WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
If you must change out the KT88 for KT120, I would make sure the filament supply can handle the extra 20% that the KT120 require. I assume you would also increase the Bias and/or B+ to take advantage of the extra KT120 max plate dissipation. In which case, you might want to increase fuse to maintain the proper margin %. For example, if you had 80mA prior and increase it to 100mA. I would use something close to a 200mA fuse. An alternative is keep it as is and some spare fuses on hand.
for such a large number of valves i doubt that Jadis will use just one 6 volt winding, that would be highly unlikely, like in the carver silver sevens, the tubes were arranged in threes and the filaments are series connected....so unless we can confirm, then hold off...
The Jadis amp uses two 12 volt filament windings using the 6 volt KT88 tube filaments in series.Very doubtful the filament windings will tolerate the increased current for long term reliability.Those amps are well known for reliability issues because Jadis uses 12AX7's in parallel to drive the outputs.They use 470K ohm resisters to load the input G1 grid,the tube manual lists the max load for that input grid as 50K,this leads to problems with bias stability and red plating caused by current flowing in the G1 input circuit.Jadis tests their tubes to insure they have properly aligned grids at least when new,after a few hours that can and often does change.This fault in the design is well known and information on it can be found with a web search.I have repaired a good many of these amps and owned several as well,the only tube I would use in the outputs are KT77s as they will tolerate the 470K G1 load.Cheers and good luck.
I have replaced three power transformers over the years,all of them with the larger power transformers,that the owners installed KT120 tubes in their amps.By all means it is your amp and do as you wish with it but those power transformers are very expensive.Opening the larger power transformer cover revealed that the core was the same between the smaller first generation and the larger later versions only the outer case was bigger.
even if you can use the same traffo for the kt120, i do not see any real gain here as the psu voltages remain the same, the OPT are the same, only the tubes changed......power wise you do not gain anything......except perhaps that feel good feeling....