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I have been promising a current test of some of the current production 12AX7 / ECC83 offerings and have just completed my testing.
Gain and Transconductance. Some folks confuse these two things. Maybe this will help explain:
Gain is the factor by which an amplifier multiplies a signal. It is the product of the output voltage at the plate divided by the input voltage at the control grid. Gain is a unit-less factor since both measured signals are voltages. (voltage)
Transconductance is a measurement of the ability of a tube to transmit current. It is the product of the output current in amps divided by the input voltage at the control grid. Transconductance is significant in that it shows us the difference in current capability that is generated by variations in plate impedance between tubes. (Current)
When I test tubes I also measure standing plate emissions current for preamp tubes that will be used in the phase inverter position. Plate current is one factor that many tube testers do not measure.
150 tubes were tested. 15 of each type. The above lines on each tube are the average of that particular tube.
Click on "read more" to continue.
If you look at the hard cover Tube Amp Book from Groove Tubes you will find pieces by me. There are also QA numbers on some current production tubes that are quite good. There are two reasons for this. One reason was that only one factor of four (plate resistance, plate current, true gain, mutual conductance / transconductance) was used in some cases. The other reason is that some tubes tested were GT Gold Series which were already tested for some factors. I look at all these factors currently and publish the WORST number of the four factors. If any one factor is off the tube is not right. If plate resistance is off there will be all sorts of issues that range across sonic, life, heat, reliability, crosstalk across triode sides, low gain, high plate current and high heat and more.
These tubes come from the same places that most amp builders procure their tubes from. They are NOT from any of my favorite vendors who test and grade tubes. These are supposedly tested by the manufacturer which is sometimes doubtful based on my observations.
I have the tubes shipped to friends in various spots in the USA and then sent to me so the vendor does not know they are for me. I do not want cherry picked items, I want what amp makers receive. There are a high percentage of tubes which are microphonic. Some vendors will let you ship them back for replacement. Some will not.
When I was at GT for a half a dozen years they had a no questions asked 180 day warranty on preamp tubes. That was pretty cool. Their reject rate before they were put in stock was generally 50% or higher. They did do very good testing on preamp tubes (Gold Line ... Silver series were a different story). On Ei long smooth plate the reject rate at GT was commonly 90%. GT eventually stopped even trying to offer these.
What I learned from these tests is as follows:
1. Know and trust your tube vendor. Tubes procurement is a crap shoot at best and the odds are far better in Vegas.
2. There are more models and variants of tubes made today than there were five years ago. The quality is worse.
3. There are multiple versions of the same tube with either different names, a slight cosmetic change such as shape of the getter, gold pins, etc. Gold pin tubes had a higher cost but were generally more close in tolerance to spec. Not good for the most part, just not as bad.
4. Know and trust your vendor.
5. My heart goes out to amp repair folks and amp manufacturers. It is difficult to find good tubes for one amp. Finding enough for production must be virtually impossible at times.
6. Know and trust your vendor.
7. The most consistent tubes AND most close to design spec were Ruby Tubes Chinese 12AX7C tubes. I am not too surprised as Tom McNeil works very closely with the Shuguang factory and his testing parameters are held to closer tolerances it seems.
8. Tubes that could be abused the most, vibrated, run at over voltage; Sovtek 12AX7WA and JJ ECC83S. Both short plate designs. The 12AX7WA is the workhorse of the industry as it is cheap and the low gain makes it more forgiving in noisy designs.
9. Know and trust your vendor
10... and lastly .... know and trust your vendor.
Two vendors I use and trust, and have for years:
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I just added the above to include the Tung Sol Reissues from New Sensor. These tubes are VERY similar to the EH and EH Gold pin. The item in the list is called the ECC803S. It has gold pins. It may be more highly selected than their non gold pin version. The mica supports for the internal structure are star micas which may or may not lead to less microphonics than the less pricy version which sells for around $15 retail as opposed to about twice that price for the ECC803S version.
5/9/13 Added two more popular 12AX7s to the report. If you have a request on a common production 12AX7 you wish to be tested please let me know.
I also have many power tube tests published in the past. Power tubes are a bit pricy to test as you need at least a dozen of each from the same run so these tests are not done as often.
I will update this post with new data as it is available.
I mostly use JJ's but sino-chinese are good for high gain apps. got any info on the sino tubes?
Below is my response to the question.
Sino is not really a company. It is a conglomerate of smaller tube companies in China. Their offerings are very inconsistent. Chinese is really Shuguang for the most part for the most reliable and consistent Chinese offering. Even then they can be short lived and inconsistent unless the contractor who uses them as their OEM is on top of them every minute. When I was at GT I had face to face meetings with them every month for years. The 6L6GE was a great tube. The 12AX7M started out as the greatest 12AX7 maybe ever made and turned into the worst nightmare and most inconsistent and unreliable tube ever made. Their 6L6 tubes are generally of very old design that was cheap to produce then and very poor now. They are generally rebranded as "PM" tubes and some other bargain brands. They have a very short life and are very weak.
Did I even answer the question here? Probably not. The answer really is there are no specific Sino tubes. What you buy today is not the same tube as you will buy tomorrow.
Hmmm... I think I will copy and past this into my blog piece for others that wonder why I don't test Sino tubes.
Where do tubes actually come from? Here is a link to a piece on my GAB website that is a bit old and may be a bit out of date but there is some good info in there if for nothing more than for history:
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5/22/13 Update - Phase Inverter tip.
If your tube vendor can measure plate current this is what should be done for tubes for PI use rather than use mU or transconductance measurements. It is current, not gain, that provides the drive for your output tubes.
In the case of a 12AX7 the proper spec is 1.2mA. Too many tubes today fall far below at 0.7-0.9. Low plate current will make the amp loose, flubby, indistinct. They will also distort earlier and pass this right to your output tubes giving you less headroom than the amp was designed to provide.
If a 12AX7 has too high of plate current, above 1.4mA, this is also bad. It indicates that the plate resistance is too low and the tube was not made properly. It is a reject. In this case the plate current is high because of improper plate resistance. The tube will not have the proper gain and will run hot.
10mA is the spec for a 12AT7 as a side note.
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The Northern Electric 12AX7 is made with the highest quality materials. From the glass bottle to the gold plated pins, everything is designed to give you unsurpassed audio quality. This tube has very large smooth plates. This design is much like a Telefunken 12AX7 / ECC83 smooth plate but a bit shorter. This design provides plenty of gain while maintaining an extremely low noise floor. It has been torture tested in a JTM 45, a Deluxe Reverb, an Orange 50 watt combo, and Bandmaster Reverb, with no failures or noise problems after hundreds of hours in clubs and studios. It doesn’t matter if you use an American or British design, this tube works.
In our listening tests it was easy to hear that the tube did not change the voice of the amplifier as some preamp tubes do. The Northern Electric enhances the sound across the entire spectrum. Our testers have reported no noticeable spikes or weakness in the lows, mids, or high end. The highs are very smooth with a balanced midrange response and warm, full bottom end.
6/13/13 - A bit of dialogue and my reply on Z-Talk
From the Tube Store website: "Northern Electric had a rich history of producing high quality audio products and vacuum tubes in Canada. The original owners, Bell Canada and Western Electric, formed this company to produce telephone and audio equipment including vacuum tubes. Western Electric licensed their designs, processes, and technology to Northern Electric for the first fifty years of their production.
We are now re-introducing this classic brand and bringing to you the highest quality audio tubes possible. We are working together with a small, foreign tube manufacturer to produce exceptional high quality versions of common audio tubes. And while this manufacturer is not well known, they are the “Ferrari” of tube manufacturing and design.
The manufacturer’s immaculately clean factory houses state of the art and high precision equipment. All of the design and production is overseen by a living legend in tube design, Zhe Shen Liu.
We are excited and pleased to present our new reissue Northern Electric audio tubes."
So am I reading this correct Myles, Made in China, shipped from Canada?
If that is what their data says I will not question it.
The issue I have with Chinese tubes built under a specific contract is that the initial runs are fantastic as the maker goes out of their way to meet promises and establish the brand. As time goes on things decline. In my own experience it is not an up and down path but more of a down path. In the case of the GT12AX7M the path went from the best 12AX7 ever made to the worst ever made in a span of about four years over a dozen or so production runs.
At the moment the tube is a winner but will this remain the case? Only time will tell. The Chinese can make some great products but their consistency is not good at all. I would not be surprised if the formula on the materials change to maximize their profit (the factory) while charging The Tube Store the same price. I would not be surprised to find the tubes tolerances drift as labor time is reduced and watch microphonics rise and specs drift. If they are shipping a lot of these the changes will come sooner than later. Perhaps the factory has finally learned that consistency is the key and when you have a good thing going to leave it alone. This seems to go against Chinese business culture but I hope I am wrong.
As a side note I am going to copy and paste your info into the blog piece along with my reply here. We will see what happens down the road.
|The test results for these are the last entry, 7/22/2013 |
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|6/10/2013||Northern Electric 12AX7||1.3||1610||0.0165||60.6||97.58||$57.95 but they work. QA = 34% - xlnt|
|7/23/2013||Northern Electric 12AX7||1.0||1420||0.0158||63.3||89.87||New test. QA now 51% - moderate|
Currently testing the TAD 7025S. It is a $35 tube that is supposed to be a copy of the famed Mullard.
So far things look like .... well .... TAD works with Shuguang and TAD used to steal GT parts (or Shuguang would back door them to TAD on their 6L6). Lawsuit ensued and GT won.
Bottom line .... these TAD 7025S tubes sure look and trace like the last and worst version of the GT 12AX7M. Perhaps the Chinese found a way to collect some more money from old GT tooling and designs? I will know more in a few days.
7/28/13 - TAD 7025S results added in last chart entry below.
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I generally test new/current production tubes but I also test tubes from friends and associates which include audiophiles and engineers.
Last week I was brought 27 Raytheon tubes made in 1962. These were NOS / NIB (new in box). Their condition was unknown as the boxes were unmarked. My usual sample quantity is generally larger than 27 tubes but there were enough for a decent test run so I thought I would add the results to my ongoing tests.
As a side note I have data compiled from 2002 and all my data from six plus years as GT Tech Support / SAG if folks want any of that info but it is too verbose to post here. This info is not available for simple curiosity but rather for those folks active in tube development or manufacture.
Bottom line on the test results was no surprises. The tubes all met design spec (actually all of them did better than design spec). The tolerances for plate current, gain, mutual/transconductance, plate resistance were all tight, proper and consistent. The curve traces were all smooth and the A and B sides of every tube were very closely matched. In the 27 samples, 22 of them had curves that were right on top of each other meaning the tubes were matched across the entire operating range, not just a single static point.
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