Custom Power Transformers

Due to limited information and misinformation available on the Internet, it would save time and money to read the accumulated explanations, presented below, given prior to building a proper working transformer for the customer.

Power transformer design considerations:

An electrostatic shield between primary and first secondary, is to keep the device the transformer is operating from interfering with the power line, not to shield the device from the power line. This shield will not make the power supply hum any less than the filter design used. It also will decrease the efficiency of the transformer and increase heat rise.

Additional taps will make the transformer inefficient, decrease regulation and heat up, due to the winding space increase to via the tap leads, and this decouples the winding. If a variable voltage is needed there are other means.

We cannot design a transformer with the DC result needed. You must have all the secondary AC RMS (working voltages) and current requirement of each winding and its use. The rectifiers filter capacitors, chokes, resistors and current flow will decide the voltage result. It is best to breadboard a power supply with another transformer close to the needed ratings with a Variac and a load resistor of proper power rating, representing the circuit current and voltage need. When all voltage, current, ripple (hum) specs are met, measure the AC voltage/s at the transformer, this is the RMS value that is used to design a proper transformer, or we can guess and maybe you'll get lucky.

Customer specifications or designs derived from computer programs will not work in the real world. If you wish the transformer built to these specifications, you build it!

Increasing winding current higher than the needed circuit current will increase the resulting voltage across the item powered due to the winding only being partially loaded. This will also increase the size and cost of the transformer.

Steel or any metal container will increase heating and primary current due to it acting as an external shorted turn. It will not decrease the hum field only divide it up around the perimeter of the metal container. Rated primary current must be keep correct if transformer is to be in a metal container. The use of nonmetallic containers is suggested.

A multiple winding transformer can distort the field of the transformer primary and can increase its heat rise, even if its VA rating is met. The need to replace the M19 type laminations to M6 (grain orientated) will decrease the heat. When a transformer is built and tested and the heat rise is unfavorable, Electra-Print Audio co. has the right to change the laminations to comply with safe and proper operational practices without customer’s permission. The price of M6 is higher.

All power transformers will run warm to fairly hot. This is normal operation. The laminations act as shorted turns and will express a little heat. Requesting a perfectly cool running power transformer will be ignored.

Multiple winding transformers can be very large and impractical to wind and will need to be divided up into separate transformers. The lamination size only allows a certain amount of wire that can be used. You will be notified if transformer will need this problem corrected when quoted.

The size will be determined by its design and not by the customer. A 400 watt transformer cannot be built the size of a walnut.

The use of exotic or weird materials suggested by the customer will be met with considerable reaction.

The transformer primary MUST be fused properly, total VA rating divided by line voltage equals line current drawn, times 133% is the fuse rating ( example, 200va/117v =1.7A times 133% is 2.2A or 2A fuse). A SLO BLO for solid state rectifiers or a standard fuse for rectifier tubes or slow warm up. A transformer failure is mostly due to external circuit shorts, overload and improper fusing. A larger fuse or no fuse at all for better sound is a recipe for disaster. A VERY DANGEROUS FABLE HAS BEEN INCREASING AS TO DIFFERENT FUSES OR NO FUSE OFFERING A BETTER SOUND, THIS IS NOT TRUE AND ANY RETURNED BURNED OUT TRANSFORMERS WILL BE NOT BE HONORED. A fuse that is blowing out repeatedly is telling you there is a problem to locate.

If a power transformer is left on continuously it will continue to heat up and could fail. One of the other fables is, leaving an amplifier on continuously for better sound. This will only increase your electric bill, wear out your tubes faster and maybe burn your house down.

If two windings of same voltage are to be paralleled for the purpose of increasing current, it should be understood as to the proper way to connect these windings in phase or the transformer will burn out. Please indicate this need in design so phasing color code can be applied. We cannot read minds.