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Model 70 speaker transformer wireing - Printable Version

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Model 70 speaker transformer wireing - Tubesforme - 07-14-2016

I am replacing the transformer on a speaker for a Philco model 70 which uses a single 47 output tube. I have the transformer. The supplied speaker is from a Model 20. I also have the Philco schematic. My question is this: I know that the two wires, from the transformer are for the voice coil. I also know that there are two terminals used on one side of the transformer and that is the primary. Opposite the two primary terminals are two which are unused. I know that they are for use as tie points for the speaker connecting cable. Here is the situation. I also need to put on a new speaker cable. From the schematic for the Model 70 it looks like I should only have three wires to the speaker plug. I suspect that one of the primary terminals goes directly to the speaker cable to connect to the 47 plate. The other secondary connects to one side of the voice coil and to another speaker cable lead which goes to supply point "C" B+ for the radio. The other field coil lead goes to point "D", which is connected to the bleeder resistor. I can easily trace the correct speaker plug connections from the chassis. However is there a correct and an incorrect way to wire the speaker field coil? While I am very experienced with radios, field coils are almost a new thing for me! Thank you.

RE: Model 70 speaker transformer wireing - Steve Davis - 07-14-2016

Take a look here:  http://www.philcoradio.com/phorum/showthread.php?tid=14991   Scroll down a ways and you will see speaker wiring diagrams, the upper one would be right for your 70. Since the 70 speaker does not have a hum bucking coil, the polarity of the field coil doesn't matter.

Steve

RE: Model 70 speaker transformer wireing - Tubesforme - 07-15-2016

(07-14-2016, 11:44 PM)Steve Davis Wrote:  Take a look here:  http://www.philcoradio.com/phorum/showthread.php?tid=14991   Scroll down a ways and you will see speaker wiring diagrams, the upper one would be right for your 70. Since the 70 speaker does not have a hum bucking coil, the polarity of the field coil doesn't matter.

Steve

Thank you Steve. that is exactly what I was looking for. I had been told just to use the transformer that was on the 20 speaker and use only 1/2 of the primary and it would work. That I doubted. The impedance would be way off. I was able to get the correct transformer, but wasn't sure about the proper wiring. Once again I am indebted to you. Thank you.

Michael (Tubesforme)

RE: Model 70 speaker transformer wireing - morzh - 07-15-2016

Michael

You were told correctly, the 2766 transformer from 20 is used both as push-pull (like in 20 for 71A or in some later models for 45) and an SE (for 42/47 tubes). It has the ratio of 100:1 and is used for 0.65 Ohm voice coil impedance.
The 2673 transformer, that is used in 70 model, has 90:1 ratio and is used for 0.85 Ohm voice coils.
These are pretty close and the 2766 would work just fine in a 70.

RE: Model 70 speaker transformer wireing - Steve Davis - 07-15-2016

According to this page that Ron posted  http://www.philcoradio.com/phorum/showthread.php?tid=14991  the 20/70/90 speakers have a .7 ohm impedance voice coil.

Steve

RE: Model 70 speaker transformer wireing - Steve Davis - 07-16-2016

Mike, I do agree, the impedance match of a 20 speaker and output transformer is close enough. But, in order to achieve a close match, the plug should be rewired so the B+ is connected to the end of the primary rather than the center tap.

Steve

RE: Model 70 speaker transformer wireing - morzh - 07-16-2016

Steve

Yes. The CT has to be disconnected from its pin in the plug and the other P (ftom the fat pin) has to be soldered in its stead.