getter problems

[Wakculloch]

I have a JCM900 with 5881's and they have problems!
One of the valves has lost all the getter on the inside of the glass. It hasnt gone white, as when the glass has cracked, but has completely dissappeared.
The other one has gone black when looking from the outside, but the inside still has a shiny mirror look.
Neither anode appears to have overheated,(hard to tell what happened before i got the amp as the fuses had blown). I've put new 5881's in( 6P3S's) and all seems well.
Has anyone had similar problems, and is it preventable.
Appologies if this has been covered before; did a search and found nowt.

[Steve Conner]

Hi,

I think the Sovtek tubes just do that as they wear out. I have some old Sovtek 6550WEs here. Two of them have that same missing getter appearance, and they are weak, can't make 50 watts in a 50 watt amp, and go into thermal runaway trying.

I think what happens is the getter disappears as it mops up gas, and once it's all gone, the tube goes gassy. The fuses may have been blown by that tube running away. My suspicion is that they don't put as much getter in as they did in the old days.

[Wakculloch]

Thanks for that Steve.
I have to say that despite they're ultimate demise, they havn't done too badly. About ten years of use and abuse.

[stokes]

100% on what Steve said,but it is not just Sovteks that do that,I've changed RCA Blackplates,as well as other brands that had the same symptom.All tubes begin to lose the silver look to the getter as they age and get weaker,usually they will start to turn brown and then fade away.Ten years?Probably should have been changed 5 years ago.
"My suspicion is that they don't put as much getter in as they did in the old days".Got that right,as well as all the components and quality control.