List of Audio Tube Measured Noise Performance

Those are all good comments and suggestions. The current production tubes I tested were not pre-selected in any way. They were purchased in small lots from 3 different retail online tube vendors in the US.

Next time I update the list, I'll include standard deviations (either in addition to, or in place of min/max). I agree that is a better way to judge consistency of a manufacturer. And it is needed to calculate the statistical significance of the measured difference between tubes.

I've read Merlin Blencowe's paper, which is excellent. He also saw a lot of variation between tubes, but did not try to break it down by manufacturer, I believe. His paper has convinced me to test the D3a and similar Russian 6J52P, which I'll add to my list in the future.

We'll see how the list evolves as I test more tubes in the future - I'll post to this forum whenever I update the list.

Scott

This is a great start, Scott. Maybe the manufacturers will start paying attention to these results, once they are more statistically robust. I know I will, my active gain MC phono stage uses 12AX7s. Well done.

I don't get this. Noise from tubes is in my experience a variable related to sampling, and of course high gain tubes will tend to have more noise those that afford less gain, when used in a gain stage. Then too, the circuit design is critical and so are the voltage and current chosen to run the tube. These elements and more constitute a calculation of noise that has so many variables as to make the outcome unpredictable in advance. IMO, you have to measure noise in-circuit and experiment to determine whether substitution of one 12AX7 (for random example) with another 12AX7 results in audible or measurable improvement.

"High gain tubes will tend to have more noise than those that afford less gain"

Truer words were never spoken but the real question is how much noise is introduced by different types of the same tube (high or low gain).

Hi Lewm,
Thanks, you have raised some important issues. It is not true that high gain tubes are necessarily noisier than low gain tubes. Tubes, like transistors, are characterized by their input-referred voltage noise (EIN) and input-referred current noise (IIN). The test circuits that I have used have a low input impedance, similar to the impedance of a moving magnet phonograph cartridge, and in such circuits only EIN is important. The high gain 12AX7 has a lower EIN than the low gain 12AU7 and therefore will have a higher signal-to-noise ratio, as an example. Since a lower gain tube amplifies both the signal and the noise less, sometimes people perceive the lower gain tube to have less noise, sort of like turning down the volume control. But the tube with lower EIN will actually produce a better signal-to-noise ratio, regardless of its gain.

The circuit does influence EIN to some extent, but in my test circuits, as in most low noise circuits, the circuit is designed such that the tube EIN dominates the noise performance. The test circuits I’ve used are designed to provide the optimum bias current for lowest tube EIN, and are also designed to be representative of the way the tubes are used in low-noise circuits. Therefore, my measurement results should reflect the way signal-to-noise ratios in audio equipment will be affected by the tubes under test.

Please note that these measurements only characterize the intrinsic noise performance of the tubes (hiss), not their susceptibility to “hum” when AC is used on the tube heaters. In most high-performance audio equipment, tube heaters use filtered (and sometimes regulated DC), so their susceptibility to hum when AC is used on the heaters is not a concern. But since many musical instrument amplifiers do still use AC on tube heaters, I do plan to characterize this “hum susceptibility” in the future – it will be a separate measurement and a separate table in my listings.
I hope this is helpful.
Best regards,
Scott