List of Audio Tube Measured Noise Performance

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

Hi Everyone,

I updated the measurement list on my website.

1. I included a few more samples of Shuguang 12AX7B and Shuguang 12AT7.

2. I included new measurements made on the JJ EF86 (they call it EF806S). I've read in many places that pentodes are noisier than triodes, but I've never measured it before and was very curious to see how much. The EF86 measurements make it one of the higher noise tubes I measured, but not the highest. I also measured it as a triode, because I've read that a triode-connected pentode should produce less noise. At least in this case, it did not seem to be true. Within the accuracy of my measurements, it was the same. Hmmm......

3. Finally, I have begun to compute sample standard deviation (sigma) instead of posting the min & max for each type. Standard deviation is a better way to compare data sets of different sizes.

Scott

Since my last post here, I’ve updated the list on my website to include more tube types (such as the European D3a), and many more samples of the original tubes. The list at this point includes 95 samples and 15 different brands and types, along with some comments on the measurements and on specific tubes. And it includes the average noise, standard deviation, and number of samples of each type, so statistical significance of the measurements can be computed (if you are so inclined).

You can see the list on my “downloads” page at:

http://tavishdesign.com/pages/downloads

I’d be interested in hearing about other tubes or brands you think should be included.
Scott