Bought these tubes for a Fender Hot Rod Deville because the Groove Tubes that came in the amp blew out. The GT tubes were their 6L6-R which are really Sovtec 5881s. Those tubes were ok but not what I really thought they should be plus I couldn't see paying for over priced tubes because they tested them and stamped their label on them. So I settled with these Winged "C" 6L6GC tubes to try them out. Nothing different about the amp the pre amp still has the GT 12ax7s in them and it was biased to the proper setting and I can honestly say what a difference it has made to this amp. I have tried other tubes in other amps with no real huge impact on me as far as a difference in sound. These tubes opened the amp to a more articulate and punctual sound. To be able to here every note as crisp and clear as this with the vintage sound was just is amazing. I cannot speak for everyone but these tubes have really made a big difference to this amp. I really was thinking of selling the amp because of the muddled and strangled sound it had. I knew it would get a little better with new tubes but I am blown away how much it helped. This amp is now easily one of the best amps I have ever played with. Don't worry about losing gain either there is still a lot of gain to be had. I will post a video on youtube soon. I will also be changing the preamp tubes over to Tung-Sol 12AX7 tubes as well soon. Best change from an amp I have ever heard.