Instructables

Guitar Tube Amp From Scratch (Part 1, Transformers)

Step 3: DC Resistance on the Output Transformer

Found Transformers Set Up.jpg
Found Transformers Math I.jpg
 All we need is a small AC wall wart (anything between 4 and 10 volts AC would work), a multi-meter (we'll only need the AC voltage and the ohm-meter), a 1 Ohm / 1 Watt resistor, and that transformer of unknown quality.
 If the wires on the transformer lack any distinctive coloring, it would be handy to create labels.  This can be as simple as stripes of color using markers or folded tape with numbers.
 Using the ohm-meter, test all the possible combinations of wires to see what connects to what.  Usually the reading will be very low,  as a high DC resistance is rarely desirable in a transformer.  Draw out the connections and make note of the resistances. 
 On the output transformer I found four separate windings: two with two connecting wires each and two more that each had a center tap.
  For example: The yellow wire was 17.6 Ohms away from the red wire, but 8.8 Ohms away from the red/yellow.  I checked the resistance between the red/yellow wire and the red (8.8 Ohms) and confirmed that the red/yellow wire was directly in the middle of the red and yellow wires.
  Notice that the black-red-green winding has a slight imbalance.  This is fine.  The DC resistance is a good indicator but it only hints at the winding ratio; and the winding ratio is what we're interested in.
 
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