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It seems supply of NOS, originally boxed 6SN7, has almost completely dried up at the moment. It is weird, we would have to buy 6SN7 now, for a higher price than we sold it, just some years ago. What is still for sale, is single pairs by private Ebay sellers. These are often said to be "never used before". But what many people are doing, is buy such tubes, use them for a few years, and then put them back in the old boxes to sell them on Ebay for a higher price as they paid 5 years ago. To prevent buying such used "NOS" tubes, tube dealers are always looking for many pieces of the same. That is much safer, but it is exactly this type of NOS, which is totally and completely gone, I would say since the year 2000. Sometimes we can buy collections from people who want to get rid of it, and when these have been stored long enough, sometimes a life time, there can be some good finds with it. But other sources for real NOS 6SN7 are rare.

Chinese NOS, with metal base, I remember it was the cheap junk of the 1990's, and they cost almost nothing from Shuguang, as the old factory was still running. Such cheap stuff, only fools would buy it. But who is the fool... I whish I stocked 1000 pcs. These are heavily sought now. Russian 6H8, I could buy those for 30 Euro in boxes of 100. So not per tube, but 30 Euro for the whole box. That was long ago. But at that time, there was little use buying it, because you were lucky to sell a few pairs per year, and get 5 Euro for it. Besides, cheap tubes by audiophile definition sound bad. But now look what happens today! People are t buying them, specifically asking for REFLEKTOR brand, or metal base Chinese NOS, because they say, there is nothing like it. If you understand this behavior, you understand more than I do, but one thing I can say for sure: Prices for really good 6SN7 NOS are rocketing, and all the usual stories about NOS pricing apply here. Including the fact that everybody is surprized that prices go up.

Here is something important with 6SN7 NOS, I know this is so, but I can not really prove it. I expect some of the gas, bound in the Barium Getter, over many decades gets released, and one way or another, finds it's way the cathode. Don't call this impossible, because in the getter is really quite a lot of gas bound, and already the tiniest bits can damage the cathode. This is just my explanation. But I can say, It is a fact, long time stored 6SN7 has usually some loss of emission, and they recover not fully from that, no matter what you try or do. At the same time, these are still fine tubes, no problems with them of any kind. Just they test always at 95% of average, seldom at 100% and never much above.

In any case, when testing NOS 6SN7, they all have in common, that plate current is just a tiny fraction too low. Not much, but it is. It will improve by burning in, but not be fully cured. I think this is a thing we just need to accept. This is NOT a problem, but it is a small issue.

An EXCEPTION to this quality issue are the so called CHROME TOPs.This is a wrong word for them, because it is not Chrome, but the older type Aluminum getters. They look like Chrome indeed. Also they are not always chrome TOPs, but also Chrome SIDES exist, as found with some VT231 from WW2 production. These are simply the BEST. Gettering technology was much different from modern Barium getters. These tubes were vacuum pumped, but a tiny bit of gas was left intentionally. This gas made it possible to out glow the complete tube, with mild RF energy, just enough to create a plasma in the tube. A Plasma is nothing but a lighting gas, same as in neon lamps, or Geissler tubes. Then, when out glowing was completed, the RF energy was increased, and at that point the Aluminum getter would begin to melt, and flash against the glass. This moment of evaporation is the gettering moment. It is only NOW when the gettering function is strong. That is because a cloud of aluminium gas, is mixed with the other gassed in the tube, and these get all absorbed. Right after the gas is gone, for that reason (BECAUSE) it is gone, the plasma disappears, and the process stops itself.

This creates a perfect, high vacuum. Unlike Barium getters, this vacuum does not seem to degrade during almost 80 years of storage. This technology was obsoleted however around 1945, and replaced by the more comfortable Barium getters which are easier and faster to produce..

So any Chrome Getter tube is old indeed, before or from the WW2. Chances they are still good are high.