Does this sound like a shorted preamp tube?

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?

 

Thanks John!

I've gotta tell you, I'm so grateful you're on this board and take the time to answer questions like these for the electronically challenged like me. Over the years I've seen you help countless people straighten out their problems. I knew when I posted this question that you'd be the one to take the time and respond. You're one of the guys that make this board the great place that it is - thank you!

 

I didn't try that but just replacing that last tube seemed to do the trick. I just found it strange that the tube was working correctly in another amp and failed in this one. I must have damaged it somehow when putting it in or, as John pointed out, one of the pins was bent and not making contact. Either way problem solved. Thanks!