Well, there is no clear answer. Example: ECC88
The old datasheet says 15mA, but that's just a figure under the "typical operation conditions". From our long term experience, new old stock (NOS) ECC88 tubes - out of the box - can have ANY anode current ("emission") ranging from as low as 10mA to as high as 20mA.
Vintage tubes are hand made, with tiny differences on each tube. And so are the electrical specifications. Unlike a mechanical part, the tube has no "exact" electrical figures. Have a look at a metal screw, 10mm of diameter, and 100mm long. You can buy a box of 100pieces, and they will all look exactly the same. They will all have 10mm of diameter, and 100mm of length. There is no exception. And tubes? Well, that's a whole different story. You can buy a box of 100pieces, and you will get 100 different measurements. Let's stick with the ECC88-example. You can't say: 15.0mA should be a NEW tube, and everything below can't be used. That's not how it works. At the end: 10mA could be fine.
We have unboxed hundreds of new ECC88/E88CC tubes, and none of them had exactly 15.00mA. Some had 8mA, and some had 22mA. And yes, for some odd reasons, some NOS tubes were "dead" aswell. Really, it's that disappointing in the life of a tube seller. And "balanced systems"? Whereas both triodes have the same current.... well, that does not exist on practise. Balanced tubes are rare - and hence the price.
You see, there is no clear answer on that question. The datasheet value is something you can rely on. But if a NOS tube has reasonably less, then it is fine too. Remember: New tubes often have a low test result (as low as an excessively used tube). And some USED tubes test "as strong as a new one". Both could be considered "normal" for some reasons.
This is very important: From our own experience with high end tube audio equipment, the "highest emission", "closest match" and "tightest balance" doesn't always give you the best sound. It also depends on construction details, and the schematic. We have seen this a hundred times. When plugging in an ECC88 of 15.25mA/15.40mA the sound is awesome, and when you plug in a totally different brand with (let's say) 10mA/14mA the sound yould dramatically improve. Also, if the anode current is too high, your audio gear could produce distortions. If the anode current is too little (typically less than 6mA on an ECC88), it may sound "noisy" or distorted. That's why we don't offer tubes with abnormal high / low anode current. If you need tubes with odd anode currents, please contact us (but please do not ask for faulty ECC-tubes, as we mechanically destroy them, in order to prevent third parties to cheat with such tubes, or entirely ruining the reputation of used tubes).
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