If you have a car radio that mounts in the vehicle with the tubes upside down this can save you mass trouble.
me a couple of times.
Radio worked fine on the bench but in the car wouldn't work.
Testing the tubes in the normal fashion shows all the tubes test good.
Now put the tube in the tester and TURN THE TUBE TESTER UPSIDE DOWN, and test the tube.
Yes! I had the short appear ONLY with the tube upside down in the tube tester.
This was a tip from a very old electronics publication. I fought that radio for hours and it worked perfectly on the bench, but the minute it was turned upside down, as it was mounted in the car, it failed. Fortunately I remembered this tip and I solved my problem. With the tube and tester inverted, the short light came on. Replacing that tube fixed the radio.
A real oddball rare problem, but I did have it happen to me.