Hi to all,

Search doesn't work yet. I post my question here.

I have a set which is missing it's field coil.
I tried to wind a field coil instead of using a resistor.

I had a roll of copper wire of size 38.
Too small i suppose ?
I decided to wind a field coil with size 38 but in bifilar winding, until i reach the DC resistance i need. (1,3K)
Then i inserted a steel core of 3/4" diameter in the coil.

Testing the radio: i have the exact right B+, but my coil is heating too much. I measured 140v across the coil at 70mA.

What is the usual field coil AWG size wire in old speakers ?

Thanks
Vianney

Marv is the resident coil winding expert, so he will surely check in soon.

But, in the meantime, here are some thoughts. First, the steel core shouldn't be necessary, but won't hurt.
The heating power is V x I = 10W. If you use heavier wire, and achieve the same current and voltage drop, the heating power and temperature rise at the inside of the winding may be about the same. So, if the coil doesn't burn out, I would run it. If it does burn out, then you'll have to rewind it, obviously.

I did a field coil (over 3 miles of wire) for a Majestic (GG) 90B with #32 per measurements on the original wire. Actually fit into original speaker frame with a metal core (pole piece) Wouldn't think a fixed resistance of 1.3K would get any hotter with #38 than #32, although would be dispersed over a larger area using #32. May be the combination of core cross section and wire size (more flux lines). Any time you drop a voltage across a fixed resistance, it'll generate heat. I don't recall doing any heat measurements on my rewind, but the radio still works, occasionally on a yearly basis.

marv

:Marv is the resident coil winding expert, so he will surely check in soon.
:
:But, in the meantime, here are some thoughts. First, the steel core shouldn't be necessary, but won't hurt.
:The heating power is V x I = 10W. If you use heavier wire, and achieve the same current and voltage drop, the heating power and temperature rise at the inside of the winding may be about the same. So, if the coil doesn't burn out, I would run it. If it does burn out, then you'll have to rewind it, obviously.
:
:
:
:

Marv, somewhere I seem to recall reading that the main problem with using finer wire is it breaking during the winding process. If true, then the original poster seems to have surmounted that issue.

Doug,
If as stated, he used a bifilar technique, would have been less breakage. I used #38 rewinding the HV secondary on a power xformer for, a Heath BC1 AM tuner 2700 turns on a 1 inch core. I had rigged up a hand cranked winder, with electronic controls and motor driven jack screw mechanism for guiding and advancing the wire. Worked pretty well, but dealing that small wire is a real pain. Was successful getting it to fit orig EI core laminations, but took 3 trials and lotsa waisted wire. If he is rewinding just to use as a choke, far less restrictions.

marv

:Marv, somewhere I seem to recall reading that the main problem with using finer wire is it breaking during the winding process. If true, then the original poster seems to have surmounted that issue.
:

Hi,

I used a DeWalt drill with variable speed to wind my choke.

And the wire, even bililar, has broken several times during the winding process.
I had to resolder every time. 15 times at least.

I dunno how many turns i did, but each winding has 2600 ohms. Then in parallel makes the 1.3k i needed.

The #38 copper wire has been taken from an old car distributor coil.
The coil had a DC resistance of 10K.

My set works very well with that choke, but overheat after 30 minutes of operation, too hot for the fingers.

The size of my choke is small compare to the original field coil.
I believe i should have use size #32 instead of #38.