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Replacing a field coil speaker - Printable Version

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Replacing a field coil speaker - bridkarl - 03-28-2026

I haven't done this before, but it's time I figured this out. Not a Philco but a Fairfield Morse. As I understand this I replace the field coil (circled on schematic) with a resistor of the same value and high wattage and put a permanent magnet speaker where the speaker goes. There's a cap/resistor in series across the speaker which I assume stays. Am I missing something??

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RE: Replacing a field coil speaker - RodB - 03-28-2026

You will be removing the inductive part of the filter which does the biggest amount of filtering, so, to make up for it the filter caps 15 and 25 will have to increase. Caution should be used on cap 25 because the 5Y3 may have a limit to the size of the filter input capacitor.

RE: Replacing a field coil speaker - RodB - 03-28-2026

I wouldn't go any higher than 12 mfd on cap 25.

RE: Replacing a field coil speaker - bridkarl - 03-28-2026

I gathered that since that would be necessary to reduce ripple. I'll have to work out exactly how much after I check the tube specs.

RE: Replacing a field coil speaker - RodB - 03-28-2026

And the resistor/cap, 48 & 49 are there for audio transformer protection and tone control. You can increase the value of the cap to get more bass. Sometimes there will be a switch to add capacitors to change the tone, called tone control. It's actually bypassing the highs.

RE: Replacing a field coil speaker - RodB - 03-28-2026

I see part 25 is already 16 mfd so I wouldn't increase that. The other one maybe use 47 mfd. If there's hum then put a 22 mfd in place of the 16.

RE: Replacing a field coil speaker - bridkarl - 03-28-2026

Good. Thanks. I'll get some parts ordered and in the meantime start on this fairbanks morse 5A using parts onhand which is more complete. Aside from needing to be reconed and the usual cap replacement it seems ok

RE: Replacing a field coil speaker - MrFixr55 - 03-28-2026

+1 on what Rod B says. I would use a 1K 10W resistor to replace the field coil. Better to raise the capacitance after the resistor than before, but per the RCA Receiving Tube Manual Ver RC14, 1940, for a 5Y3, an input cap in the filter (Cap between cathode of 5Y3 and center tap of transformer, the most negative point of the supply) of up to 40UF is tolerable. Per the manual, "When a filter input capacitor with a value greater than 40 MFD (uF) is used, it may be necessary to use more plate supply impedance than the minimum value shown to limit the peak plate value to the rated value".

RE: Replacing a field coil speaker - morzh - 03-28-2026

I would not be changing C25 no matter what the tube's specs are.
C19, on the other hand, could safely double. Or triple.

RE: Replacing a field coil speaker - bridkarl - 03-28-2026

I have an appropriate resistor on order.
While I wait I moved on to this other radio.I'm reconing the speaker. Sort of interesting 1930s design - the spider is cotton. You can see where the wires are underneath to the spider. Apparently the cone is glued to the voice coil and that oval moves the cone?? Not like a modern design. Sort of interesting - if the cone breaks you can replace it without soldering wires. just glue in a new cone

       

RE: Replacing a field coil speaker - morzh - 03-28-2026

Yeah, that one needs more than just the cone.

RE: Replacing a field coil speaker - bridkarl - 03-28-2026

I'm told the oval is the spider. I've got a new cone on order. Hopefully I can get it installed and centered correctly. Very useful learning this skill - I see why reconing as a service is so pricey.

RE: Replacing a field coil speaker - morzh - 03-28-2026

Yes the oval is the spider.

Or, in other words, anything which is elastic, rippled circle in today's speakers, piece of cardboard (like in Philco's K speakers), the 45 rpm (Philcos, Atwaters etc) to which the cone is glued to, is the spy-deer .

RE: Replacing a field coil speaker - Arran - 03-28-2026

Karl;
Depending on the needs of the power supply you can either use a wire wound resistor, in smaller radios, or a filter choke in the larger ones. This one is sort of borderline being a seven tube set.
Is the field coil open on that speaker you plan to recone? What I use for repairing speakers is thin strips of film negative, though you could probably use any stiff plastic of the appropriate thickness as shims between the voice coil and the pole piece. I think that spider is likely cloth impregnated with phenolic resin, or something similar, which is what it has adjustments on either end, with the corrugated cloth or paper ones you can adjust them by using shims, and moistening the spider.
Regards
Arran

RE: Replacing a field coil speaker - bridkarl - 03-28-2026

There's continuity on the field coil. I have any number of pieces of plastic - even a cut down playing card would work.

The coil seems well centered. The shim seems more to keep you from gluing it and the cone together.

The spider has little metal bits under the nuts holding the basket that I guess are for adjustments.

Nice piece of design really.