Il Primo Bis (12AX7+EL34/6CA7)

The empirical approach that I used tu build “Il Primo” was replaced by a more rigorous approach in “Il Primo Bis”. Here, I tried to use a more formal and rigorous process to the design and value settings of the various parts of the vacuum tube amplifier. Especially, the Power Supply Unit was significantly revised, and component values were chosen with more confidence and appropriateness. This more rigorous approach is the one described in my book.


  • Push-Pull stereo integrated amplifier
  • Max Output Power 2x ~35W
    • @ 1 Volt peak input signal, with 6dB global negative feedback
  • Power Stage: 2x 2 EL34/6CA7 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 40 mA anode current bias
  • Output Transformer: Hammond 1650HA
    • Primary: 6.6K Ohm C.T., 40% screen tap
    • Secondary: 4, 8, 16 Ohm
  • Power Supply Unit Transformer: Hammonnd 373FX 650V c.t@460mA
  • Global Negative Feedback: max 6dB
  • THD (measured with Sovtek 12AX7 LPS + JJ EL34)
    • at 1W: 0.058%
    • at 5W: 0.11%
    • at 10W: 0.17%
  • Two only capacitors in the signal path
  • 4, 8 Ohm Speakers
  • Input Impedance: 100K Ohm
  • Output Impedance: 1.45 Ohm
  • Damping Factor: 5.6
  • 3 selectable input sources

Frequency Response

frequency response



Vacuum tube amplifier schema

>>click on the image to enlarge<<

Power supply schema for the vacuum tube amplifier

>>click on the image to enlarge<<

Construction layout

>click on the image to enlarge<<

List of components

Component: Design values: Quantity: Description: Chosen component:
Power supply:
TR1 650V C.T. @460mA, 6.3V C.T. 1 Power transformer Hammond 373FX 650V c.t@460mA, 50V tap, 6.3V@9A 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 1 Ground lift Wima MKS4 011uF/400V, polyester
C22 4.7uF 100V 1 Bias circuit reservoir Nichicon KW 4.7uF 100V, UKW2A4R7MDD, condensatore elettrolitico radiale
C23 8uF 100V 1 Bias Smothing Filter Nichicon FW 10uF 100V, condensatore elettrolitico radiale
C26, C27 6.8nF 100V 2 Heater short to ground Wima FKS2 0.0068uF/100V, polyester
D1, D2, D3 UF4007 3 Rectifiers
R1, R2 120 5W 2 Power stage smoothing filter
R3, R4 10K 0,5W 2 Splitter stage smooting filter
R5, R6 82K 0,5W 2 Input stage smooting filter
R7 180K 2W 1 Heater elevation
R8 15K 0,5W 1 Heater elevation
R33, R34, R35, R36 33K 0,5W 4 Bias circuit voltage divider
R37 10K 0,5W 1 Bias circuit voltage divider
R38 15K 0,5W 1 Bias Smothing Filter
R45 120Ω 0.5W 1 Ground lift
VR3 220k 1 Trimmer for bias circuit Elecsound 3296W 200K, vertical multiturn, 3296W-1-204
VR4, VR5 25K 2 Trimmer for bias circuit Elecsound 3296W 25K, vertical multiturn, 3296W-1-253
Power Stage:
TR2 6.6k Ohm C.T. 40% U.L. Tap 2 Output Transformer Hammond 1650HA 6600 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 0.22uF 400V 4 Coupling cap Mundorf MCAP EVO OIL 0.22uF 450V, polypropylene
R21, R22,R23,R24 4.7K 0,5W 4 Grid Stopper
R25, R26,R27,R28 82K 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 EL34 4 Power tubes JJ EL34, Matched Quartet
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 220pF 500V 2 Feedback circuit step net cap Silver Mica 220pF/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 6.8K 0,5W 2 Feedback circuit R1
V5,V6 12AX7 2 Input and splitter tubes JJ 12AX7, matched pair
VR1 100K Log Stereo 1 Volume


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