Brown Inner Glass Staining on Power Tubes--On pulls-- how much is too little?

[VE3BRL]

Some of the more desirable Broadcast and Amateur Power Tubes offered at Flea Markets and on E-bay show slight to moderate brown staining. I suppose it is depositions of thorium or tungsten on the inner glass surface. yet some of these tubes actually have  a useful life left  I am told--- Is there any relationship between the amount of staining and Useful Tube Life, or is the presence of any  staining POISON? Is there a relationship , for example : that over 1000 hours of constant use  staining occurs :-- and one of the criteria for replacing Broadcast Tubes is this staining? Jim ve3brl

[The Slab Bacon]

Jim,
     The "coffee stains" on the inside of power tubes is a deposition of cathode material blasted onto the glass by electron streams missing the plate. It is usually some indication how the tube has been run. In my opinion it is more how hard it has been run rather than how long it had been run.
      I have a handfull of spares for my 4x1 transmitter, all have different degrees of coffee staining on them,( except for 1 new one), and all seem to perform about the same in my transmitter except one. I have one that has little to no staining, you have to hold against a piece of white paper to see it. this one is so piss beat that it will barely make 200w and dies on positive peaks. It will probably be made into a slimotron one day. All of the ones with coffee stains perform about the same as the new one in my transmitter.
      The tubes in broadcast rigs have to be changed periodically in time intervals. (if they dont have a failure). Most of the time the broadcast pulls have more than enough usefull life left in them for what we do, especially as we dont run them as hard as they did in commercial service.
      The final in my 4x1 rig was badly coffee stained when I got it, and has been running happily for 5 years now in my transmitter. the amount of staining is really no indication how much life is left in the tube. They can look terrible and still work quite well. However the amount of staining is a pretty good indication how long / hard the tube has been run. The only real way to tell is to stick it in a transmitter and see what it will do. Also if one is making an offer for one, the amount of coffee staining
should have a definate negative effect on the amount of the offer!
                                                              The Slab Bacon