• Item: "Il Terzo" - EL84/12AX7  
  • Type: Push-Pull Integrated Amplifier  
  • Posted: 23 September 2018  
  • Builder: gamato  
  • Country: Italy  
  • Comments: 2  

"Il Terzo" - EL84/12AX7

Il Terzo, which in English means “The Third”, is the third vacuum tube amplifier I built. It uses EL84 power tubes and 12AX7 vacuum tubes as input and phase split. Its design is precisely discussed and described here:https://www.vtadiy.com/book/chapter-6-step-by-step-design-of-a-push-pull-tube-amplifier/

Features of the project are:

-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%
-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


  • Aros71
  • 24 September 2018 at 11:47PM
  • It looks great. You're using the same OPTs as me :)

    Interested to see how you got enough open-loop gain to allow 16dB of gNFB, I'll study your schema carefully.

    Everything is point-to-point - no PCBs?
  • gamato
  • 3 October 2018 at 05:22AM
  • Hi Aros71,
    sorry I did not notice your comment before.

    Yes I did not use PCBs. All point-to-point.

    Discussion on how I obtained 16dB of gNFB is here: https://www.vtadiy.com/book/chapter-6-step-by-step-design-of-a-push-pull-tube-amplifier/6-3-global-negative-feedback/

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