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
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!
Just change the output tubes first. Listen for SQ. Don't forget to adjust the bias voltage (if your amp isn't cathode biased). The small signal tubes may still be fine!