When it sounds like a tube amp needs rejuvenating,


how can you determine whether the power tubes or the output tubes need to be replaced ?  Or both ?   

 My amp is the VAC Phi 300 running 4 Psvane CV-181's up front and two quads of Shuguang Black Treasure KT-88's behind them.  

Thanks -
-john 
128x128dr-john
oh also - when a tube can no longer hold bias, it's time to replace it., right ?  

-john 
Short of catastrophic failures, tubes that are past their prime can sound “tired” or lackluster; you start noticing treble doesn’t have the life and sparkle you were used to. The best approach is to buy a new set of tubes before they are needed, then swap the new ones in periodically to see how they compare to the old set.  If you don’t notice an improvement, put the old set back in.
thanks - yeah, I've noticed a lack of air  and space around individual instruments - in other words, the 'magic' I love about my system. 
Part of the tube amp "thing" is the fun of collecting tubes and trying various ones in different combinations...many tube heads do this and wind up with a lifetime supply to mess around with...I agree utterly with swapping in fresh tubes being the way to absolutely tell that tube retirement time is here. I usually have a small ceremony when retiring a tube...cake...small certificate...
When the noise level behind the music becomes noticeable! Power tubes usually go first. Small signal tubes can last way longer - 10X that of output tubes!