Getter halo turned dull and brownish: tube still good?

[slidincharlie (Carlo P)]

Hello fellow ampagers,
I've been away from these pages since many months...
I have a question about a tube.
I have a pair of RCA 6L6 blackplates bought used, that I have been using for some time in my SF Vibrolux Reverb. Now I have taken the tubes off the amp and have seen that the getter halo on the side of one tube is normally silver-blackish, while that on the other tube is dull and pale brownish (see photo below). I don't remember if the halo looked like this or if the colour changed during the recent use.
I must add that this amp developed a bad short (with flames) in the power tubes area some time ago. I have replaced one socket, but I can't tell for sure if the tube with the dull getter stayed in the burned socket.
Is the dull brownish color normal? May I use safely the tube or should I trash it?
Carlo

[Dave Curtis, dB AudioTech]

You can use it, and it may last a while, but there's probably not much life left in it. Not to mention it's probably not matched well at all with the other one. Get a new pair.

[Gtr_tech]

Yep....when the getter flash is brown and you can see through it when you hold it up to a lamp that indicates high mileage. I've seen EL34s where the getter flash is completely gone....just a darkened area where it once was. They still "worked", but not well.

Could also indicate problems with the vacuum.....impurities in the envelope tend to tear up the flash.

[Dave Curtis, dB AudioTech]

Yep....when the getter flash is brown and you can see through it when you hold it up to a lamp that indicates high mileage. I've seen EL34s where the getter flash is completely gone....just a darkened area where it once was. They still "worked", but not well.

Could also indicate problems with the vacuum.....impurities in the envelope tend to tear up the flash.

Definitely could be a microscopic leak...

As you likely know, the getter flash adsorbs impurities in the vacuum (a LOT when they initially flash it). Throughout the life of the tube, normal operation creates impurities. The hotter it is run, the more are created and the flash fades quicker. If one tube was run hotter than the other, it would lose the flash quicker.

Check the bias when retubing your outputs.

[slidincharlie (Carlo P)]

Thanks for the feedback.
I have a bias balance + bias adjust setup in this amp, so I can't actually tell how much the tubes are mismatched.
Anyway, although I use this amp rarely on stage (I tend to use smaller amps), I don't want to risk a failure during a gig... I'll put another pair of tubes in there.
Thanks,

[Gtr_tech]

Thanks for the feedback.
I have a bias balance + bias adjust setup in this amp, so I can't actually tell how much the tubes are mismatched.

Thanks,

Sure you can. Just remove the output tubes and fire the chassis up and leave it in stby. Adjust the -V on pin 5 of the sockets with the balance control until they match. Mismatched tubes will have differing V/I readings when you pwr up with tubes in.

[slidincharlie (Carlo P)]

Ah, ok
Didn't think about it...