Bucking Transformer DIY (Drop 240V to 220V)



The problem with many china made tube amplifiers from the likes of eBay is the voltage requirement being 220V. Do not believe the voltage written on the back of most china HiFi. My now sold Mr Liang 845 SET amp was marked as a 240V unit but checking the 6.3V heaters showed them to be running high at 6.85V and the 845 heaters at 11V. This means all the voltages can be 10% higher including that HT !! A problem waiting to happen. A higher voltage may not be an issue for a transistor amplifier or CD player that is well made with over rated components. However with vacuum tube amps the heater voltage can be a problem in that it will shorten the tube life if they are running hot. Also the heat output can be an issue and the HT being higher may stress other parts.

This is an elegant solution to Buck the mains and drop 15 to 25V without any bulky step-down transformer or Variac. But first some other and crude methods to drop the mains voltage... The first is not recommended.

 

Power Resistors (Very Crude – Not Recommended)

Possible solutions to drop the voltage include using low Ohm power resistors in series with the mains side primary of the transformer. This is only ok if the unit does not pull too high a current off the mains. This should work on lower power units such as a tuner, transistor headphone amp or DAC etc. A cd player pulling 0.5A for example could use a 40R power resistor to drop 20V on the mains side.

I x R = Volt Drop
0.5A x 40R = 20V Drop
I^2 x R = Power
(0.5 x 0.5) x 40R = 10W !! - Wasted as Heat

So the resistor will dissipate 10W. Purchase a 25W metal body wirewound and mount it to the back of a metal chassis inside the unit in a very clear area or on a small to medium heat-sink. I am not sure what effect it will have on the sound through, I have not tried this myself. It is not an elegant solution. ... very crude, wasteful of energy as heat, but it will work. I would not use this method myself unless power use of the unit is extremely low. This method would be ok for items pulling around 100mA without heat issues. To drop 10V for example (240 to 230) then a 100R 2W resistor would work for 100mA load disipating 1W as heat wasted.

 

Variac or Step Down (Bulky)

A better method is to use a variac (or auto transformer) and set the voltage out to 220V. Problem solved. However these are not cheap to buy new and can be heavy and bulky. Some auto transformers also need to be mounted in a case adding to costs. One advantage of this method is that the core will isolate any DC on the mains. The mains should have zero DC but sometimes poor mains can have some and you typically hear this as louder than normal buzzing in the transformer although poor laminations can also cause this.

 

Bucking Transformer

The last method is to use a regular mains step down transformer wired in 'bucking' configuration as shown in the schematic. One side of the secondary winding is connected back to the primary Live. Depending which side of the secondary is connected back, it will either 'boost' or 'buck' the mains. That is it will 'boost' (increase) the mains voltage by what ever the secondary voltage rating is, or 'buck' (decrease) the mains voltage. In this example I used a 240 to 20V step down transformer. The 20V AC secondary will 'buck' (decrease) the mains by 20V. Live is taken off the remaining secondary and neutral is just connected through. The secondary winding must be rated greater than the current demanded by the equipment supplied as it is in series with the supply. The secondary will be carrying mains level voltage so avoid cheap nasty transformers with poor laminations and insulation.

 

Example :
Our Audio Equipment is rated at 1.5A @230V (EU unit for example)
So we need to drop UK 240V to 230V European
We require a 240 to 10V transformer (12V is more common so that is fine)

So :- 12V @ 1.5A = 18VA
We need some headroom to stop the laminations saturating so double this value at least for better regulation.
Requirement = 36VA minimum (240 to 12V). Higher VA is always better cost permitting but no need to go silly.
 

Testing

I tested this on a Mr Liang 845 tube amplifier which uses around 1A or just above, therefore a 2A secondary rated transformer for 'bucking' was ideal. That is around a 50VA transformer which is quite small, a fraction the size of a variac. Do not be surprised at how small the final bucking transformer may be since it is not the same as a step down transformer.

You could buy a nice fully potted transformer from RS or Farnell which negates the need for a case. Just insulate and heatshrink the primary and secondary wires having fitted your mains lead in and IEC output. Alternative is to use any old suitable rated transformer such as those used to power Christmas decoration lights, outdoor lights etc. Any AC secondary voltage 15 to 25V should do the job and be sufficient to drop the mains voltage enough to get you closer to the desired 220V.

I used 24V 60VA plastic boxed transformer (a good quality one) which I bought from a boot sale and it works a treat wired as above. My heaters then measured 6.2V which is good enough. The whole amplifier ran much cooler with a more sensible HT voltage. Thanks to <Dave H> for the original idea.

 

Bucking Transformer Circuit
Bucking Transformer DIY