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

Frequency Response

frequency response

Vacuum tube amplifier schema

 >>click on the image to enlarge<<


Amplifier Schema for Il Terzo – 12AX7/EL84

Power Supply Schema for the vacuum tube amplifier

 >>click on the image to enlarge<<

Power supply Schema for Il Terzo – 12AX7/EL84

Construction layout

 >>click on the image to enlarge<<

Layout Schema for Il Terzo – 12AX7/EL84

List of components


Design values:



Chosen component:

Power supply:        
TR1 500V C.T. @300mA, 6.3V C.T. 1 Power transformer Hammond 370KX 500V c.t@322mA, 50V tap, 6.3V@6A section, 5V@6A Section
C1, C2 100uF 450V 2 Power stage smoothing filter F&T Fischer & Tausche 100uF/450V, axial electrolitic
C3, C4 47uF 450V 2 Splitter stage smooting filter F&T Fischer & Tausche 47uF/450V, axial electrolitic
C5, C6 4.7uF 450V 2 Input stage smooting filter F&T Fischer & Tausche 4.7uF/450V, axial electrolitic
C7 22uF 500V 1 Reservoir F&T Fischer & Tausche 22uF/500V, axial electrolitic 18x30mm
C8 10uF 100V 1 Heater elevation Nichicon FW 10uF 100V, radial electrolitic
C21 0.1uF 400V   Ground lift Wima MKS4 0.1uF/400V, polyester
C22 1uF 100V 1 Bias circuit reservoir Nichicon KW 1uF 100V, UKW2A010MDD, radial electrolitic
C23 8uF 100V 1 Bias Smothing Filter  Nichicon FW 10uF 100V, condensatore elettrolitico radiale, UFW2A100MED
C26, C27 6.8nF 100V 2 Heater short to ground Wima FKS2 0.0068uF/100V, polyester
D1, D2, D3 UF4007 3 Rectifiers  
R1, R2 330 5W 2 Power stage smoothing filter  
R3, R4 5.6K 0,5W 2 Splitter stage smooting filter  
R5, R6 12K 0.5W 2 Input stage smooting filter  
R7 150K 1W 1 Heater elevation  
R8 15K 0.5W 1 Heater elevation  
R33, R34, R35, R36 150K 0.5W 4 Bias circuit voltage divider  
R37 10K 0.5W 1 Bias circuit voltage divider  
R38 27K 0.5W 1 Bias Smothing Filter   
R45 120Ω 0.5W   Ground lift  
VR3, VR4, VR5 10K 3 Trimmer for bias circuit Elecsound 3296W 10K, trimmer vertical multiturn
Power Stage:        
TR2 8k Ohm C.T. 40% U.L. Tap 2 Output Transformer Hammond 1650E 8000 c.t. 40% screen 4,8,16 Ohm
C13, C14,C17,C18 0.47uF 100V 4 Grid leak short to ground cap Epcos TDK 0.47uF/100V, MKT metal polyester
C15, C16,C19,C20 0.1uF 400V 4 Coupling cap  Mundorf MCAP EVO OIL 0.1uF 650V, polypropylene
R21, R22,R23,R24 1K 0.5W 4 Grid Stopper  
R25, R26,R27,R28 200K 0.5W 4 Grid leak  
R29, R30,R31,R32 10Ω 0.5W 4 Cathode Probe   
R39, R40,R41,R42 1K 1W 4 Screen resistors  
V1, V2,V3,V4 EL84 4 Power tubes JJ Electronic EL84 (6BQ5)
Phase splitter stage:        
R17,R18,R19,R20 100K 0.5W 4 Cathode/Anode Resistors   
Input Stage:        
C9,C10 0.22uF 100V 2 Coupling cap Mundorf MCAP EVO OIL 0.22uF 450V, polypropylene
C11,C12 150uF 100V 2 Catode bypass cap  Sprague Atom 150uF/50V, electrolitic axial
C24,C25 500pF 500V 2 Feedback circuit step net cap Silver Mica 500pF/500V,
R9,R10 47K 0.5W 2 Grid Stopper  
R11,R13 150Ω 0.5W 2 Feedback circuit R2   
R12,R14 680Ω 0.5W 2 Cathode resistor   
R15,R16 220K 0.5W 2 Anode resistor  
R43,R44 2,2K 0.5W 2 Feedback circuit R1   
V5,V6 12AX7 2 Input and splitter tubes Sovtek 12AX7 / ECC83 LPS
VR1 100K Log Stereo 1 Volume   


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6 thoughts on “Il Terzo (12AX7+EL84)

Add yours

  1. Thank you, you have inspired me. For some reason I’ve bought a cv2666 off eBay to build an amp. Whilst looking for hints on circuits, I’ve found your article. I like the simplicity of your circuit. I’m thinking of using your prestage to power the dual tetrode. Would you mind please?
    Thank you for posting your article.
    Cheers Tim

    1. Hi Tim,
      Note that the CV2666 can also work with positive grid (Class AB2), according to its datasheet. This prestage circuits is not meant to work with positive grid of the power stage, where the input impedance of the power stage goes down a lot. So, if you use it, configure your power stage to work only with negative grid (Class AB).

  2. Thank you so much, I wasn’t aware of that. I haven’t built any valve circuits for forty years, so relearning so much. Your son clearly has taste!

  3. Hello,

    Wonderful project! Congratulations, I wish I could build one for myself, but first I have to study and learn to read schematics properly. Built a few guitar amps, but this is a different field of expertise. Is the construction layout accurate?

    best regards,

    1. Hi,
      Layout should be fine
      Up to now, nobody mentioned issues on it. However, if you have doubts or if you find something weird, do not hesitate to contact me.
      Given that you built guitar amplifiers, reprducing it should not be a problem.
      Let me know!

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