In the “old days” UK mains voltage was 240 volts AC and Ireland plus mainland Europe was 220 volts AC.
With the tolerances back in those old days UK mains voltage could be as high as 264 volts AC, so running an amp that was set for 220 volts AC in the UK could mean that the amp was being run at 20% over voltage. That means your valves have a heater voltage that is 20% too high and the high voltage on the valves was 20% too high.
The EU made regulations and stated the mains voltage throughout Europe was to be 230 volts so we all think, these problems have gone away. NOT TRUE.
The reason is that, just think of all the cost of changing all the transformers in the power networks across Europe to accommodate this rule. Just too expensive and not practical. So the power networks did nothing to their electricity networks.
As with all political decisions, they came up with a way to get around this regulation.
In the UK, they set different tolerances on the maximum and minimum voltage that is allowed on the mains. The Uk mains voltage tolerance is now set as maximum 254.4 Volts AC and a minimum of 216 volts AC. the rest of Europe now has a maximum of 242 volts AC and a minimum of 206.8 volts AC. So you can see there is an overlap and it what is called 230 volts AC. In actual fact nowhere has a 230 volts AC supply !
Modern designs that are rated for the pretend 230 volts AC supply are designed to hand the big changes between 206.8 and 254.4 volts AC, but vintage equipment was not designed for this wide range.
So for vintage amps, set the mains voltage selector to 240 volts AC in the UK and Northern Ireland and for Southern Ireland and the rest of Europe set the mains voltage selector to 220 volts AC. Remember to set the voltage with the amp disconnected from the mains!