110 (or 120) vs 220

For an amplifier to be designed to run on either 120 or 240 it must have two fuses, one for the hot and one for the neutral. When it is running on 120, the fuse on the neutral is just dead weight and does nothing but provide continuity. When it's running on 240, that "neutral" is now doing duty as a hot conductor and can pull short circuit current under a fault -- hence the reason for the second fuse. Don't forget to change out the fuses to a lower (usually half) amperage since the current draw is now half at 240 volts.

If there in only one fuse then do not run it at 240 regardless if there is a split primary with a switch or jumpers capable of conversion or else you will run the risk of what jea48 outlined above.

Also, you should not run other equipment off a 240 circuit. Theoretically, you can pull power off one leg and feed it to a 120 outlet and run a neutral  back to the panel but that is against code because it unbalances the circuit. The correct way is to leave it dedicated. That circuit will have a 2-pole circuit breaker feeding a 250 volt receptacle (6-15 or 6-20) which has the blades rotated at a 90 degree angle (horizontal) so that you cannot accidentally plug a 115 volt appliance into that outlet, resulting in a pretty fireworks display.

Your electrician knows how to wire everything up if you decide to go with 240.

Ok. If the amp isn't designed to do both with no modifications from me then I don't want to do this. (I have a call in to the manufacturer).

I have a CPAP machine and it will automatically switch from 110 to 220 it I switch the power cord. Guess it isn't so simple for an amp!

For the interconnects, I was referring to a converter that you plug into the wall and then you plug the ancillary equipment into it. The converter takes 220 volts from the wall and converts to 110 for the equipment to use.

My reason for doing this is that my tube amp is rated at 35 watts / channel. I'm buying new speakers after having my Paradigm 5se's for over two decades.

I naively thought if I doubled the voltage I could ~double my available power (~quadruple at 4 ohms?) and that would vastly increase the pool of speakers I could choose from (e.g., Totem, Magnapan).