I just changed all 5 preamp tubes in one of my amps. All of the replacement tubes were out of other amps that were working fine. After powering up I played a little and started hearing loud, intermittent popping noises that gradually increased to a loud and steady hum. I immediately shut the amp off and started replacing the tubes one by one. Each time I powered up I got the same popping and then a steady hum. I also noticed that each time the popping occured and when the sound changed to a hum the power tubes glowed blue. On the fourth tube I noticed that the socket was loose, so I opened it up and tightened it down and looked for loose wires but didn't see any problems. I tried it again and got the same hum. Finally, after replacing the fifth and last tube the hum dissapeared and the amp seemed fine. I've never had a tube short out on me and just wanted to make sure this sounds typical of a preamp tube going bad. I'm about to sell the amp and don't want to stick someone with a problem. Is there anyway to test a tube for shorts just using a meter?