what cooks a tube?

[hodad]

I have a Peavey VMP2 mic pre that seems to have a habit of cooking a particular tube, so I'm wondering what the causes of this might be (the tube gets white inside, by the way.) The tube is also closest to where the power comes from the PS, & I'm wondering if this might be somehow related to the problem.
But generally, what are the causes of fried tubes?

Tom

 

[CJ]

What kind of tube?
Does it fail completely or just lose gain?
If it's white inside, that usually means you lost the vacuum.
That would mean some type of mechanical trama.

Go.

 

[hodad]

12AT7. Signal distorts easily, but passes. I'm not sure why two tubes in the same position would lose vacuum--the vmp2 doesn't get moved around much--but maybe. Either Peavey or the previous owner attached some soft rubber strips to the inside top of the chassis to keep the tubes in their sockets--it's possible this might lead to some unpleasant stresses. Seems a little unlikely, but maybe.
FWIW, this tube is not as white as the one it replaced but is whiter than the 12AT7 on the other channel, which makes me think it's a gradual decline.

 

[pucho812]

what company of tube? Could just be a bad batch.

 

[NewYorkDave]

CJ's right, a white fog inside the glass usually means a loss of vacuum. Even a bent pin can cause this to happen. Are these Chinese tubes, as Peavey usually supplies? The QC on those used to be horrible--I don't know if they're any better now.

 

[hodad]

The 12AT7's are philips. I'm going to take a closer look at things to see if I can figure out what could have caused the problem.

Tom

 

[analag]

If I could dive through my computer I could fix it in 2 minutes, maybe less. But seriously you can cook a tube with garlic, peppers, onion with a little olive oil in the skillet...I like mine well done

Analag

 

[mcs]

[quote author="CJ"]If it's white inside, that usually means you lost the vacuum.
That would mean some type of mechanical trama.[/quote]
Not nescessarily. I have seen a tube "go white" from electrical causes. It was a line output tube in a B/W TV though. The original tube was white. I replaced it, and turned on the TV (I was young and stupid...). As soon as the tubes got hot, I heard a loud whistle, and the new tube was white also...

I don't think that sort of behaviour is normal in a preamp though...

Best regards,

Mikkel C. Simonsen