As Hifi mentioned: most testers do not apply full operating voltages(B+) to the tube being tested. Tubes, under actual operating conditions, will occasionally either short, draw high screen current or open a connection internally. In the event of a short or high current draw; your circuitry may suffer some consequences. The only way to minimize the odds of that occuring is to burn in the tubes at full voltage and cull the weak ones prior to their use, as is Upscale's practice with their power tubes, ie:(http://www.upscaleaudio.com/rare/tpm.htm).
