Tube tester?

While true that most testers don't test at real world voltages it is important to leave the tube in the tester for 5 min or so.  If you put it in and test it cold your results will be poor.    Ultimately your ears are the final say.   That fact that a tube seems to have strong emissions it can still sound poor

Depends on how old the tube is and how long it sat on the shelf. Testing NOS that have sat for decades will almost all certainly fail a gas test.

Even new tubes that have sat around for a couple of years may fail the gas test.

Tubes that fail a gas test often recover after 10-20+ continuous hours on the heater. If it doesn't, it likely never will. Some tubes may test gassy after weeks of inaction if they have pin/envelope micro fractures from repeated insertion cycles or 'porous' glass.

As far as sonics, all bets are off. Tubes vary immensely. A particular tube may sound fine in one circuit and not well in another. Additionally one tube may drag down its siblings in a circuit or channel.

AudioPhools lack of evaluation rigor makes most recommendations specious.

Good advice from oddiofyl about warming up the tube sufficiently in the tester.  I recommended the Orange tester as a reasonably-priced modern alternative, but, someone who uses it at a dealership told me that it does not operate at high power and may give a poor rating to certain types of output tubes (e.g., KT88) unless the test is run several times in quick succession so the tube has warmed up sufficiently.  

If you are not really experienced with tubes, a tube testers can be a good thing and a bad thing, sure you can weed out shorted tubes that aren’t any good, but you may also be throwing out tubes that are gassy and are not bad. My advice would be to get on the diy tube audio forum and do some research about tubes, tube life, and what are the pros and cons of tube testers.

Schematic, BoM, CAD dwg w dimensions and build photos for

available @ ieLogical Valve Tester