It is very hard to damage a tube tester... but an interelectrode
short can do the trick. Usually the meters have a diode in parallel
with their movement to limit current to a safe value.

Hi Chuck - I am sure that's what I did. I fried 2 zener diodes in the meter
circuit, and also took out the bias pot (wirewound) in the process, plus of
course blew the internal fuse (soldered in place)
..
I still don't know exactly what I did, but CTech did a great job
repairing and recalibrating it to factory specs so I'm now very careful
to not do stupid stuff again!

Dave