Ef50 Single Ended Power Amplifier


The EF50 is an RF power pentode which in this circuit is being used as a triode.
To find how to use a pentode as a triode, click here.
The maximum peak audio output power is about 0.5W without distortion.
The cathode heater voltage for this tube is 6.3V. The supply for this is omitted from the schematic for clarity.
T1 is a 240V to 12V mains step-down transformer. The 240V side connects to the tube while the 12V side connects to the loudspeaker.
B+ voltage should not exceed 350V.
You can find the EF50 datasheet, with the pinout here.




  • V1-EF50 pentode wired as triode
  • V2-EF50 pentode wired as triode
  • T1-240V:12V mains step-down transformer
  • R1-1K
  • R2-700K
  • R3-100K
  • R4-100R
  • R5-700K
  • R6-10K linear potentiometer
  • C1-100nF
  • C2-100nF
  • SPK1-8 ohm loudspeaker
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