If a new electron tube has a build in defect, it will probably occur within the first few hundred hours of use. To give an example: micro cracks in the glass, will not give a problem during decades of storage, but may develop a vacuum defect after some 500 hours of first use. When a tube will stay good for this time, you can expect such a defect will not develop at all. For many other defects, the effect is similar. The situation with guarantee on vacuum tubes can be disappointing when vendors give only 3 months guarantee. This is often to short, to reach 500 hours. For instance, when a tube is used for 12 hours per week, it would take 42 weeks to reach 500 hours, and the 3 months guarantee would be already over. To make sure, all defects are covered by the guarantee, we have a standard guarantee time of 12 months or 2000 hours (whatever comes first) , and it can be extended to 5 years by internet registration. Please note this: Apart from visible mechanical failures, the major defect mechanism with older tubes, is a vacuum defect. For this reason, at EML we include vacuum defects in the 5 years enhanced guarantee. (See below text for the guarantee conditions). Upon registration, the buyer will receive a personalized response, with the tubes series numbers, buying date, buyer's name and relevant details.
With all tubes, regardless the brand, high plate dissipation, or high current, will result in lower lifetime. The safe operation area is below 75...80% of maximum plate dissipation, and "long life" application is around 60% of maximum. In some special cases, a tube can be used at maximum power, and lifetime is still maximized as well. With such tubes it can be seen in the data sheet, when the typical dissipation and maximum dissipation is the same number. For instance the RCA 2A3 is specified like this, but Western Electric 300B is not.
The best lifetime will occur at the specified heater voltage, a safe electrical circuit, and no oversized capacitors for rectifier tubes. 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. This is an individual value for each tube, and the required grid voltage for this is written on the tube box. If you wan to judge the condition of an older tube, you should test it at just that same Grid voltage, as we measured when the tube was new. 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 a normal sign of use, and no defect. This will continue during the entire 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% plate current. There is a general conception that such tubes 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 becomes audible, when the tube starts to distort at loud music, and was not doing so before. When you can measure the plate current, the area for a "good" tube is above 70%. Below this is not always the end of lifetime, or a bad tube. Tubes with values of 40% have been reported to work 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.