Audio amplifier power supply transformer problem
The right way to get the needed turn numbers when rebuilding a transformer is to calculate from a known ORIGINAL winding the right turns per volt -factor. I guess your mains AC side winding has too few turns.

Beware: If you test a transformer with no load you can accidentally break it by switching the mains AC input OFF. The inductive peak can break the insulations if you happen to be able to separate the transformer from the mains input fast enough (=no arc in the switch, but one inside the transformer) A load or another still connected device in parallel with the primary would absorb the peak. See this: How does the inductor &#39;&#39;really&#39;&#39; induce voltage?