Help, strange glowing in KT88

[taotao]

Hi Forum,

in my DIY Ella amp, one powertube shows a strong bright light.
I tried to photograph, but pictures have gotten unsharp.
But anyway one can notice, where this bright spot is located.
It's only one from 4 tubes showing this light. Bias is for all tubes equal (conservatively 45mA).

Could it be the tube will go to fail in the nearer time?
And what's causing this spot?

FYI: this spot wasn't there, when tubes were new.
I only noticed two days ago, bias was quite on the high side (around 79 mA, don't know why. I adjusted it to the above mentioned value)

I appreciate your input.

Pictures:
http://i852.photobucket.com/albums/ab87/olafzen/DSC01576.jpg
http://i852.photobucket.com/albums/ab87/olafzen/DSC01577.jpg


Olaf

 

[tomchr]

Try setting the camera to macro mode if it has one. It's usually indicated with a flower icon and will allow you to focus at close distances. - Or shoot in high resolution from further away and crop the image.

It looks like one heater is running hotter than the other. Could be a hot spot that has developed... But it's pretty tough to see on the pictures.

Tom

 

[Ty_Bower]

Try setting the camera to macro mode if it has one. It's usually indicated with a flower icon and will allow you to focus at close distances.

Good photography tips. I'll also add that it makes it difficult (if not impossible) to focus on close up objects if you use the zoom (telephoto) feature of the camera. Keep it at the wide angle setting (1x) for best results.

 

[6h5c]

Looks like the grid is glowing, that's not good...

Ray

 

[taotao]

Oh, well, "not good" that also comes in my mind.
Tomorrow, I'll try to take better pics.

Thanks for first responses.

See ya
Olaf

 

[ray_moth]

What happens if you swap the tubes arouind?

 

[taotao]

Hi Members,

the glowing moves with the tube, when I put it in another socket.
So the tube is faulty.

Here are more tries with macro modus (not perfect, maybe OK for recognizing, what's the problem):

http://i852.photobucket.com/albums/ab87/olafzen/DSC01590.jpg
http://i852.photobucket.com/albums/ab87/olafzen/DSC01591.jpg
http://i852.photobucket.com/albums/ab87/olafzen/DSC01592.jpg
http://i852.photobucket.com/albums/ab87/olafzen/DSC01593.jpg
http://i852.photobucket.com/albums/ab87/olafzen/DSC01595.jpg

Thank you


Olaf

 

[audioot]

i think something is wrong with the cathode/filament on the upper side
is the valve working correctly?
audioot

 

[taotao]

Hey Audioot,

yes, tube is working correctly. It doesn't distort (audible) more than the other channel.


Bye
Olaf

 

[audioot]

probably the cathode sleeve is a bit shorter en you see the bare filament glowing
in that case you have nothing to worry about
greetz from belgium
audioot

 

[taotao]

Ehm, but it shurely wasn't before. Realy strange.

 

[6h5c]

The sudden change in bias indicates something's wrong. Try to find out where the glow is coming from exactly. See if you can look inside the tube, through the round holes in the anode. Can you see if the grid wire inside the tube is glowing? Is the glow situated below or above the upper mica disc?

Regards,

Ray

 

[audioot]

sorry
i dit not notice de sudden change in bias
must be someting wrong with the valve
audioot

 

[Audiofanatic]

Just take this tube out before it's too late.

Hi,

I've had this before and IIRC the filament wire was shorted and the tube started glowing like a lamp! Before you know this tube will cause more damage to the rest of the electronic inside your amp, so best thing to do is take that tube out and replace it with an other one.

The fact that the tube was switched and the glowing tube was the same one that glows indicates that this tube is not good.

Don't wait until you're amp starts sending smoking note's!

Just my 2 cents.


Best regards,


Audiofanatic 😉
P.S. This happened to me in 1990 so I don't remember anymore what was wrong with the filaments.

 

[taotao]

Thank you all for replies.
I'm going to replace that tube. Should be safest idea.

See ya

Olaf