41-255 Output Transformer - Printable Version
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41-255 Output Transformer - Raleigh - 09-19-2015
I'm working on a 41-255. Both primary legs of the push-pull output transformer (P/N: 32-8120) are open with respect to the center tap. I searched the Phorum to find advice on a replacement and found this thread: http://philcoradio.com/phorum/showthread.php?tid=13098&page=2
On the basis of this information I ordered a Hammond 125B. When it came I measured the primary resistance and noticed that the two legs of the primary had different resistance with respect to the center tap. 124 ohms / 172 ohms. The original did, too, (170 / 190), but with the Hammond the difference is much greater.
The schematic marks which 41 output tube the 190 ohm resistance leg of the 32-8120 goes to (the tubes are not wired identically.) I know that winding inductance, not resistance, is the critical factor, but otherwise the workings of transformers are a mystery to me. My question for those who do understand transformers is: does it matter which primary wire of the Hammond I attach to which 41 tube?
RE: 41-255 Output Transformer - OZ4 - 09-19-2015
Somethings not right. According to Nostalgia Air the 41-225 does not have push pull output. Which radio are you working on?
RE: 41-255 Output Transformer - klondike98 - 09-19-2015
He's working on a 41-255 http://www.nostalgiaair.org/pagesbymodel/408/M0013408.pdf
RE: 41-255 Output Transformer - OZ4 - 09-19-2015
I must be missing something.
http://www.nostalgiaair.org/Resources/395/M0013395.htm
RE: 41-255 Output Transformer - OZ4 - 09-19-2015
Sorry I can't read yet.
RE: 41-255 Output Transformer - Mondial - 09-19-2015
No, it does not matter which primary wire goes to which 41 plate. Each half of the primary should have the same number of turns.
The reason why the resistances are different is because of how the transformer is wound. The primary is basically wire wound on a bobbin. At the beginning of the winding, the bobbin diameter is small. As the winding continues, the diameter increases as more wire is wound, so for the same number of turns a longer length of wire is needed. Since the second half of the winding has more wire for the same number of turns, its resistance is higher than the first half.
The way expensive transformers get around this is called a bifilar winding. Both halves of the winding are wound simultaneously as a pair of wires, so for the same number of turns the wire lengths are equal and therefore the resistances are the same.
RE: 41-255 Output Transformer - Raleigh - 09-19-2015
Great! Thanks for the explanation.
RE: 41-255 Output Transformer - codefox1 - 09-20-2015
Now the sole exception. If you have two radios, each with PP output, but the primaries are reversed, and play them in the same room, they will be out of phase and will not sound as good as if they were the same. I think nobody worried about this ever in the past, and as a practical matter, the number of audio stages might or might not be the same for any two sets as each stage inverts the signal.
Of course you will really notice the difference in a stereo setup if you miswire your speakers.
RE: 41-255 Output Transformer - Raleigh - 09-20-2015
Codefox, can you reverse the secondaries to correct this, or does it only have to do with the primaries being in phase?
RE: 41-255 Output Transformer - codefox1 - 09-20-2015
Well, glad you asked, because it does matter when there are feedback circuitry attached to the secondary windings, and I was not thinking of that when I responded earlier. Generally we want negative feedback, and if you reverse the primary windings on an output transformer you will get a terrible hown when you fire it up. So whenever you are faced with this hook up a junk speaker and variac to sort it out. I suppose you could use a signal generator and scope to get it right the first time, or you can do it my way.
RE: 41-255 Output Transformer - Raleigh - 09-20-2015
And what is your way?
RE: 41-255 Output Transformer - codefox1 - 09-20-2015
Well, I power the set up slowly, and if I get a howl, I know I have made a mistake. Incidently the polarity of the secondary sometimes matches the norm (black ground, green 4, purple 8 if multiple secondaries) and the primary (Red-B+, blue-plate1 and if PP, brown plate2.) But not always. Some single ended sets had a reverse winding on primary to obviate a larger filter capacitor, and that can be confusing as well. Don't think Philco used them but maybe they did.
Anyway, best to leave well enough alone when fixing up an old set, and take it easy if doing a restoration. Schematic may give you a rough idea of whether you have replacement hooked up correctly.
RE: 41-255 Output Transformer - Radioroslyn - 09-21-2015
Not seeing any negative feedback around the output transformer. Typically It's used to enhance the low frequency response of the audio amp. What you will see on the diagram is a connection from the secondary of the output transformer back to the cathode of the input audio stage though a few resistor and a cap. This does have a connection to the secondary but it goes to the terminal strip on the back of the set.
Terry
RE: 41-255 Output Transformer - Raleigh - 09-21-2015
Right, I don't think there is any feedback from the secondary. The connection to the terminal strip was so you could attach an output meter to measure the audio while aligning.