That's essentially what a curve tracer does, it just goes one step further and varies the plate voltage along a specified range for each step in grid voltage.
The point I was trying to make is that a tube may work fine in one circuit and fail in another. For any test to be valid the tube needs to be tested under the conditions that it will be actually used in. For an audio amp, things like distortion are not usually tested in any kind of tube tester, let alone microphonics.
If you are buying used tubes, there is always a gamble involved. The probability of success is lower when the buyer and seller are separated by the internet, or other non person to person transaction.
If I find a box full of random used tubes at a hamfest or other type of flea market, and they are cheap ($1 each or less), I will pull out all the sweep tubes, 6SN7's, 12BY7's, 12AX7's and others that are useful to me, and don't look obviously used up or dead. I have found that 50 to 90% of these tubes will be good enough for something I am making. There will ALWAYS be some surprises, both good and bad.
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