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How does the sound of a power tube change, over time?

I have a pair of Rogue M-180 monoblocks that each use a quad of KT-120 power tubes.  I'm considering a new set of power tubes (same brand, Tung-Sol)  

I realize this question will depend on the specific tube type and amp, but I'm curious about the common experiences.  For those of you experienced owning the same tube amp over years and replacing the power tubes with the same brand of power tube, what did you gain, sonically?  E.g. as power tubes age does the amp suffer in terms of frequency extension, dynamics, volume, etc? 
c_s
lowrider57's avatar
lowrider57
5,411 posts
 
as power tubes age does the amp suffer in terms of frequency extension, dynamics, volume, etc?
Yes, what I notice is a loss of top-end extension and a dullness to the sound, as in a loss of dynamics.
But this change due to aging tubes happens gradually, so it’s not something you notice immediately.

That's why I've been looking for an affordable tube tester.
rockadanny
1,224 posts
 
Does this happen with amps which employ auto-biasing?
atmasphere's avatar
atmasphere
11,751 posts
 
Does this happen with amps which employ auto-biasing?
Yes.

I find that triode power tubes tend to go downhill a bit slower than tetrodes and pentodes.
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