Thought this would be an easy R&R operation - but Noooo! Problem: Found an open primary in the output transformer of a Philco 41-250 - a phase inverted, push-pull output (the weird thing is, it's open on both sides of the ct). So, went to my AES and Radio Daze catatlogs to select a replacement and found that the only specs I can match are the physical dimensions. There is not much data on this radio, save the schematic which gives the primary resistances: 190 ohms->ct->170ohms. The catalog specs are for impedance. Now, I can crunch the numbers if I have all the formula variables, but I don't know coil inductances or voice coil impedance. I'm stuck with three options: Hammond 124D, 125C, or 125H???? I'm many of you guys have replaced these units, so before I make a random guess and end up sending the replacement back, I appeal to the experts!

I would go with one of Hammond's 125 series univeral p-p xfmrs - see RadioDaze or AES. The 125B is rated for 5W output, $26.83 at Radiodaze. You can play around with various hookups, but it shouldn't be too critical.
Doug

:Thought this would be an easy R&R operation - but Noooo! Problem: Found an open primary in the output transformer of a Philco 41-250 - a phase inverted, push-pull output (the weird thing is, it's open on both sides of the ct). So, went to my AES and Radio Daze catatlogs to select a replacement and found that the only specs I can match are the physical dimensions. There is not much data on this radio, save the schematic which gives the primary resistances: 190 ohms->ct->170ohms. The catalog specs are for impedance. Now, I can crunch the numbers if I have all the formula variables, but I don't know coil inductances or voice coil impedance. I'm stuck with three options: Hammond 124D, 125C, or 125H???? I'm many of you guys have replaced these units, so before I make a random guess and end up sending the replacement back, I appeal to the experts!

Stephen,

The recommended plate-to-plate AC impedance for the 41 tube in push-pull operation with fixed bias is around 12000 Ohms. The secondary impedance is most likely 4 Ohms. You may find a 'universal' output transformer at the two places mentioned and you can try the various taps to get the best sound quality and volume.

Radiodoc
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:Thought this would be an easy R&R operation - but Noooo! Problem: Found an open primary in the output transformer of a Philco 41-250 - a phase inverted, push-pull output (the weird thing is, it's open on both sides of the ct). So, went to my AES and Radio Daze catatlogs to select a replacement and found that the only specs I can match are the physical dimensions. There is not much data on this radio, save the schematic which gives the primary resistances: 190 ohms->ct->170ohms. The catalog specs are for impedance. Now, I can crunch the numbers if I have all the formula variables, but I don't know coil inductances or voice coil impedance. I'm stuck with three options: Hammond 124D, 125C, or 125H???? I'm many of you guys have replaced these units, so before I make a random guess and end up sending the replacement back, I appeal to the experts!

Stephen:
Try this link:

http://radioremembered.org/outimp.htm

Lewis

:Thought this would be an easy R&R operation - but Noooo! Problem: Found an open primary in the output transformer of a Philco 41-250 - a phase inverted, push-pull output (the weird thing is, it's open on both sides of the ct). So, went to my AES and Radio Daze catatlogs to select a replacement and found that the only specs I can match are the physical dimensions. There is not much data on this radio, save the schematic which gives the primary resistances: 190 ohms->ct->170ohms. The catalog specs are for impedance. Now, I can crunch the numbers if I have all the formula variables, but I don't know coil inductances or voice coil impedance. I'm stuck with three options: Hammond 124D, 125C, or 125H???? I'm many of you guys have replaced these units, so before I make a random guess and end up sending the replacement back, I appeal to the experts!

The impedance at the primary is related to the square of the turns radio multiplied by the impedance of the load on the secondary.

I seem to recall a very rough rule of thumb. Measure the DC resistance of the speaker's voice coil, and multiply by two. That gives a rough idea of the speaker impedance at 400Hz, which is the freq at which speakers are typically rated. In fact, a speaker's impedance, while perhaps nominally 4 or 8 ohms, will vary much with audio frequency, which tends to throw things into a cocked hat.

Ideally, according to theory, making the load impedance at the xfmr primary equal to the plate resistance of the output tube(s) maximizes audio power to the speaker. But, in practice, I think the load impedance is usually desired to be less, maybe half?

So do what Philco probably did to 67+ years ago when they designed this set - try different taps on a universal xfmr, and see what works best for you.
Doug

Thanks guys, guess I'll get the universal transformer with multiple pins and play around with it. Have changed many a single end OT with old parts but It's probably time to get up close and personal with really tuning one for best performance. Also, Thanks Lewis for the great website on OT Impedance determination. Carr's "Old Time Radios! Restoration and Repair" didn't go into enough practical detail. What would we do without you guys.....except have a room full of boat anchors!!