I have started my next project, a bucking transformer. I initially had problems finding a suitable transformer until I remembered an old 13.8v power supply I had for powering an AM CB radio back in the late 90s. I dug it out of the garage and tested it's output, 19 volts, which would make it ideal for use with 220v devices. However, quite a lot of these Chinese tube amps are 230v and I also worried that I would suddenly have a use for that power supply so I opted to search for another transformer. This is what I found:
"Custom audio 64W multi-purpose power transformer Multi-voltage input secondary multi-winding output voltage adjustable" https://www.aliexpress.com/item/32982855405.html
The 'B' version seems to have all the right specs to reduce the voltage to 220v and 230v except for the 2 amps output. I would have thought that the amperage would vary across the output voltages so if 32v is 2A, then the 20v and 10v output I need would be 3.2A and 6.4A respectively? I went ahead and ordered the transformer anyway and plan to use this schematic that I modified from here https://sound-au.com/articles/buck-xfmr.htm
Completed the wiring today and it works as designed:
Well my eBay project box finally arrived today and I cut all the holes and mounted the hardware. Will do all the wiring tomorrow or Friday. All the bits
Mounted the terminal block I salvaged from an old electric wall heater.
Transformer mounted
Holes cut into lid. Handles, 220/230v switch, voltage display and mains outlet fitted.
Proof of concept. Mains voltage was 242VAC at the time of testing:
The transformer arrived today. Coincidentally, so did the cheap 80-500VAC digital display I ordered from a different Aliexpress seller. While all the output voltages of the transformer match those pictured by the seller, the input voltages are only 230v and 240v - not the 220v to 250v as pictured. So if you intend to order one, make sure you specify the input voltage you require. Now to find a suitable case for it.
Update: I did a quick check of the outputs and they are all 1 to 5 volts more than specified. General specs: Dimensions: 8 x 8 x 7cm (3.14 x 3.14 x 2.75in) Weight: 1.84kg (4lbs)
Now the bad. Despite being wrapped in thick foam padding, one side of the mounting brackets are badly bent. There was no visible damage to the packaging so it likely occurred prior to being shipped to me. Hopefully the windings are intact.
Now I am worried. This transformer is pictured showing 220v, 230v, 240v and 250v inputs (see below) yet the seller messaged me to ask if I want 220v or 230v? They followed up with "we send input 230-240V to you". Now I don't know what they are sending to me 🤔
Useful for many Chinese amps where they lie about the voltage on the transformers. A brand that is notorious for this is Yaqin. When you specify a certain voltage, they "convert" the voltage on the stock transformers by spray-painting the specified voltage onto them with white paint. White paint is very cheap in China, so it's much cheaper to convert them that way lol.