How to differentiate real NOS, from the rest.

it is sometimes not so easy to differentiate real NOS from other conditions. NOS is expensive, and we see at the moment almost any trick in the book to sell so called NOS which is not.

The unexperienced buyer of course tries to prevent buying second class stuff, but has no other option than to look for the obvious things. Like clean colorful boxes, photo shopped pictures, particular details, shiny getters, some Forum talk, or "green" result on some obscure tester. All these things mean very little. I have seen so often used tubes that look and feel like NOS. Just not TEST that way. Tube testing is the only way, and that means at leats test for PLATE CURRENT and TRANSCONDUCTANCE simultaniously. Better is test additional also for GAIN, PLATE IMPEDANCE and GRID LEAKAGE. If all these things are good, then the TUBE CURVES are normally good without testing those. Besides, tube curves are hard to judge, but there is nothing against having those as well if one tube cost 1500 Euro.

The following are no good ways to identify a NOS tube, though it must be said a good tube passed this test of course. It is just not reversible, meaning if the tube passes it is an NOS TUBE. Because the question was about NOS TUBES.

  1. Transconductance is"strong" on a Hickok tester. This tester is made such that is lets pass any tube which is working good. NOS or used. It was the sole purpose of this tester.
  2. Tube tests in the green on a tube tester which has no parameters. So the needle scale is red/green, representing bad/good. Such a tester shows a good used tube way in the green.

The following is a general way to differentiate NOS from used tubes: Go by parameters only. This means the result of a tube tester must be taken at the manufacturer's recommended settings, and no other. It must be verifyable by any other tube random tube tester which can do so as well. That is a solid result.

Here is to judge such results:

For an NOS tube there is always this balance:

  1. Plate current is below 100% but transconductance is above 100%.
  2. Transconductance is below 100%, Plate current is above 100%.
  3. Plate current is above 100% and transconductance is above 100%.
  4. Plate current is below 100% and transconductance is below 100%.
  5. The whole lot is at 100%. +/- very little.

Condition 1 + 2 are the best. Balance can be 20% off. So plate current is only 80%. That is fine! As long as transconductance is above 100%. Reason for wanting to have this balance (so the one up, the other down) is that gain and plate impedance often are fine in that case, and these two produce the sound differences between tubes.

Condition 3 is fine when not more than 120%. Above 130%, gain and impedance start to become wrong.

Condition 4 is for used tubes. Below 70%, impedance starts to become wrong.

4) Plate current is below 100%, transconductance is below 100%. Such a tube is useful, if not one of the two is below 70%. Just not NOS.

If an unknown tube is in condition 4, increase heater voltage slowly, up to 110% . If now the tube moves to condition 1) or 2) it is a fine used tube. If NOS do this, they are not fine at all. They are suspected to have cathode problems.

If it moves to 3) now you can not say much. This is rare though. More often they move to 1) or 2)

Last but not least. Fine NOS comes often in factory boxes of 100pcs. This an additional hurdle to fake or fool people. Though we have seen factory 100pcs boxes faked by a professional, family owned company in Switserland before, this stays rare.