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electro-dynamic speaker to permanent magnet.

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electro-dynamic speaker to permanent magnet. Empty electro-dynamic speaker to permanent magnet.

Post by acbaker123 on Thu Sep 17, 2020 9:45 pm

my 1930 echophone s-3 has a shot speaker that i made sort of work but i would really like to replace. i think it has an electro-dynamic speaker thoigh i did not do much research. is there any way i could replace it with a new permanent magnet speaker. the old speaker has a coil on the back with 3 wires coming out and going into the chassis somewhere, and two more for the output transformer which is mounted to the speaker. anyone know how i could do this?

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Post by acbaker123 on Fri Sep 18, 2020 10:37 am

i think that i can't just omit the field coil because it does something else important, but i might be able to separate the coil from the old speakerand leave it in, then attach a new speaker to the output transformer. are there any problems with doing this?

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Post by Chas on Fri Sep 18, 2020 1:31 pm

acbaker123 wrote:i think that i can't just omit the field coil because it does something else important, but i might be able to separate the coil from the old speakerand leave it in, then attach a new speaker to the output transformer. are there any problems with doing this?
See:  http://www.nostalgiaair.org/pagesbymodel/702/M0039702.pdf

The speaker field is in the negative return and is tapped for bias for the 45 tube. Because the field shunts the filter choke there will be some increase in hum if the field is replaced with resistors. This can be compensated by increasing the value of the 2nd filter condenser to 30mf or so.

Sorry, despite shrinking the image of the schematic I could not get the board to accept the image that could be readable...

IMHO I would re-cone the speaker, even if the spider is damaged it is possible to cut a new spider of every day materials. Use a reversible glue and shim the voice coil. Depending on the speakers construction a jig may have to be made to properly glue the voice coil to the cone. If the voice coil is damaged it is also possible to rebuild/rewind. All with simple home tools. You will need GC speaker service cement and one maybe two solvens Acetone (mostly and Toluol, Personal protection such as filter masks, a clean well ventilated area where one is not disturbed. Old speakers were assembled with colodion solvent glue and hide glue for certain permanent bonds. Colodion glue is dissolved with acetone. Use disposable long body eyedroppers to apply a bead of solvent around the rime of the basket, determine how to extract the cone with or without taking out the voice coil. Many re-cones are done by leaving a lap joint at the cones apex, shim the voice coil cleanup the landing zone for the surround prep each surface to be glued with a thin coat of G-C cement AND LET THE CEMENT DRY!. Now position the cone and find a ring shaped weight of enough ounce to keep the new cone pressed against the lap joint at the apex yet leave room to access the joint with the eyedropper. Be sure the windings of the voice coil are centered in the magnetic gap, not too far in or out. Use clothespins or other similar method to keep the surround in contact with the landing zone. Once all pre-glued area are in contact, flow toluene with an eyedropper to all the joints. That will activate the glue and it will bond. Give about a couple of hours to cure/harden. Then remove the clips, and the center cone weight, then pull the shims. Glue a new perimeter gasket or use a new felt gasket (make) felt from Zoro Tools works good. For felt glue use contact cement, coat felt twice and landing zone once. Let contact cement dry. The contact cement is unforgiving so install carefully.

Be careful how much of the apex is cut on the new cone, if the cone sits too deep the pleats will hit the basket and bass tones will be muffled and buzz.

New cones with pleated surrounds are available from ebay vendors, you may have to buy two cones. Foam surround cone will work IF the speaker is pedestal mount and a thick felt gasket is used to space away the speaker from the baffle.

To replace the speaker measure the DC resistance of the field with an analog ohmmeter. get 10 watt resistors. Remove output transformer and install on chassis or on new speaker...   Chas
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Post by acbaker123 on Fri Sep 18, 2020 1:40 pm

the cone is in decent shape and so is the coil, but the paper ring that holds the cone to the metal frame was completely gone, i tried to replace it with strips of stiff paper, i did not go all the way around because it was a stiffer paper. i dont know if this is what is making it sound bad or if the cone itself is stiff.

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Post by acbaker123 on Fri Sep 18, 2020 2:45 pm

is the second filter you are talking about c15 on the schematic, the 2mf one

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Post by Chas on Fri Sep 18, 2020 8:30 pm

Yes, without the surround to hold the cone in position the voice coil (form) will rub on the pole pieces, pivoting on the spider.  I did a quick search of ebay, since you did not determine the speaker size I guessed at 10" about all that can be driven with a single ended '45...

See here check all the dimensional information before purchasing any cone...

https://www.ebay.com/itm/10-Waldom-Speaker-Cone-W-5422-1-1-4-VC-hole-2-1-4-Depth-Rubber-Edge/112995639739

Or A vendor and speaker type I have used, no worries on the tiny apex opening, this gets cut back for a lap splice to the OEM voice coil form.

https://www.ebay.com/itm/SPEAKER-CONE-10-FIT-EV-JBL-C-VEGA-PEAVEY-JENSEN-DP-VINTAGE-SPEAKER-REPAIR/131937238463


G-C speaker cement, pricey so shop around, then again I got mine at a close out for $10/qt.

https://www.ebay.com/c/1804437986

Eyedroppers

https://www.ebay.com/itm/FINGOOO-25pcs-3ML-Plastic-Disposable-Transfer-Pipettes-Eye-Dropper-Set-Graduated/363018084229

Acetone & Toluol from Home Depot or Lowes, solvent is very costly to ship to an individual.

You can make shims from recycle plastic bottles or brass shim stock. Use multiple shims of known thickness and build up evenly on four sides of the voice coil at the center pole-piece. Avoid distorting the voice coil, just enough shims to hold the voice coil in position, centered...

--------------------------------------------------------------------------------------------

Yes, C15 can be raised starting at 10mf, to control hum caused by lack of inductance... Do not raise the mf values of the capacitor that is attached to the rectifier filament. If that value is raise the plate voltage will rise...  Chas
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Post by acbaker123 on Sat Sep 19, 2020 3:47 pm

the resistors and cap is only if i want to convert it to a permanent magnet speaker right?. can you elaborate more on how the resistors are installed. i figured they just replace the field coil in their respective resistances.

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Post by Chas on Sat Sep 19, 2020 4:24 pm

Measure the resistance of the field coil including the tap as the bias voltage source is the tap. IF the radio can be energized for a 1/2 hour, safely, measure those resistances when the coil is warm, power off of course. But also measure the voltage across the coil before turning off the radio.

Use an analog meter, many digital meters have problems measuring the resistance of a large inductor.

With those two values one can determine the proper resistance as well as the starting wattage to use using ohms law. Add 50% to the wattage and then adjust higher to the next manufactured wattage. In selecting the resistance, consider the total resistance and select two 5% values based on the nearest 5% available. The bias can vary +-5% and the '45 will be fine... Hopefully, if there is a problem with the bias too low, flipping the values will be correct.

You may want to actually measure the value of bias at that tap on the speaker field.

I have not seen a full, balanced (L/L-C-C) pi filter used in a radio. Apparently there is no hum balance devices used. In all possibility, this design may be more effective in lowering hum (120hz) than using balance pots in the filaments, would also be less costly.

Remember to add capacity to the second filter cap and to carefully remove the output transformer and mount to the frame of the PM speaker.

I suggest that the original speaker be carefully boxed and kept with the radio. Tapped fields are very hard to get... In the future someone may want to re-install it...

Caveat, the new PM speaker may have a foam surround that balloons forward, if the speaker is to be face mounted that balloon structure will strike the baffle and sound awful. Use a semi-hard felt gasket, this can be found from tool suppliers like Zoro Tools sold by the foot. Choose a semi-hard felt as glue will not soak too deeply and not crush too much. Chas
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Post by acbaker123 on Sun Sep 20, 2020 10:23 am

i replaced the 45 with a 1619 using an adapter, does that change anything?

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Post by Chas on Sun Sep 20, 2020 12:26 pm

The 1619 is considered a "direct" replacement.

However, since there will be changes to the grid bias, consider altering the bias to a level more suitable for the 1619. That should yield a somewhat "louder" sound and/or less distortion. Look up the operating conditions for the plate voltage the radio offers for the 45 and adjust the bias to allow a plate current at or slightly above the rating for a 45. Do not allow more than 10% greater plate current to flow as that will put more heating stress on the output transformer.

Adding a 10 ohm 3 watt resistor in series with the 45/1619 plate will allow measuring plate current with a portable ammeter. Use an accurate resistor or use the actual value for the current calculation. Remember both leads to the plate shunt resistor will have B+. The resistor can be left in place

You can also place the resistor from the center tap of the 45 filament to ground (return). You must lift the existing ground for that connection. Depending on how the radio was put together lifting that connection may not be practical but it does provide a margin of measurement safety. A 5 watt resistor should do the job but a 10 watt would be better in that it would not be likely to fail open and would run cooler. Place that resistor away from any caps and other wiring by at least ainch or more.

That resistor can remain in place even if the 45 is returned to the radio.

Reminder, to shift the bias for the 45: Alter the ratio of the values of resistors chosen to replace the field coil. Do not alter the total resistance.  Chas
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Post by acbaker123 on Tue Sep 22, 2020 12:45 pm

Are there services that can Re-cone speakers?

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Post by Tony V on Tue Sep 22, 2020 2:45 pm

Look in the Useful Links section of TRF and you will find several speaker re-coning services that are available..
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