12AX7 vacuum tube comparison: THD measures

We all know vacuum tubes are different. If you change type or brand you get different characteristics and different audio quality. However, what is the real difference between different vacuum tubes? Here I report the results of a comparison of 12AX7 vacuum tubes, when used with the same Hi-Fi amplifier. Specifically, I measured the Total Harmonic Distortion (THD) of different 12AX7 (ECC83) vacuum tubes at 1 kHz, at various output power levels.

Below you will find the settings for the tests. If you prefer, you can skip these details and jump directly to the results.

Test settings

12AX7 vacuum tube comparison test were executed with “Il Terzo“, one of the amplifier I built. It uses EL84 output tubes, configured in ultralinear push-pull, with 16dB of global negative feedback, able to deliver 12W maximum output power. Input stage and phase split stage use 12AX7 tubes. Phase split stage is a concertina configuration, directly coupled with the input stage. The schema of the input and phase split stage is shown below. See “Design of the input and phase splitter stage of a vaccum tube amplifier“, for additional details.

Schema of the input and phase-split stage

Load of the input stage is 220k Ohm. Grid bias voltage is obtained using self-bias with a 870 Ohm resistor connected to the cathode. The concertina has a load of 200k Ohm divided between 100k Ohm to the anode and 100k Ohm to the cathode. Grid leak resistors of the power stage (not shown in schema above) are 200k Ohm each. The grid of the concertina is directly coupled to the anode of the input stage, which provide also the correct bias voltage.

Power supply for the input stage is 280V, and that of the concertina is 290V. This configuration produces the loadlines and operating points below. Both concertina and input stage work in a fairly linear area.

Loadlines and operating points of the inpit and phase-split stages

Results:

Disclaimer:
12AX7 vacuum tube comparison results presented here have to be interpreted uniquely taking into consideration the input and phase split stage configurations of my own design of the Il Terzo vacuum tube amplifier, used for the tests. In particular, these tests do not pretend to provide general results for all possible configurations. There are certainly other configurations where produced results, and relative ranking among the tested tubes, will significantly differ from those obtained here.

I tested four different 12AX7 vacuum tubes:

12AX7 vacuum tube comparison results are depicted in the plot below where, for each test, the THD at 1 kHz is reported at various output power levels, raging from 1W to 10W. Two vacuum tubes were tested for each type (left and right channel). The graph shows that small differences were measured between two different tubes of the same type. However, more significant differences were measured among different type.

12AX7 vacuum tube comparison: THDs  when used with “Il Terzo

It is important to point-out that THD, measured during 12AX7 vacuum tube comparison, is not the THD directly produced by the tested tubes. Rather, what is reported here is the THD produced by the entire amplifier, using the tested tubes. THD measured is affected by various aspects, including the output power tubes, the global negative feedback, and the schema of the amplifier. The reported values allow comparing how the tested tubes behave with this specific amplifier. Completely different results might be obtained with a different amplifier design.

Let’s discuss the results obtained.

12AX7WB by Sovtek

THD resulting from 12AX7WB tubes, made by Sovtek, was generally high at all output power levels. THD at the minimum tested output power (1W) is around 0.7%. This is a value higher than THD measured on all other tubes at their maximum output levels. THD produced with 12AX7WB goes up to 1.7% at 5W, and arrives up to 2.6% at 10W, with one of the two tested tubes.


ECC83S-12AX7 by JJ Electronics

The two tested ECC83S-12AX7 by JJ Electronics provided us with a THD of respectively 0.14% and 0.16%, at 1W output power level. The THD went up respectively to 0.32% and 0.4%, at 5W. At 10W output level, the THD, with the two tested tubes, was respectively 0.75% and 0.93%. Note that at 10W output power level, the measured THD is now lower than 1%.


12AX7 Genalex Gold Lion

THD slightly improves, when using 12AX7 Genalex Gold Lion. When the amplifier provides 1W output power, THD goes down to 0.11% on one tube, and to 0.08% to the other. THD measured at 5W output power is respectively 0.27%  and 0.28%, with the two tubes. When output power delivered by the amplifier is brought to 10W, the THD that measured is respectively 0.43% and 0.57%. THD is significantly moving far from 1% at 10W output power, with these vacuum tubes.


12AX7LPS by Sovtek

Finally let’s give a look at the results obtained by 12AX7LPS, produced by Sovtek. These vaccum tubes provided us with the best results. At 1W output power, THD now goes down to 0.06% and 0.05%, respectively on the two tested tubes.  Also at 5W output results were excellent, with a THD of 0.15% and  0.12% respectively. At the maximum tested output power level THD is 0.25% and  0.18%. These values are generally one order of magnitude better than 12AX7WB, also produced by Sovtek.


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