White light in vacuum tubes?

nathanm · « **on:** 25 May 2004, 08:31 pm »

What causes a tube to glow with a bright, white light? I had this one tube (since replaced) which, when the amp was first turned on, would flash very brightly and then die out, start to hum a bit and then flicker a bit more. I didn't keep it in there very long after that, but I was just curious what is occuring when that happens. The tube was a KT-88.

Dan Driscoll · « **Reply #1 on:** 25 May 2004, 11:31 pm »

Quote from: nathanm

What causes a tube to glow with a bright, white light? I had this one tube (since replaced) which, when the amp was first turned on, would flash very brightly and then die out, start to hum a bit and then flicker a bit more. I didn't keep it in there very long after that, but I was just curious what is occuring when that happens. The tube was a KT-88.

An instantaneous white flash sounds like an arc.

nathanm · « **Reply #2 on:** 26 May 2004, 02:30 am »

What's arcing though? Is there something that's broken, some pieces that are touching inside there that shouldn't be?

Dan Driscoll · « **Reply #3 on:** 26 May 2004, 09:05 pm »

Quote from: nathanm

What's arcing though? Is there something that's broken, some pieces that are touching inside there that shouldn't be?

Assuming the tube functioned properly for a while, then something probably broke. The windings on the grid and plate are pretty small and turn-on shock can cause them to break, especially if the tube has been well used.