Hi Oliver
Do not try and power it up again until you determine whether it's 240v or 110vSome versions had a voltage selector switch on them, to right of the output jack. If yours doesn't have a voltage selector then it should have a fuse holder.
Would seem strange to convert it to 240 and leave the american plug on it....
Are you referring to the plug that would plug into a power point or the power socket on the back of the organ.
A lot of international touring bands from that period of time had a 110 volt model for touring the USA.
If you've got a multlmeter you could check the DC resistance of the power transformer primary winding and see if it matches the other unit that's wired for 240V.
Unless you've got a technical background then it's probably best to get a tech to look at it.
The hum could be due to the filter caps drying out, another good reason to get a tech to have a look.
Lastly if you can get a hold of a 240v to 110v stepdown transformer then you could plug it into that, switch it on and if it doesn't fire up it's probably already been converted to 240v.
One other possibility is that the original linear power supply has been replaced with a switchmode power supply which will run on either voltage.
This is a classic organ from the 60's so that means it could be up to 50 years old. If it's in good physical condition then I'd spend a bit of money on it, take it to a tech, get it checked out and have the power supply recapped.
Then I'd sit down at it, and play The Animals, House of the Rising Sun, or the Doors version of Light My Fire and dream I was back in the 60's... when things were oh so groovy!
Good luck with it.
Greg