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

Vacuum tube amplifier schema (LTSpice simulation screenshot)

Schema Il Primo Bis

Power supply schema for the vacuum tube amplifier (LTSpice simulation screenshot)

PoweSupply-IlPrimo Bis

Frequency Response

frequency response




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