[Patrick]

Other than resistance value, is there another consideration to a field coil?

Here's why I'm asking: If I have, for instance, an 8" speaker with a field coil resistance, of say, 5000Ω, at around 15W, can I replace it with an 8" speaker with the same wattage, but only 1000Ω? And just add a 4000Ω resistor of around 10W, with possibly increasing the size of the filter cap, also?

[morzh]

Yes you can do that. In case your fiedl coil is used as a filter.
If used as a load you do not need to increase the cap.

The wattage depends...10W is good to start number.

[Radioroslyn]

I got a question. Does it matter how much of the total current passes thru the fc and creates magnesium vs current that pass thru the resistor and just make heat??

I didn't go to skool for this stuff just a hobbyist...

[Chas]

For the radio in question:

1-If the field coil is shunt connected across the "B" power source, positive to negative then it is not part of the filter network for that source.

2-If the field is series connected in the positive of the source with one or more filter caps on either side the resistance is important to the supply and the inductance part of the filter network.

3-If the field coil is in the negative of the positive supply with a center-tap transformer the field is part of the resistance required for developing negative bias.

The these solution could be applied in most instances...

#1 Replace the speaker with a PM type if the resultant "B" voltage is too high add a sutable shunt resistance, but it does not have to be as low as the old field coil.

#2 Replace the field with a similar value resistor, it will usually be 10 watts. Increase the value of the last filter cap by 50%. Do not increase the value of the first cap as that will often incense the B+ and put a strain on the rectifier.

#3 This one is a bit tricky as there can be filter caps that reference either side of the filter but these are primarily to insure there is no hum in the bias, some sets also obtain a reference AVC level there too. For most receivers the field in this location can be replace by a resistor. If there are bias resistor shunting the filed or the field has a center tap resistors can be adjusted to provide the same bias. Be aware all current for the "B" system returns via these resistors so appropriate wattage's must be used...


For auto radios with the 6-volt field these can be replaced outright with a PM speaker, YMMV as there may be exceptions.

It is nice to have a PM speaker solution IMHO it does require these carefully made changes I feel that it is better to re-cone or rewind the fields than to do all the labor and sometimes marginal results from extensive electrical mods. YMMV... chas

[Patrick]

Looks like this may be more involved than I thought, since it's center tapped. Here's the actual schematic. The speaker was completely missing from the radio. I don't want to use a PM, 'cause I want it to closely match the other two speakers.

[Image: https://i.imgur.com/4QJXQlj.jpg]

[morzh]

Terry,

Yes it does matter.
The speaker will work all the way up to the nominal current, but obviously not as loud if the magnet is not as strong, and the magnet strength (Field (or induction) B) is proportional to the current (and inversely proportional to the distance).

So when using a speaker with a different field coil, as long as the current is met the speaker will feel OK. Of course this current might not be what the radio wants if the field coil is used as the filter, if, let's say, the field coil wants to see 30mA and the radio wants 70mA - in this case we are overstressing the coil as it will dissipate more power than it was calculated for.
But if one is using two similar speakers with coils both wanting, let's say, 70mA, but one is being 1K and the other - 500 Ohm, and you add another 500 Ohm to keep the voltages the same (as the sch counts on the coild dropping 70V) your speaker will be OK and work at full capacity.

It is easier to do with the speakers with coils used as a load and not the filter, as then you could adjust the current independently and not upset the voltages, as lon as your load is comparable to the original one.

[morzh]

Patric,

what you could do is this:

Measure your voltage with the 6100 Ohms resistor in place of the speaker. Then you will know your current for the speaker. Approximately it is 35mA or so.
Then you could possibly take any speaker that fits the size and has a field coil that is calculated to a similar current and use it with additional resistors to create the divider instead of the centertapped coil.

Or you could try to superimpose a divider on a speaker across the field coil, but then it has to be of much higher value than the coil itself and the current drawn bu the tubes' grids could act as additional resistance to GND, so you will have to try to adjust it to bring the voltages there where they want to be.

[tbone]

What a crazy design. Two field coils in the power supply, and another that’s tapped and used as a voltage divider.

The Schematic shows that the field coil from the tap to chassis is 3700 ohms. So in theory you can make a voltage divider out of 3700 and 2400 resistors, or a combination of available values to get you close. Then you could use a PM Speaker.

A speaker field with an approximate DC resistance of 2400 and a 3700 ohm resistor could possibly work, but as previously mentioned, there may not be enough current. Using the difference in voltages between the plate and screen of the 58 RF tube you could calculate the current through the 2400 ohm section. If I did it right, that would be 28-33 Ma. Hard to say if This would work on a different speaker.

Just a thought.

[morzh]

Tony

I would also recommend a perm speaker and a divider but then Patrick said he wants a Field Coil type, though I am not sure I understand "closely match to the other two speakers" part. Speaker is an acoustic transducer and the way you make the magnet, perm or electromagnet, is not necessarily super-important. However if this is a firm decision, then we have to work within the frame of the owner's desires
Which means either speaker and another resistor (or two resistors in series) or a speaker and two resistors in parallel. I think the former though.

Myself I would go with a perm. Let alone the fact one could by perm speaker way, way cheaper than a good FC one.

[Patrick]

The reason I want to match the other speakers is purely for aesthetics, as indicated by the attached pic. This is the way the radio should look from the rear. You can see how a PM speaker would stick out like a sore thumb.

[Image: https://i.imgur.com/S8g1GUl.jpg]

[morzh]

Well......an FC speaker with the FC not covered like it is here would do exactly the same.

   

Whereas this PM one wouldn't.

   

[Chas]

If the rear view of the radio and its speakers are accurate, then it appears the bad field coil is a speaker of which the field coil is easily removable and potentially rewound. Even if the cone is bad, the design of the speaker seems to support a re-cone as well. Rewinding and re-coning are not rocket science somewhat tedious IMHO less tedious than a "re-design" of a voltage divider as simple as it may be... chas

[Radioroslyn]

<The speaker was completely missing from the radio. I don't want to use a PM, 'cause I want it to closely match the other two speakers.

[Patrick]

Thanks, Terry.

[morzh]

Patrick, what diameter is it?

https://www.ebay.com/itm/vintage-SPARTON...SwL8pcw7GL