Each pair comes has the factory test data on the box.
Sample: Picture

Defect rate

If a new electron tube has a build in defect,it will probably occur within the first few hundredhours of use, in the guarantee time. To give an example: micro cracks inthe glass, will not give a problem during storage, but will give a vacuum defect after appr. 200 hours. When a tube will stay good for this time, you can expect this tube will never develop a vacuum defect at all. For many other defects there is the same situation. 

Safe operation

The lifetime of electron tubes depends mostly on the use conditions. With all tubes, regardless the brand, highest power use, will result in lower lifetime, and vice versa. The safe operation area is below 75...80% of maximum plate dissipation, and "long life" application means around 60% of maximum dissipation. In some cases, a tube can be used at maximum power, and lifetime is still maximum as well. With such tubes it will be written in the data sheet. For instance all EML 2A3 versions are such tubes.

Lifetime

Thebest lifetime will result from the specified filament voltage, normal use, and no oversized components, like too large capacitors, and use of electronic circuits that are no experiments. The plate current is factory tested after the burn-in, and when the tube has become stabile. This level is called 100% when the tube is new. So it is an individual value for each tube, and the required grid voltage for this is written on the tube box. It will stay at or close to that level during customer burn in. In the beginning the plate current can change (up or down) very slowly, or stabilize for most of the time. After it stabilizes, the customer burn in is finished, and normal wear out will start to begin. You will observe the plate current going down very slow, each few hundred hours. So if the plate current (referring to the original test value) has decreased, this is just a sign of use, and no defect. This will continue during the lifetime. Amplifiers with Auto bias, will hold the plate current fairly good. Amplifiers with adjustable bias, will need regularly adjustment. That's what the adjustment is for, because these have no Auto bias. When the end of lifetime is getting nearer, the plate current will start to go down quicker than before. Generally, you can say when the tubes sound good, that's what they are. Only you can not judge the remaining lifetime in the tube by that. On a test bench, the measured values of fairly used tubes can be at 70%. There is a general conception that such tubes are have still enough lifetime left in, to make a well designed amplifier function normally. This value of 70% is found on most tube testers as the level where the "good" reading begins.

End of Lifetime

This can be heard when the tube starts to distort at loud music, and was not doing so before. When you can measure the plate current, the area, where you can say a tube is "good" is above 70%. Below this is not always the end of lifetime. Tubes with values of 40% can sound normal, depending on the amplifier. Below 40% problems will probably occur. So you can feel safe when buying used tubes at or above 70%. Not below that. 

This is what we give guarantee on:

Guarantee Conditions: