The short answer is yes!  Tubes can fail over time or they can fail 
in a heartbeat.  Your finals dropping current after they are tuned 
could be a symptom of tired tubes - or is there perhaps a tube driver 
(12BY7) or the like?  It too could be dropping in emission resulting 
in lowered drive to the finals.   I suppose a quick check would be to 
put in your backup set of 6146s and see if you see the same 
symptoms.  And if you have a spare driver, try the same.
Good luck.
Pat, K9JAUAt 07:46 AM 4/11/2015, you wrote:
>Hi all:=20
>
>What are the clear symptoms of failing tubes?=20
>Is 1 of them when you have your plate and load adjusted for initial max =
>output and each time after that, the output gets less and less?=20
>Will a tube just go like a light bulb, 1 minute you got full output then =
>boom, no output?=20
>I'm thinking once again about my fuse blowing issue and according to =
>NC4L, the 102 guy in FLA,=20
>this is a sign of failing tubes because of miss tuning or too long key =
>down in the tune up cycles.=20
>He calls it thermal run away and it contaminates the screens and deforms =
>them.=20
>The only way to fix this is with a new set of tubes, which I have.=20
>Well not brand new but a good set of backups.=20
>
>73:=20
>Michael DE VO1AX=20