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Philco 20 speaker wires became snack for new puppy - Printable Version

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Philco 20 speaker wires became snack for new puppy - WallaceRoger - 01-29-2026

I had the chassis and speak out of the cabinet for refinishing and it seems the new puppy decided the speaker plug was delicious... and chewed it right off the wires. I opened up the plug but of course these wires aren't color coded so I have no clue which wires go to which pin. The plug is "keyed" with the one smaller pin so obviously I can't just put them back any old way and hope for the best.  Icon_e_biggrin  This is the original great condition speaker for the cabinet, too.

I assume I can test resistance back to the speaker itself to determine which wires pair up, but not sure if they'd go top pins, bottom pins, sides....

RE: Philco 20 speaker wires became snack for new puppy - MrFixr55 - 01-30-2026

Hi Wallace.

Cleaning up in my backyard, I was amazed at the things that "passed" through our dog.  Ms. Fixr had a cat also (Notice that I did not say that it was "our cat").  One day she thought that the cat sat on a piece of tape and tried to pick it off her little cat butt.  1 foot later....

OK, the speaker wire has 4 pins.  Attached is a pic of one of my Philco 20s, that I "experiment" on, as the cabinet is a total mess.  The pic is of the speaker plug that I made out of a tube socket. Refer to the pic below:
   

From the outside view of the chassis rear apron, the 2 smaller pins at the bottom go to the plates of the output tubes.  which is which is not important. The upper right large pin is the output transformer primary center tap and one leg of the field coil.  This point is denoted "D" on the attached schematic diagram below.  The upper left pin is Point J on the schematic, the other end of the field coil. The schematic of the Audio section:
   

Hope this helps.

RE: Philco 20 speaker wires became snack for new puppy - WallaceRoger - 02-02-2026

Just so I'm clear, it looks like the two wires that should test to 500 ohms resistance are for the two 71a tubes, located on the bottom two pins, and their order isn't important?

Is the order important for the two field coil wires (top two) coming off the speaker? It looks like these should show 3100 ohms resistance, but if the order is important how do I determine which wire should pin to D, for example?

I may just be overthinking this but sometimes I'm a knucklehead and have learned to be risk averse!

RE: Philco 20 speaker wires became snack for new puppy - MrFixr55 - 02-02-2026

Hi Wallace,

No reason to be ashamed of being risk adverse, as some of these parts are hard to replace. Risk adversity is less exciting than taking risks but it is also way less expensive.

Refer to the schematic of the audio section that I previously posted. Yes, the 2 wires with a 500 Ohm reading between them go to the 2 bottom pins. Order should not important. Yes, between the other 2 wires should be 3100 Ohms.

But the order of those 2 wires IS important. Measure resistance between one of the 2 wires to the 71A plates and each of the other 2 wires that go to the field coil. One of those 2 wires goes to both the field coil and the center tap of the output transformer. Resistance between that wire and either of the 2 leads to the plates of the 71As is roughly 250 ohms, depending on which plate lead you are measuring to. It is normal that the readings between this wire and each of the 2 plate leads are not the same. This is lead D and goes to the upper right of the socket (viewed from the outside. It is supplied by the power supply and delivers power to the field coil and the output transformer.

The other lead will measure approximately 3350 Ohms when measured against one of the plate leads. This is Lead J which supplies a lower B+ voltage from the field coil to other parts of the radio.

I don't think that you would smoke anything. However, reversing the wires would greatly lower the B+ voltage to the output stage and sound level and fidelity would be greatly affected.

RE: Philco 20 speaker wires became snack for new puppy - Arran - 02-02-2026

Greg;
I will have to look at the full diagram for a model 20 again, but in most tube radios the primary of the output transformer is connected to the B+ ahead of the rest of the radio since the audio section consumes the most current, and requires the most voltage. Pin "D" connects to the output filter cap in part #26, which is in a metal box, and #27 which is a filter choke. If would have been nice had they illustrated what goes to which pin of the speaker plug socket the way Mr. Fix shows. It's also NOT a smart design since the input filter cap of the power supply can still be energized with the speaker unplugged, leaving the B+ supply with almost no load, a lot of sets have a jumper on the speaker plug that disconnects the cathode of the rectifier tube from the input filter cap. That being said the model 20 uses a pair of #71As so the B+ is going to be well under 200 vdc, so it may only reach 250 vdc without a load, but Philco designed a lot of sets this way right into the mid 30s with much higher voltages involved, this may explain why so many of their sets had hard wired speakers.

https://philcoradio.com/library/download/service%20info/wiring%20diagrams/vol1/split/Part13-Philco%20Wiring%20Diagrams%20Vol.%201.pdf

Regards
Arran

RE: Philco 20 speaker wires became snack for new puppy - WallaceRoger - 02-02-2026

Perfect, this is everything I needed. Many thanks, guys!