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Sorry, there is a bit of a 6SL7 hype at the moment. We sell more 6SL7 as we can buy. These are all we have for sale at this moment.

6188 is a miraculous tube, fully upwards compatible to 6SL7.

From the official datasheet (link here) the difference can be seen when comparing it with the commercial version. Here is a quote from the 6188 datasheet:

• ... made for applications that require extreme stability, such as balanced amplifiers....
• ... lower spread of characteristics such as Plate current, Transconductance and gain...
• ... low noise (limits are in datasheet)
• ... Higher grid resistor possible
• ....Guaranteed imbalance between two systems maximum 10%

Look at the above... This means there is NO NEED for selecting those in pairs. Just take two, and it's a pair!

JAN SELECTED. If it is those that you can buy, there even better. JAN testing is the craziest there is. The manufacturer has to quality for this approval, and document anything they say and do, and accept unannounced control visits. From each lot, they have to take a sample quantity. Stress those tubes much above maximum limits... Read the rest here.

I have never seen NOS Siemens 6SL7 until recently at the same famous auction site where miracles can be bought along with new socks and a pair of shoes. NOS Siemens appeared suddenly out of nowhere, in massive quantities, in crispy new boxes, that look like made yesterday. The printing is so clear and bright, also that looks like printed on it just yesterday too.

Not with all tunes, but sure with 6SL7, the plate system inside, unveils the manufacturer. Though Philips ECG and Sylvania look pretty much alike, this is NOT the same quality. Sylvania 6SL7 is mint quality, but Philips ECG 6SL7 has a bad reputation for sudden vacuum loss. These Siemens are are displayed with this text: "must have been made by Sylvania, for Siemens". Oh, yes...now I see.. so that explains it. Everything is good now.

Stop here! What does this seller say? What means "must have been...". What makes this true? And when not, it is false. Does it really explain THIS? Or does it explain something else? Let's better do our own thinking.

This is reality: 6SL7 was a "junk-that-nobody-needs" tube ever from 1950 until the year 2000. For half a century, they were regarded crap. And even in 2018, while I write this, it is not the tube everybody is desperately looking for. Though 6SL7 is similar to ECC83, which NOS version is very expensive, and 6SL7 is even older.

So with 6SL7 there was enough supply and little demand for 50 years, which includes the 1960's and 70's when tubes were simply disposed of when they were in the way. You need to understand what that does to supplies in stock rooms. A dry, maintained stockroom, has some yearly costs, and people clean it up and look around for free space. And when space is money, 50 years is a VERY long time. So during those 50 years, these cheap crap tubes get always kicked out by somebody. Most of the time on the junk yard, or perhaps they go to somebody who thinks it a pity to throw it away, but he finds out soon they can't be sold. So after all to the junk yard, or perhaps he stores them at a place which cost almost nothing. Like in the basement, a barn, or an attic. For 50 years.....

This is the explanation, why most 6SL7, if in QUANTITIES, are normally in bad optical condition. These lots had to survive decades as forgotten junk.

Knowing this, there is something weird about somebody maintaining a dry storage for 60 years, storing tubes not a soul wanted to buy. And today, after they became expensive after all, they finally can be sold, and of course they come out crispy like now. So who believes this, thinks these tubes were stored for 60 years in a clean and dry stock room and all of that time, nobody needed them, and there was always enough room there, and nobody kicked them out. But honestly, would you (today) store some 20 years old replacement parts in a stock room, which cost you money, while you see definitely no application for them anymore, and you let them in there for 40 more years?

About the other option, they recently are reprinted by some faker, let me say it like this: When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.

Sylvania 6SL7. Military box, with 1966 date code and more information. Lovely, much sought brown base, and colored printing. I am sorry, these are expensive, but tube quality, and old packaging is as NOS as can be. "W" stands for military quality. So this is NOT what was produced by Philips ECG, "last time buy" from the 1980's, which usually has a black base, und used to pop up in the 1990's for very low cost.. This here is Sylvania, finest 1960's date code, military material.

40,-

Mint Quality tubes. Believed to be NOS, but white boxes. Appearance, stamps and test data perfect and fresh like new. Perfect matched pair. I estimate these are from the time at the WW2. Really old, and test like NEW.

Amplitrex report tube #47 - Amplitrex report tube #48

6188 is a superb beautiful tube, upwards compatible to 6SL7. From the official datasheet (link here) the difference can be seen when comparing it with the commercial version. This is a tube which needs no selection for balancing. Here is a quote from the 6188 datasheet: '... made for applications that require extreme stability, such as balanced amplifiers....'

Moreover, this is a black plate tube, and unlike most other 6SL7 I have seen, it has flat anodes.

They were factory selected for AVARAGE initial plate current of 2.3mA +/-0.3mA. Please read this data sheet carefully, as it is written. Do not read it how you would "like it" to be. What is written there is: AVARAGE initial Plate current 2...2.6mA. Can this value be exceeded? Individually: Yes, because it is an average specification. Can a whole lot of 100 tubes also exceed those numbers? No, because the average is guaranteed to be somewhere in between 2....2.6mA.

So with any random tube, you can expect: The data sheet says 1.55....3.04mA.

Now you may think that is a big difference. But it is not, because having a tube with one system of 1.55 and the other of 3.04 is exceptional. Though possible. Also note, this is measured with fixed grid voltage, but for a tube with gain of 70x. So of course smallest differences in grid voltage will cause this already.

So the higher the gain, the more terrible the tubes "seem" if you use fixed grid voltage. Don't confuse yourself with that. So.... put the tube in an auto bias circuit, at 250V, and BANG it will draw 2.3mA. That is because the HIGH gain will correct it. And if not, then the tube is bad.

So take any UNSPECIFIED 6SL7, it will have only an avarage of 2.3mA, but they will not tell you how much deviation of the avarage is allowed. When you habe a box of 100 pcs, how much deviation of the avarage is possible, it is unspecified. And unlike 6811, individual tubes have no specification at all.

Lower gain tubes, like 6SN7 will show less difference, but have also less auto bias effect. In the end, result is the same.