07-05-2018, 12:25 AM
If your tube tester has a headphone jack, it is for checking just these sorts of problems. With the headphones in, and the controls set properly, you should hear virtually nothing. A whooshing sound, or hiss means a gassy tube. If you tap the tube and it rings like a bell for a bit after the tap it is microphonic. If you tap it and it pops and crackles you probably have an internal short that the shorts test didn't catch.