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