"],"equivalents":[],"similar":[],"pins":[{"pin_number":"1","function":"Filament","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","function_code":"h","notes":null}],"specs":[{"key":"Vh","display_name":"Heater Voltage","value":"1.5","numeric_value":"1.5","unit":"V","param_group":"heater"},{"key":"Ah","display_name":"Heater Current","value":"1.05","numeric_value":"1.05","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":"-14.5","numeric_value":"-14.5","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"6.2","numeric_value":"6.2","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"7300","numeric_value":"7300","unit":"Ω","param_group":"typical_operating"},{"key":"gm","display_name":"Transconductance","value":"1.15","numeric_value":"1.15","unit":"mA/V","param_group":"typical_operating"},{"key":"mu","display_name":"Amplification Factor (μ)","value":"8.3","numeric_value":"8.3","unit":"—","param_group":"typical_operating"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"8.1","numeric_value":"8.1","unit":"pF","param_group":"capacitances"},{"key":"Cgk","display_name":"Grid-to-Cathode Capacitance","value":"2.8","numeric_value":"2.8","unit":"pF","param_group":"capacitances"},{"key":"Cak","display_name":"Anode-to-Cathode Capacitance","value":"2.5","numeric_value":"2.5","unit":"pF","param_group":"capacitances"}]}],"synced_at":"2026-04-07T12:08:13.667Z"}