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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