03-11-2016, 09:42 AM
One thing to remember is that the 37-670 has a triode class A-B output stage. That's why the old driver transformer has a step down turns ratio. As you increase the volume, the 6F6 grids begin to draw grid current on the positive audio peaks and go into Class B. The grid current loads the driver stage, so a step down driver transformer provides a low impedance drive signal needed for low distortion.
The winding resistance readings do give some indication of the turns ratio of the old transformer. Assuming similar wire gauge on both primary and secondary, the fact that the secondary has a much lower DC resistance implies that there are less turns than the primary, (34 ohms vs 200 ohms) which would indicate a step down ratio.
Step down driver transformers are no longer made, but you can try connecting the 124D in reverse ( using the primary as the secondary and vice versa. The will give you a 1.5 to 1 step down ratio, which would be closer to the original.
It will work both normally connected or with the windings reversed. So you can really try connecting it both ways and see which way sounds better.
The winding resistance readings do give some indication of the turns ratio of the old transformer. Assuming similar wire gauge on both primary and secondary, the fact that the secondary has a much lower DC resistance implies that there are less turns than the primary, (34 ohms vs 200 ohms) which would indicate a step down ratio.
Step down driver transformers are no longer made, but you can try connecting the 124D in reverse ( using the primary as the secondary and vice versa. The will give you a 1.5 to 1 step down ratio, which would be closer to the original.
It will work both normally connected or with the windings reversed. So you can really try connecting it both ways and see which way sounds better.
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