12AU7 / 5814 tube pre-amp


After finished my simple 12AX7 tube preamp, I decided to try other medium mu tubes, including this 12AU7 and also the 6922.


The design is exactly the same as the 12AX7 preamp, it is a single gain stage triode tube with cathode follower. Schematics :


Gain stage loadline :

Cathode follower loadline :

I am not going into the details that how the loadline is chosen and how the resistor values are calculated, please reference to my 12AX7 preamp page if you want more information.


The gain for this amp can be calculated by this formula :

Gain = (Tube Gain x Plate load resistor)/(plate load resistor + plate resistance + ((tube gain +1) * cathode resistor)).


With 12AU7, the plate resistance = 7.2k, tube gain = 17. Using these value in the above formula, the calculated gain is 10.


The output impedance of the 1st gain stage is equal to the plate load resistor paralleled with the plate resistance, so in my case it is about 5.81k, which is a bit high. So a cathode follower is needed to reduce the output impedance. With the cathode follower, the output impedance would roughly equal to 1/(tube's transconductance). For 12AU7, its transconductance is about 3100 micromhos. So the cathode follower's output impedance would be at about 322ohms


Components selection : 5814A tube from JAN Philips.


The construction is the same as my 12AX7 preamp, only the tube and resistors values are different.

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 RMAA measured THD is 0.058%. The THD is higher than the 12AX7. However, the harmonic contents are different, the 2nd harmonic distortion is a bit higher at -65dB. The 3rd harmonics is lower at -105dB.


Because the gain of this preamp is only 10, this helped to improve the signal to noise ratio. This results in a noise floor of about 10dB lower than the 12AX7. If you want a low noise tube preamp, this would be your choice.




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