Someone tell me what caused this valve to look like this please?

[allante666]

This was in a Vox VT30 preamp (they're a SS amp with a valve preamp as you probably know). Symptom was no audio. Checked all voltage rails and all were correct, changed this tube because it looks like it does!
New tube cured the fault.
Just out of curiosity does anyone know why it would look like this? I'm not that up on valve stuff I stick to SS but can find my way around when pushed. I'm just curious now.
The only thing I did do was replace the electrolytic in the heater supply. Not because it measured wrong but simply because its top was looking a bit swollen.

[nsubulysses]

THe tube has lost it's vacuum. The glass is cracked somewhere.

[allante666]

I wondered if that might be the case but couldn't see any cracks. Thanks for that.

[Leo_Gnardo]

I wondered if that might be the case but couldn't see any cracks.

Sometimes there's a leak where the metal pins pass thru glass. It doesn't take much - any leak even the tiniest will doom the tube.

[Jazz P Bass]

Loss of vacuum.
Like Leo said.

I will admit, I have not seen this often on preamp tubes.

[drewl]

I usually see it in real old amps, amps that have been dropped or banged hard enough.
Rapid temperature change can also cause it.

Always try to let your amp warm up a bitter if it's really cold in winter before powering it up.

[Justin Thomas]

I've usually seen it in preamp tubes where an owner pulled the tube for some reason, then didn't realize the pins were spaced unevenly when they tried to put it back in. If the pins are bent, you can cause very small chips or cracks where the pins meet the glass, and it may take a few minutes or hours for the air to leak in. Owner can have it in before it turns white.

Be careful straightening those pins!

Justin

[loudthud]

When there is an air leak, the heater will usually burn out.

[Mick Bailey]

The only thing I did do was replace the electrolytic in the heater supply. Not because it measured wrong but simply because its top was looking a bit swollen.

Was the cap actually swollen? - many caps look bulged but can be just the plastic cover disc that bulges and the cap is fine.