Tough Dog Car Radio Puzzler and Fix

I had a car radio on the bench, tube type.

Complaint: gets noisy when it warms up after 2 minutes or so.

 Worked just great on the bench. Checked all the tubes, connections, etc. Whacked it with my hand, etc. Ran fantastic for several hours solid.
 Couldn't find any sort of problem.
 Reinstalled it in the car. Problem re-appeared.
 Again more bench testing. No problem found.
 At this point, I had spent maybe 8-10 hours of bench time on this set,plus an unhappy customer and puzzled tech, me.

Then my memory kicked in. In an old trade magazine I remembered seeing an article on the same sort of "tough dog" problem.
 The article said the tech had turned his tube tester upside down and tested the tubes. He found one bad one that way and fixed his problem.
 "Hmm, nothing to lose". I turned my tester upside down and re-tested the tubes.  BINGO!
One showed short after it heated up for about 2 minutes in the upside down position, but tested perfectly fine right side up. (Several times!!)
 Replacing that tube solved the problem.

All I can figure is the inverted position the tube was in caused the supports or elements to sag enough when it was hot to short out.

Anyway, radio was fixed , reinstalled, & out the door with a happy customer. I explained to the customer what was going on and he was amazed I even found the problem. I repaired several more units for that chap over the years.

 Oh, and after replacing a radio, be sure to re-check the antenna trimmer capacitor and tune for highest signal level. Also whack on the antenna base, as they can come loose from vibration and cause a poor ground with loss of reception.