Push-Pull Output Transformer 125E (Item: HX125E)

P-P output transformer 15 watt #125E - modification for 39 ohms

Posted by Dave Poland on Sep 9th 2016

Several E H Scott radio models from the 1930's use 39 ohm impedance voice coils. The 14 tube Scott Masterpiece model amp with 6V6's in push-pull is rated for 15 watt audio, per documentation. Available universal output transformers max out at 15 or 16 ohms.
This 125E universal transformer turn ratio ( lug 1 to lug 6) has a turn ratio of 26:1. I needed 21:1 (measured from another good original Scott transformer for this Scott speaker for 39 ohms voice coil).
For this particular replacement transformer, there is room between iron laminations and the core to carefully add a layer of wire. After experimentation, I found 12 feet of #22 gage magnet wire connected to the #6 lug and continuing to the RIGHT of lug 6 increased the ratio to about the desired 21:1 turn ratio.
Do use electrical tape on the metal to protect the wire from abrasion while winding the additional wire around the core. Wind the wire fairly tight and neat, and slip 3 or so inches of spaghetti at the end, so the end of the added wire end can be wedged between lug 1 and lug 2 to become a new lug #7. The added layer of wire is left exposed.
This transformer will not fit into the original bracket which also holds the speaker terminal strip. So I relocated the original bracket enough to mount the new transformer tightly agains the original bracket (after removing the old transformer from the old bracket).
Success - the radio sounds good and the impedance is about right.