making power from kt120 tubes

what is the main reason a amp that is rated at 60 wpc with kt88,6550's will only get about 5-10 watts more a channel with kt120's or the new kt150's. since these tubes are rated for about 60 watts dissipation each. a 60 wpc amp with two tubes per channel you would think you would get 100-120wpc. but an amp designed for 60 wpc will only see a little more power. upping the plate volts from 500 to 500-600 would make more power?? most tubes you do that with everybody says the tubes would work harder and burn out faster. but with 120's,everybody says they are not working hard at all if you set to factory bias settings. is the screen voltage the limiting factor. if so cant the amp be adjusted to raise that and get more power out of the tube. just wondering here. everybody is buying these new tubes because their amp can handle them but your not getting the power potential out of them. thanks for your input
The answer is quite simple. To maximize the output and performance of any tube, you need to design a circuit to optimize the tube’s electrical requirements. The KT88 and the KT150 tubes have different electrical requirements.
Definitely it doesn't mean that 2xKT120 per channel will throw 120wpc. It doesn't work that way. How many more watts do you want with model number increase after all?
You can mine for technical data for each tube and read it's volt-ampere characteristics to figure out what is maximum output(depending on class of operation push-pull or SE) of any of ones you've mentioned.
Some designers will choose different quiesce voltage offsets that would mainly dictate an output power of your amp.
Both the KT120 and the KT150 are rated for a max of 800V on the plate. 6550 and KT88's are rated a max of 600V. I have an Octave V110 and the plate voltage is 540V for EL34-6550 and KT88 (producing 70W). For the KT120 I flip a switch which changes the plate voltage to 620V (producing 110W). It also changes the screen voltage from 310V to 330V. So it all depends on the circuitry in the amp. At 540V the KT120's work but not as hard. So if your amp is designed for 6550's - KT88's you won't get more power using the KT120 or KT150 unless you change both the plate voltage and screen grid.