Speaking of Ebay tubes, does anyone have any info on the drop of tube emission (oxide cathode, indirect) with aging, is it a linear curve or some nonlinear curve? I got some so called NOS tubes today off Ebay and the're more like worn out old stock (WOOS) with burn marks on the glass. I'm trying to estimate the hours of life left in them from the % drop in emission or gm, for estimating a partial refund request. Tried searching the forum, but no luck. Seems like this would be important info to know for anyone buying old tubes.
The reason you found no info is that it can't be done reliably. The actual emission of the cathode may gradually decline, but what really matters is whether it emits enough electrons for the anode current you want to take. Normally it will emit lots more, and form a space charge around the cathode. When the cathode is worn out it will barely emit enough, so anode current goes down and shot noise will go up as space charge smoothing will no longer fully occur.
You can tell roughly whether a valve has reached the end of its useful life by testing first at full heater voltage, then with reduced heater voltage (say, 20% down). A good valve will only show a small drop in anode current. Snag is that this does not tell you how much life is left in a good valve.
Careful inspection of ebay pictures can reduce disappointment, but some sellers are very good at displaying only the unburnt side. The colour of the print can sometimes help too - if it was meant to be white but is now cream or yellow it means the valve has spent some time being hot.
You can tell roughly whether a valve has reached the end of its useful life by testing first at full heater voltage, then with reduced heater voltage (say, 20% down). A good valve will only show a small drop in anode current. Snag is that this does not tell you how much life is left in a good valve.
Careful inspection of ebay pictures can reduce disappointment, but some sellers are very good at displaying only the unburnt side. The colour of the print can sometimes help too - if it was meant to be white but is now cream or yellow it means the valve has spent some time being hot.
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