12AX7 tube pre-amp

since this tube amp is relatively simple, the wiring will be done on a prototype board. I drew the planned layout on a piece of paper before the actual wiring is done.


Prototype board with the pole and tube socket installed.


Wiring at the back.


Solder the components.




The tube PSU is from a web forum member, it provide the regulated 300V and 12.6V for the amplifier board.

Putting them altogether.


Measurement is done at 2V peak to peak output to a 10k load, using my EM-U 1212m PCI sound card and RMAA 6.1:


Frequency response -

Total Harmonic Distortion at 1kHz -

The harmonic distortion is exactly as what I wanted : some 2nd harmonic distortion at -70dB with very low 3rd harmonics at -100dB. The RMAA measured THD is 0.033%.


There are room for improvement for this simple design : The gain of this preamp is 51 (34dB) ,which is a bit high. The 625 ohm output impedance could be lowered, also higher output current is desired. Therefore I implemented this design with 2 other medium mu tubes to overcome the mentioned disadvantage, see my 12AU7 and 6922 tube preamp.


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