[AMRadio] How fast does an 8874 transmitting tube go soft?

[Ka9p at aol.com]

Arguably an AM question as I am gearing up to use one of 2 old  Alpha 374 
decks I've acquired as a linear behind my Ranger 2, as retirement and  an 
upcoming move suggests I should keep life simple and start unloading the  100 
pound beasts, and arguably this is one of the most knowledgeable bunch to  
ask so.......
 
I've searched and not really found an answer to the question of how fast a  
tube like an 8874 degrades once it starts showing signs of  softness on 
higher frequencies like 10 meters.
 
I have a number of used 8874s with unknown histories I got with the old  
decks that range from performing pretty much like new, at least no apparent  d
egradation in performance as you go up to 10 meters, my primary AM band of  
interest, to one's that drop from about a kw out/pair at 20 meters to 400  
out/pair on 10 (which would be most of them).
 
Assuming these aren't abused any further by me, can anyone give me a clue  
as to how fast or slow higher frequency softness advances, and is it a 
function  of time, or run time, or?  And is softness any indication of a higher  
probablility of catastrophic failure or is it fine to beat the softer  ones 
on the lower frequencies until they completely give up?.  Or is  it so much 
a function of prior history that this is a silly question?
 
The reason I'm curious is I'm considering converting one of the old 3 by  
8874 Alpha decks to something other than 8874s while in the midst of the  
refurbing, and I'm trying to decide whether that should be a sooner or later  
phenomena based on the tubes I've got on hand.
 
If there's something I should read, feel free to chide me for not finding  
it - I skimmed the Care and Feeding of Power Grid Tubes but didn't find 
anything  that helped much.
 
Of course if ten doesn't improve this is all pretty moot I guess, where the 
 heck is everyone? Last cycle a CQ with the old Viking 1 routinely brought 
plenty  of responses from the west coast, last year I had trouble finding  
anyone.....
 
Thanks, Scott