Robert asks: Hi Chris, I’m looking at importing an amplifier from the USA, what will I need to run it on a UK 240v mains supply?
Fantana: Thanks for getting in touch, Robert! Depending on what the amplifier is, your options can be varied or limited. On some amplifiers you might get away with just a wire swap inside the chassis, on others, major surgery might be required.
Your timing couldn’t be better though…I have a Benson Vinny amplifier here in the workshop that the customer wants converting to work with our mains supply. He purchased it whilst out in the States and as it is so small, he brought it back as hand luggage!
Unfortunately, it’s wired for 120v mains so cannot be plugged into our wall sockets without modification. A quick look inside shows that the mains transformer does not have a 240v tap, so a replacement must be sought and then swapped into the chassis. After reaching out to Benson, the company kindly supplied a 240v version of the unit made by transformer winding giant, Mercury Magnetics.
The first step is to remove the chassis from the cabinet, which in this case is held in by two machine screws on the underside. The chassis slides out with relative ease and we can now get a good look at the architecture and circuit topology.
I can see the six wires coming from the existing transformer, through a rubber grommet and into the chassis itself. Two of these are black, two are green, and the final two are red. Experience tells me that the red pair is the high voltage winding, the greens are our heater supply, and therefore the blacks must be our primary winding. I compare that to the replacement unit and can see that all the wire colours match up, except the primary side which now has a blue and brown pair, matching the homologated colours of our UK live and neutral mains wiring.
Before I can start working inside the chassis, I must discharge the hi-voltage electrolytic capacitors that store a lethal charge. I have a home-made discharge cable made up of a single 100k 2W resistor with crocodile clips attached at either end. By connecting one end to ground and the other to any point along the HV rail, any stored charge is safely drained away, making the amplifier safe to work on.
It’s best practice to make a detailed sketch of the existing wiring so you can have something to refer back to later. Of course, we live in modern times so a few photos taken on a smartphone provide all of the information that we need.
I can now desolder the six wires and remove the transformer from the chassis. The replacement unit is of the same form and dimensions, so bolts directly in using the same mounting holes and hardware. Once installed and the wires fed through the chassis grommets, I first connect the high-voltage red wires to their correct location, checking against my reference photos to ensure I’ve not got things mixed up. Next comes the two green wires, which in this amplifier are soldered directly to the jewel light socket.
Finally come the two primary wires – with the customer’s permission I take the opportunity to reconfigure the power switch and mains fusing arrangement to bring the amplifier in line with the latest UK safety standards. The neutral wire from the transformer is attached directly to the IEC socket, whereas the live from the socket connects to the fuse holder, then onto the on/off switch before finally connecting to the transformer. If this amplifier had a captive mains cable, we’d have to switch both the live and neutral wires.
With all of the internal work done, I can now fire the amplifier up on the test bench, and once all voltages are verified and checked the chassis can be loaded into the cabinet. Of course, being a modern amplifier a transformer swap is feasible, but if you’ve bought a vintage model and want to keep it original then I would recommend buying an appropriately sized step-down transformer – a separate unit that will convert UK 240v mains down to 120v or so. Good luck with your purchase!