Does this sound like a shorted preamp tube?

Randy

Silver Supporting Member
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3,932
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?
 

John Phillips

Member
Messages
13,038
Not really. A proper tube tester would show up things like that, but not a simple meter.

If the fault was very obvious until you changed a tube, and now it's stopped, that's a near-certainty that the tube is faulty - although it could just be that one of the pins is bent and isn't making proper contact with the socket.

The blue glow in the power tubes is simply caused by the 'signal' from the humming tube, I would guess. It should occur if you just play through the amp at the same sort of volume as the hum was.
 

Randy

Silver Supporting Member
Messages
3,932
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!
 

Blue Strat

Member
Messages
30,194
What happens when you pull all the preamp tubes and power it up? If the amp still hums you may have a power tube issue. Seems like a whacky coincidence, but coincicences do happen.
 

Randy

Silver Supporting Member
Messages
3,932
Originally posted by Blue Strat
What happens when you pull all the preamp tubes and power it up? If the amp still hums you may have a power tube issue. Seems like a whacky coincidence, but coincicences do happen.
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!
 

Blue Strat

Member
Messages
30,194
Originally posted by Randy
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!
Bad contacts are the number one problem in tube amps. Always have some spray contact cleaner handy.
 




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