Tube heats up, but doesn't glow

[Randall]

A VOX AC30 came in dead, powers up but no sound. I found F1 HT fuse blown, and replaced it. Now it comes on with bad noises, so I figured I have a bad EL84. While in Standby mode, I noticed one of the four isn't glowing, at all, but it is getting hot. I pulled it, and same thing in my tester, with the lights off, no glow. It gets plenty warm, though, and shows no conductance or shorts on my tester.

So OK, I know it's a bad tube, but how is it getting hot? and why doesn't it show a short?

[The Dude]

I'm not sure why your tester isn't showing a short, but I'm still betting there is one. Check tube pins with a DVM. Is something shorted to the filament. If the filament doesn't glow, it's a good bet it burned open and is shorted to something. Sometimes a datasheet and a DVM will tell you more than a tube tester.

[Leo_Gnardo]

Imma thinkin' you have an EL84 that's on its way to going white inside - it has a failed vacuum but hasn't oxidized to the point it's obvious. On rare occasion I've seen similar.

Of course, I could be full of beans. That's often the case.

No matter, it's kaput.

Fresh set of outputs, pat it on its head & send it on its way. Or as the dentists say, drill fill & bill.

[Randall]

No shorts found with my meter, checked every pin to every other pin, the only place it isn't open is between the filament pins. It is cracked, however, somehow I missed that before. Seems like it gets quite hot real fast, too.

[The Dude]

Looks like Leo gets the gold star.

[LiamBlack]

In the end, it worked, no more problems?

[Helmholtz]

When air gets inside the tube, it cools the filament so it no longer glows red hot. But as the filament can't reach its normal operating temperature, its resistance is lower and thus heater current (and power) is higher than normal. So the tube gets hot without glowing.