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Philco 20 Speaker Plug Question - Printable Version

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Philco 20 Speaker Plug Question - Mwryute1 - 10-26-2022

Hi:  First time poster, kinda sorta newbie.  I hope my question makes sense.  It involves attaching the speaker plug wires to the output transformer.  I had the speaker reconed.  I know there were some connections inside the basket when I sent it, but only the 2 voice coil wires when returned.  So......

If the bottom of the transformer as in the picture (part #, 2766J, oriented correctly, with the mounting holes at the top and bottom), where do I connect the field coil (and tap) and voice coil wires?  Don't know if it helps, but the top two tabs have a resistance of 450ohms (Imp. 16K), the tabs on the left side have a resistance of 215ohms (Imp. 2.8K).  The bottom and right side tabs test open.  What plug wire goes where?  Thank you so much for any assistance.        

RE: Philco 20 Speaker Plug Question - Radioroslyn - 10-26-2022

Hi and welcome to the Phorum.

Ok so what we need to do is find the center tap on the primary of the output transformer. That connects to one side of the field coil. In turn it go to #27 the choke in the HV side of the power supply. The other field coil lead goes to the primary of the interstage audio transformer and the bottom end of #18 the 1/4meg resistor. The two ends of the primary connect each plate of the 71A tubes.

Piece of cake!
https://philcoradio.com/library/images/schem/20b.jpg

RE: Philco 20 Speaker Plug Question - Mwryute1 - 10-27-2022

Thanks, that helps. Should I assume that the connectors reading around 250 ohms are the tap, since there are two that read about 500ohms? I know the 4 cords from the plug go to the four connector tabs, because I have a picture. I just don't know which ones go where for sure. I know that two are for the field coil and two are probably for the trans. primary (or maybe one for the tap?). I think the connection for the center tap may have originally been under the basket and hard to see. After reconing, all the wires under the basket are gone, except for the voice coil. As you can probably tell, I have poor spatial skills and am easily confused. This is more like remedial radio restoration.

RE: Philco 20 Speaker Plug Question - Mwryute1 - 10-27-2022

Thanks, that helps. Should I assume that the two connector tabs that read 250ohms go to the center tap, since there are two tabs reading is around 500 ohms? There seemed to be wiring under the basket originally. After reconing, it is gone except for the voice coil wires.

I know that the four wires from the plug are connected to the four tabs on the transformer. I'm just not sure which wires go where on the transformer (I've been assuming that two go to the field and two to the primary. I forgot about the center tap). As you can probably tell, I have poor spatial skills and am easily confused. This is more like remedial radio restoration.

RE: Philco 20 Speaker Plug Question - Radioroslyn - 10-27-2022

Here's a drawing that may help. And https://philcoradio.com/library/index.php/shop-talk/model-notes/the-speaker-problem-in-early-philco-cathedrals-20-21-70-90/ should give you the pinout of the plug but I would check it with an ohm meter just to be sure. The model 90 w/p/p45's should have the same hookup.

RE: Philco 20 Speaker Plug Question - MrFixr55 - 10-27-2022

Hy Myrute, 

Welcome to the Phorum!

I am not sure what connections go where on the transformer because my Model 20 did not have the "Pie Plate" speaker.  

I have attached 2 drawings, one a drawing of the speaker jack from the outside chassis rear apron view.  The other is the schematic.

The Model 20 was a game changer for Philco, starting the "Cathedral" radio craze and rocketing Philco to one of the largest (if not the largest) manufacturers of radios.  For a "low priced" radio, the Philco had 2 stages of "screen grid" RF amplification, a "biased" detector (better fidelity than the grid leak style detector), 1 stage of audio amplification and a Push Pull Audio Output for more power and better fidelity.

This speaker does not have the "Hum Bucking" coils that later electrodynamic speakers have. If hum is excessive, reverse the 2 connections to the field coil.

Search the Philco library for a great Ron Ramirez article on a circuit mod for the audio interstage transformer that improves fidelity and helps prevent burnout by capacitively couples the transformer to the plate of the first audio stage.

IMHO, the speaker is the only weak link to this radio.

Good Luck with this winner of a radio.

RE: Philco 20 Speaker Plug Question - Radioroslyn - 10-28-2022

The drawing of the speaker jack is a little confusing as in real life it has 3 large holes and one smaller.

RE: Philco 20 Speaker Plug Question - Mwryute1 - 10-28-2022

Thank you so much for the help. I think I can figure it out now.