The KT88 and 6550 draw 1.6A by their heaters (filaments)
The KT120 draws 1.9A by it's heater (filament)
As you can see the KT120 is higher and will tax the heater (filament) winding of the power transformer if it was not designed to take the extra 300mA. It very well over time cook the heater (filament) winding of the power transformer and burn it out, (fire is a possibility), which will be expensive to fix because the heater (filament) winding is a small part of the mains power transformer that supplies the HT (high tension) as well to the tubes, and usually the largest transformer on your amp.
Cheers George
The KT120 draws 1.9A by it's heater (filament)
As you can see the KT120 is higher and will tax the heater (filament) winding of the power transformer if it was not designed to take the extra 300mA. It very well over time cook the heater (filament) winding of the power transformer and burn it out, (fire is a possibility), which will be expensive to fix because the heater (filament) winding is a small part of the mains power transformer that supplies the HT (high tension) as well to the tubes, and usually the largest transformer on your amp.
Cheers George