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 |
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 |
Hi Bdp24,
Good question. The intrinsic, input-referred voltage noise of a tube (EIN) should be the same regardless of how it is measured. On my “downloads” page, I’ve provided a lot of technical details about how I made the measurements, so that anyone who wants to repeat them can do so. I’ve also compared my results to other tube noise measurements in the technical literature, which have shown comparable numbers. So I believe my measurements are correct and reproducible, and anyone who measures the same thing should get the same answer.
I visited the webpages for RAM Tube Works and Upscale Audio, and although I may have missed it, I did not see any technical details about how their noise measurements are made. I also did not see that they are posting the actual EIN or noise figure of the tubes they sell (again, I may have missed it). What they seem to be doing is selecting and grading the noise levels of tubes of a given brand and type, which is fine. Other tube vendors do the same. But since they do not specify the actual noise level in a technically precise way, you can’t make comparisons between the various tube types and brands.
It is an odd situation – no one would buy a low noise transistor unless the manufacturer or vendor specified the EIN or noise figure. But no manufacturer or vendor that I know of provides this data for tubes.
If these sites (or other sites) are actually providing measured tube noise data, please point me to it, I haven’t found it yet.
Thanks,
Scott |
Hi Lewm,
I'll put it on my to-do list. The only current-production ECC99 is JJ - is that what you use? What bias current do you run the ECC99 at?
Scott |
Hi Lewm,
Intrinsic noise is not important if a tube is used at high signal levels. It is only important in the first stage of an amplification chain, where it can set the signal to noise ratio of the system.
You mentioned that you use the ECC99 in the first stage of a phono preamp, as the upper device in a hybrid cascode. In that location, intrinsic noise would definitely matter, although not as much as the lower device in the cascode. The ECC99 is a good performer, within a couple of dB of the best current production tubes I've measured.
I've updated the project page on my website again, to include the actual noise measurement histograms for the JJ 5751, JJ E88CC, and Tung Sol Reissue (Reflector) 6SL7GT. It is interesting to compare the three different plots - they tend to suggest that JJ is having some trouble getting consistent noise performance for their E88CC. The JJ 5751, on the other hand, shows fairly tight distribution and pretty consistent low noise performance.
Scott |
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 |
Hi Lewm,
Thanks, another good question. If I understand correctly, you are asking if the intrinsic noise (EIN) of a tube really makes any practical difference in your audio equipment. It will only matter if the tube is the first active device in a signal chain, that is, the first tube in a phono stage, the first tube in the analog section of a hybrid DAC, or the first tube in a line amplifier or power amp. In that case, the tube EIN should determine the signal-to-noise ratio of the equipment (unless the equipment is poorly designed and has, for instance, a lot of hum).
Whether the signal-to-noise ratio matters depends on your signal source. Let’s take a phono stage as an example. The best signal-to-noise ratio achievable on an LP is around 70dB, I believe (it is often much worse). A tube phono stage using one of the lowest noise tubes on my list should have an 80dB signal-to-noise ratio, the way such things are usually measured (A-weighted and referred to an input level of 5mV at 1kHz). But one of the noisier tubes on my list may give a signal-to-noise ratio 8 – 10 dB worse, comparable to the surface noise on a very quiet LP. You wouldn’t want that. So it could make a practical difference, yes.
I haven’t measured the ECC99, but I have some on hand and could do so. I had not considered the ECC99 as a preamp tube, but it might be worth measuring. Thanks.
Best regards,
Scott |
Hi Everyone,
I updated the measurement list on my website "Downloads" page to include additional samples of the JJ 5751, and also an additional tube type, the JJ ECC99.
http://tavishdesign.com/pages/downloads
The JJ 5751 continues to be one of the most consistent and low-noise tubes in my study. But I’ve noticed that as more tubes are measured, the average input-referred noise tends to rise. This is true for all tube types, and it is because as more tubes are measured, it is more likely to get a high noise tube that raises the average. The noise distribution is actually not Gaussian – while there seems to be a lower limit for the noise of a given type, there is no upper limit, so there is a tighter distribution on the low side of the average and a wider distribution on the high side of the average. In the future, I may begin reporting the median as well as the average for each type.
The ECC99 is a high transconductance tube that I believe was originally intended for RF applications. I’ve experimented with it as a driver tube for my hybrid amplifier, but had not considered it as a low noise tube until Lewm on this forum suggested it to me. The ECC99 has transconductance comparable to an ECC88, but without the extremely tight electrode spacing or wide bandwidth.
I’ve only measured 4 samples of the ECC99 so far, but it seems promising. The average input-referred noise of 0.711 µV RMS puts it in the middle of my list, and I intend to measure more. One of the 4 samples had “popcorn” noise and was discarded.
Scott |
The list of tube noise measurements on our “Downloads” page has had several updates since my last post here. It now includes measurements on the Toshiba 2SK209 JFET, which is an interesting comparison to the tubes and provides a sanity check on my measurement technique.
The list also now includes the Sovtek 6C45, which has the lowest measured noise of tubes on the list, but the samples tested were also microphonic.
Samples of the Sovtek 6H9C / 6SL7GT have shown very low intrinsic noise, comparable to the best 12AX7s tested, and 3dB lower than the Tung Sol Reissue 6SL7GT. I’ve only tested 3 samples of the 6H9C but intend to test more.
I’ve also recently tested the RU-6H6, a NOS Russian tube comparable to the JJ ECC99. And there are additional samples of many previously tested types, so the statistics are becoming more meaningful.
Scott |
