Il Terzo (12AX7+EL84)

“Il Terzo” in English means “The Third”. This vacuum tube amplifier came after the “Il Primo” and “Il Primo Bis“. I used EL84 power vacuum tubes and 12AX7 vacuum tubes as input and phase split. Also here, I designed it using the approach described in my book. More specifically, the design of this vacuum tube amplifier is described in details in “Chapter 6: Step by step design of a push-pull tube amplifier .


  • Push-Pull stereo integrated amplifier
  • Max Output Power 2x ~12W
    • @ 1 Volt peak input signal, with 16dB global negative feedback
  • Power Stage: 2x 2 EL84 power vacuum tubes in Push-Pull Ultra-Linear configuration and fixed bias
  • Phase Split Stage: DC coupled Concertina with 12AX7 vacuum tubes
  • Input Stage: 12AX7 vacuum tubes with self bias and bypass capacitor
  • Class AB with 28 mA anode current bias
  • Output Transformer: Hammond 1650E
    • Primary: 8K Ohm C.T., 40% screen tap
    • Secondary: 4, 8, 16 Ohm
  • Power Supply Unit Transformer: Hammond 370KX 500V c.t@322mA
  • Global Negative Feedback: 16dB
  • THD (measured with Sovtek 12AX7 LPS + JJ EL84)
    • at 1W: 0.05%
    • at 5W: 0.12%
    • at 10W: 0.18%
    • Measures of THD with other tubes is reported here
  • Two only capacitors in the signal path
  • 4, 8 Ohm Speakers
  • Input Impedance: 100K Ohm
  • Output Impedance: 0.4 Ohm
  • Damping Factor: 18
  • 3 selectable input sources

Vacuum tube amplifier schema (LTSpice simulation screenshot)

Schema Il Terzo

Power Supply Schema for the vacuum tube amplifier (LTSpice simulation screenshot)


Frequency Response

frequency response


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