kt-120 or kt-88?

If your amp is designed for a KT88 tube and you put in a KT120 tube, it will not produce more power unless modified. A lot of KT88 amps can accommodate a KT120 tube (check your manufacture) and the KT120 tube will produce a different sound signature. Whether it will be to your liking, is very subjective.

It is tough to generalize about a tub’s sound because is behave differently in different amps and circuits.

It's not so much a power increase when swapping to 120s as a tonal difference...it's my understanding the 120s aren't taxed as much since they CAN handle more power than 88s, but there's no miracle power boost even though it might seem like there is. They often sound "larger." I like the sound of 120s in my amp, and they've lasted a long time (well over 3000 hours) with no audible signs of age.

I have been using the 120's since they first came out but my memory of the change from the 88's is that the 120's deliver more power and have more 88 magic than the 88's. Someone calls the 120's 88's on steroids. I kept a full set of 88's in case I wanted to go back but never even felt the need tp try them. Chris from the Parts ConXection told me that the 120's sound like realy good (expensive) 88's.
Denny

ARC switched most of its tube amps over to the KT-120 in November 2010. Previously, the 6550 "Winged C" was the tube de jure. My previous ARC amp was the VS-115, which came standard with the 6550Cs. I dropped in KT-120s a soon as ARC cleared the new tube.

At least as far as my VS-115 amp was concerned power output increased from 115 wpc to 128 wpc. Sonically, IMO, the KT-120s cleaned the floor with the 6550Cs. Better headroom, dynamics, bass, etc.