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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
c_s OP
15 posts
 
Thanks for the responses.  I'll be trying a new set in the Rogues soon and will report back. 
wolf_garcia
6,695 posts
 
lloydc…That's actually a great comment. You know when the tube's time is up when your other tubes sound better. Ears are certainly the best tube tester as long as they're not wooly…if so clean 'em.
billstevenson
352 posts
 
I have a cj ARTsa amp that uses eight KT120 Tung Sols. I replaced all the tubes annually, not just the power tubes.  The sound improves noticeably as follows:  Quieter - big difference probably not due the the power tubes.  Sweeter highs, cleaner overall, better delineation of lows - rosen on the strings kind of thing.  My system gets used a lot, 20 or more hours a week.  YRMV.
c_s OP
15 posts
 
I installed a fresh set of KT-120's in the Rogues a few days ago.  The improvement was greater than I'd anticipated - I guess the old tubes were older than I'd realized.  Similar to other impressions on this thread, out of the box they new tubes were notably more dynamic, articulate, with better frequency extension (and with a reappearance of midrange) As well, they've improved over a few days in terms of depth of stage and bass extension.  I.e. the M-180's are back in top form and sounding fantastic
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