• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Discoloration inside a tube?

Can somebody please tell me what these brownish discolorations inside the glass envelope of a tube means (or if it means something at all)?

I hope it can be seen okay at the picture.


  • tube discoloration.jpg
    tube discoloration.jpg
    60.4 KB · Views: 214


diyAudio Senior Member
2002-08-21 1:20 am

From the pic I'd say it's a 6550 ?
Nothings to worry about really.If you examine it carefully you will notice these spot on most power tubes were the supporting frames touch the bulb.Mostly the spots develop due to impurities in the glass and residual contaminants in the vacuum tube.
Barium usually gets like this.The getter tries to burn most of it but when you have hot spots on the bulb these spots will eventually show up.
This used to be a very awkward problem on early Chinese cathode ray tubes where these would spoil the image.
Don't bin your tube yet !

Frank: thanks very much for Your input!

BTW: this is a Valvo EL519 not a 6550.
Most of it does not look like sharp spots (though there are some "spots" which are even independend of the rest of the discoloration and therefore I would call them spots) but rather as areas which seem to grow from there where the circular supporting frames touch the glass envelope as You said.
To me it looks like kind of a crystalline structure.
Don't bin your tube yet !
If I got You right these "spots"/areas show up only when the tube was used ?
This tube at the pic is pretended to be a NOS tube! Can this be?
Unless really old and used tubes I never have seen a NOS tube with this brownish stuff.
Is there a particular designation for this kind of effect?