{"type_number":"4P1L","canonical_url":"https://tubeofile.com/tubes/4P1L","manufacturer":"USSR (various — Reflektor, Svetlana, Novosibirsk)","tube_type_key":"power_pentode","tube_type_display_name":"Power Pentode","base_name":"Loctal","description":"Soviet directly-heated power pentode designed for low-frequency amplification and signal generation up to 100 MHz. Originally developed as a military transmitter/audio tube, it has become highly regarded in audiophile single-ended triode-connected output stages due to its low operating voltage, good linearity, and long filament life. Often called the 'last DHT pentode ever made'. Reputed to be a Soviet adaptation of the Wehrmacht RL2,4P6/AL860 lineage.","is_directly_heated":true,"completeness":"high","notes":["DHT (directly heated cathode) — the thoriated tungsten filament IS the cathode. Bias is applied between filament centre-tap and ground, or via a choke/DC source on one filament leg with cathode resistor referenced to the midpoint.","Filament can be wired for 2.1V @ 650mA (parallel, pins 1+7 to pin 8) or 4.2V @ 325mA (series, pins 1 to 7 with pin 8 unused).","Extensively used in modern audiophile single-ended triode-connected output stages. Typical triode-strapped operating point: Va=235V, Vg=-18V, Ia=40mA giving ~2.5W into 5kΩ.","In triode-strapped mode (g2 tied to anode): mu≈10, ra≈1.6kΩ, gm≈6 mA/V.","Maximum anode and screen voltages are both 250V — this tube is designed for low-voltage operation, not high B+.","Requires a proper DC filament supply for low-noise audio use; AC heating produces audible hum given the DHT topology.","Originally a Soviet copy of the German RL2,4P6 used in WWII-era military transmitters and radio equipment."],"references":["https://www.radiomuseum.org/tubes/tube_4p1l.html","https://www.bartola.co.uk/valves/2012/06/29/4%D0%BF1%D0%BB4p1l-triode-curves/","https://www.mvaudiolabs.com/tubes/tube-data-library/4p1l/","https://soviet-tubes.com/product/4p1l-tube-pentode-russian/"],"source_urls":{"r_type":null,"tdsl":"https://tdsl.duncanamps.com/show.php?des=4P1L","radiomuseum":"https://www.radiomuseum.org/tubes/tube_4p1l.html"},"raw_equivalents":["4П1Л","4L1S","4LS1","4P6","RL2,4P6","4L20"],"equivalents":[{"type_number":"4L20","equivalence_type":"direct","notes":null},{"type_number":"RL2,4P6","equivalence_type":"direct","notes":null}],"similar":[{"type_number":"AL860","reason":"East German DHT pentode with similar specs but different base (Noval B9A) and heater voltage (4.8V)","has_detail_page":true}],"pins":[{"pin_number":"1","function":"Filament","function_code":"h","notes":null},{"pin_number":"2","function":"Suppressor Grid (g3)","function_code":"g3","notes":null},{"pin_number":"3","function":"Control Grid (g1)","function_code":"g1","notes":null},{"pin_number":"4","function":"Screen Grid (g2)","function_code":"g2","notes":null},{"pin_number":"5","function":"Anode","function_code":"a","notes":null},{"pin_number":"6","function":"No Connection","function_code":"nc","notes":null},{"pin_number":"7","function":"Filament","function_code":"h","notes":null},{"pin_number":"8","function":"Filament Centre Tap","function_code":"h","notes":null}],"specs":[{"key":"Vh","display_name":"Heater Voltage","value":"2.1","numeric_value":"2.1","unit":"V","param_group":"heater"},{"key":"Ah","display_name":"Heater Current","value":"0.65","numeric_value":"0.65","unit":"A","param_group":"heater"}],"sections":[{"section_number":1,"section_label":"Pentode","tube_type_key":"power_pentode","specs":[{"key":"Va_max","display_name":"Anode Voltage (max)","value":"250","numeric_value":"250","unit":"V","param_group":"absolute_max"},{"key":"Vs_max","display_name":"Screen Voltage (max)","value":"250","numeric_value":"250","unit":"V","param_group":"absolute_max"},{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"7.5","numeric_value":"7.5","unit":"W","param_group":"absolute_max"},{"key":"Ps_max","display_name":"Screen Dissipation (max)","value":"1.5","numeric_value":"1.5","unit":"W","param_group":"absolute_max"},{"key":"Ik_max","display_name":"Cathode Current (max)","value":"50","numeric_value":"50","unit":"mA","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"200","numeric_value":"200","unit":"V","param_group":"typical_operating"},{"key":"Vs","display_name":"Screen Voltage (typical)","value":"150","numeric_value":"150","unit":"V","param_group":"typical_operating"},{"key":"Vg","display_name":"Grid 1 Bias Voltage","value":"-7","numeric_value":"-7","unit":"V","param_group":"typical_operating"},{"key":"mAa","display_name":"Anode Current","value":"35","numeric_value":"35","unit":"mA","param_group":"typical_operating"},{"key":"ra","display_name":"Anode Resistance","value":"30000","numeric_value":"30000","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":"4.2","numeric_value":"4.2","unit":"W","param_group":"output"},{"key":"Cgp","display_name":"Grid-to-Plate Capacitance","value":"0.1","numeric_value":"0.1","unit":"pF","param_group":"capacitances"},{"key":"Cin","display_name":"Input Capacitance","value":"9","numeric_value":"9","unit":"pF","param_group":"capacitances"},{"key":"Cout","display_name":"Output Capacitance","value":"9","numeric_value":"9","unit":"pF","param_group":"capacitances"}]},{"section_number":2,"section_label":"Triode Strapped","tube_type_key":"power_triode","specs":[{"key":"Pa_max","display_name":"Anode Dissipation (max)","value":"7.5","numeric_value":"7.5","unit":"W","param_group":"absolute_max"},{"key":"Va","display_name":"Anode Voltage (typical)","value":"235","numeric_value":"235","unit":"V","param_group":"typical_operating"},{"key":"Vg","display_name":"Grid 1 Bias Voltage","value":"-18","numeric_value":"-18","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":"1600","numeric_value":"1600","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":"mu","display_name":"Amplification Factor (μ)","value":"10","numeric_value":"10","unit":"—","param_group":"typical_operating"},{"key":"Pout","display_name":"Output Power","value":"2.5","numeric_value":"2.5","unit":"W","param_group":"output"},{"key":"rl","display_name":"Load Impedance","value":"5000","numeric_value":"5000","unit":"Ω","param_group":"output"}]}],"synced_at":"2026-04-07T12:10:40.634Z"}