[chalupa13]

Hello, I've got Philco 77 radio with an open field coil. I tried to unwind it to see if the break was close to the top and completely destroyed it. Would it be possible to rewind it from scratch? The spool is just cardboard with heat tape over it and the wire seems to be 32awg. It's a lot of wire probably around 1lb, I don't know how much wire it uses, but do I need to be exact with it? Can I just wind it to about the same size it was before? Any potential damage I can do to the radio? Thanks,

[morzh]

The field coil is often used as a part of the DC filter, and also as a way to drop the voltage to the needed value.
Changing it will affect the result of filtering and the voltages in the schematic.

While there is certainly some tolerance built into any tube radio circuitry, it is still a good idea to adhere to the initial specifications as much as possible.

The specification of the DC resistance are usually known: they are listed in the documentation and are often printed in the schematic.
The type of wire could be easily determined by gauging the wire from the old coil.
Then find out what specific resistance (Ohms per Foot) the wire of that gauge has.
Then see how many feet you need to achieve the target resistance.
Then using the same or the same type bobbin wind the calculated length of the wire onto that bobbin.
If done neatly, the resulting inductance and the magnetic properties (as well as the thickness of the winding) will automatically go pretty close to the original specifications.

I do not think you could easily damage the radio, but much lower resistance will result in the hightened B+ voltage and so in the tubes running hotter.

[chalupa13]

Thank you for the explanation. The field coil has resistance of 3.2k Ohm according to the parts list. That's a lot of wire at 32awg about 20,000 feet, so I am assuming back in the day wire was not as good as today's standards so probably had higher resistance? That's why I thought I'd go by weight instead of resistance, I can always add a 5w or even 10w (just to be safe) resistor in series to compensate for the resistance drop, or is it a bad idea? Thanks,

[Radioroslyn]

Best I can calculate it's about 15,000' will get you to 3165 ohms which is about 4 pounds of wire. Using less wire and adding resistors is not a good idea. It lessen the filtering and magnetic pull that the coil can develop.

[Forrest]

One other option is having it rewound there is a club I believe in Alabama that has a machine to rewind them it is expensive I believe about $65.00. Person to contact would be Dee Haynes
205-841-4630.
Forrest.

[morzh]

I used Dee's services twice. Good guy.

As for the quality of wire not being the same....no. Copper is copper is copper, and a AWG32 wire will have 160 Ohms per 1,000 feet today as it had 80 years ago.

[chalupa13]

Thank you all for the suggestions. I think I figured out what wire I need. I weighted the old spool and it was just under 2.5 lb. I matched that weight with wire sold today and came up with awg of 33 that gives me almost exactly 3200 Ohm for 2.5lb of wire, approx 15,500'.

[chalupa13]

One more question. It it ok to use plastic spool for this coil? Does it get hot? I can 3d print one myself but material won't hold up to more than 80C. The old one was made of cardboard.

[radiorich]

chalupa13,
Some 3D printers can print in Nylon that would be the best but I sure ABS would be good I would not use PLA .
I have a Flash Forge Pro in my classroom where help teach .

Sincerely Rich

[chalupa13]

Just wanted to report that my rewinding was a success. 2lb of 33awg did it for this one. got it to 3280 ohms.

[Radioroslyn]

Very good!
I cheated I don't know if you saw it but there was a "rebuilt G speaker" on epay. My model 87 has been vacationing out in the shed only to find that the field and opt are open. Grrrr!! So I bought it. Looked great, field rewound, new paper cone, and the opt has a new paint job. Checked it over with the ohm meter and looked good. Did a few repairs on the 87 chassis. Mostly the input cap in the p/s has shorted and tried to takeout the 80 but caught it in time before any serious damage. Speaker sounds awful! Cone off center and buzzing at certain frequencies. I'll probably put an original in it.

[Stan the Man]

Hi guys,.. I am just about to start my own field coil rewind. Philco 76 with a "G" model speaker. Rich, I am very curious to know how well your new field coil works.
Did you wind it neat, or scramble wind it? Any tips?

Stan

[chalupa13]

I've made a jig to wind the coil from plywood and 1" pvc pipes. I also used battery powered drill to wind the coil. It came out ok, but not perfect. Still works great.

[Stan the Man]

I was somewhat relieved to see that you also came up with #33 wire. I've had a spool of it sitting here waiting on me. I bought a cheap coil winder, as I plan on doing more than this one field coil. I just have to come up with something to hold my free spool, then I'm ready to go. I also have an output transformer to attempt.

Stan

[radio1]

Looking at it,  there are many different schemes utilizing the field coils in these old radios. It leads me to questions of how they used it. In this case you could substitute resistors for the field coil. In looking at the Philco 70 circuit, replacing the coil with only resistors (which is what I have at the moment), it would seem the screen before the plate (supplied by the coil) would allow the rf ac, from grid to plate, to ride the DC right back into the output transformer, so I wonder this, does the the field coil stop this ac(rf) from entering the output transformer (other sections from which it is intertwined? Very perplexing to me. In substituting just resistors, does it require a choke also? I put the scope on it and saw this DC rail ride the RF back to the output transformer when using resistors and not field coil, does the field coil block screen rf on the dc rail?