Il Primo (12AX7+EL34/6CA7)

This is the first vacuum tube amplifier I built. I started learning about the art of vacuum tube amplifier design and build in 2014. After a long process of trial and error I built “Il Primo”, which in English means “The First”. It uses EL34/6CA7 power tubes and 12AX7 as input and phase split.

Characteristics:

  • Push-Pull stereo integrated amplifier
  • Max Output Power 2x ~30W
    • @ 1 Volt peak input signal, with 6dB global negative feedback
    • @ 0.5 Volt peak input signal, with no 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: Custom
    • Primary: 7.6K Ohm C.T., 40% screen tap
    • Secondary: 4, 8, 16 Ohm
  • Power Supply Unit Transformer: Custom 650V C.T. @400mA
  • Global Negative Feedback: max 6dB adjustable
  • THD (measured with Sovtek 12AX7 LPS + JJ EL34)
    • at 1W: 0.05%
    • at 5W: 0.08%
    • at 10W: 0.24%
  • Two only capacitors in the signal path
  • 4, 8, 16 Ohm Speakers
  • Input Impedance: 100K Ohm
  • Output Impedance: 1.2 Ohm
  • Damping Factor: 6.5
  • 6 selectable input sources
  • Bluetooth input

Vacuum tube amplifier schema (LTSpice simulation screenshot)

Schema Il Primo

Frequency Response

risposta in frequenza

THD

THD

Gallery

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