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 Post subject: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 1:25 am 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
How difficult is it to do this? I cannot find a field coil speaker and audio for my Fairbanks Morse 72 project. If its too difficult, i may just have to part this thing out/ pass it on to someone else. It doesnt have tp sound amazing. Just has to work. What would i need to do this conversion. I'd need to find a PM speaker (will any speaker work?), an audio transformer (probably impossible to find), and what else?


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 2:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1383
Location: Cambridge MN USA
Check the schematic. Most field coils were part of the power supply, between the first and second filter caps electrically. If so, you'll have to find a power choke to put in there. Not sure what the rating would need to be, but try 1.5-2H. There were a few radios where the field coil had its own power supply. If that's the case then just leave that part disconnected.

Scott Todd


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 2:59 am 
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Joined: Mar Sat 14, 2009 5:56 pm
Posts: 3865
Location: RI, 02885
Do you have any idea what speaker assembly was mounted on your radio? Do you have a part number? I have an 8" Fairbanks-Morse speaker with transformer but do not know from which model it was removed. I received the speaker with several other items and never saw the radio from which it was removed.

_________________
I'm not a hoarder, I'm a caretaker of scarce commodities


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 3:03 am 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
Bugman wrote:
Do you have any idea what speaker assembly was mounted on your radio? Do you have a part number? I have an 8" Fairbanks-Morse speaker with transformer but do not know from which model it was removed. I received the speaker with several other items and never saw the radio from which it was removed.



Absolutely no idea on a P/N. Bought the chassis as a project. Someone else had said a 10 watt or more transformer and 1000 ohm field coil are needed. Plug in the chassis is a 5 pin socket. A 227 fits it perfectly (using a dud tube to keep junk out of the holes)


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 3:21 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2135
Location: Santee Calif. 92071
If the field coil was used as a filter choke in the B+ power supply you can use a 1000 ohm wire wound resistor in place of it. Up the value MFD wise of 2nd filter capacitor to make up the loss of filter choke. 2nd filter capacitor can be 100 MFD but still need the high voltage rating or higher.


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 3:26 am 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
Id still need to try to find an output, correct? And could i frankenstein in a one thats close to spec if need be? Thus is more of a "saved from the dump" rather than a restoration.


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 3:39 am 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
Schematic


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 4:15 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2135
Location: Santee Calif. 92071
Can use original output transformer with a 4 to 8 ohm PM speaker. Do need 1000 ohm resistor to make the center tap of power transformer make the ground connection


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 5:26 am 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
There is no output transformer on the chassis. Nowhere to put one on the chassis either. The original was built onto the speaker from what ive been told.


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 5:54 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2135
Location: Santee Calif. 92071
Okay got you. Use the universal single ended audio output transformer from Radio Daze. Has taps for PM speaker. Still need the 1000 ohm resistor in the center tap power transformer.

http://www.radiodaze.com/universal-sing ... be-output/


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 6:05 pm 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
thunderbird281 wrote:
Okay got you. Use the universal single ended audio output transformer from Radio Daze. Has taps for PM speaker. Still need the 1000 ohm resistor in the center tap power transformer.

http://www.radiodaze.com/universal-sing ... be-output/


Out of curiosity, if i found a 1000 ohm field coil speaker without the xformer, could i use one of those replacement xformers with the old field coil speaker?


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 8:17 pm 
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Joined: Dec Wed 02, 2015 9:42 pm
Posts: 100
Location: Santa Clara, CA 95051
I've done this with a Zenith with some slight problems. It doesn't really matter if you have a field coil in series with the center tap of the transformer to ground or a wire wound resistor. I have found that the B+ often increases with a resistor and modern electrolytic capacitors. I often have to adjust the resistor values accordingly to get the proper B+ down line.
Any speaker with a field coil that is in the ball park will work. I have also adapted donor speakers with easy success. I have used a replacement speaker that had less than the anticipated resistance by adding wire wound resistors in series to take up the slack. It isn't all that critical to be absolutely spot on. Use the B+ values downstream as an indicator. Also, the resistance of a field coil increases slightly as the field coil warms up. Case in point, the radio I am currently working on drops 5+ volts from the B+ after it has been on for about half an hour. The field coil goes from cold to slightly warm to touch after that much time. Don't panic if you are 10 or so volts off as you experiment.
The output transformer does not have a center tap on the primary, so you'll need to take that into consideration when choosing a replacement. It doesn't matter if you are using a electromagnetic speaker or a permanent magnet speaker. The transformer should be matched to the output tube characteristics, and the voice coil impedance, if you're buying something new off the shelf.
Now I go from all smart and full of wonderful advice to totally stupid.
Are there pilot lights in that radio? If so where are the located on that print? (Rider, Fair-Morse 8-29) And where is the heater circuit. I've been at this a couple a three years now and I don't recall a print that does not indicate the heater/pilot light winding on the main transformer.
Good luck with your project
Tomie


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Sun 13, 2017 10:46 pm 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
Tomie wrote:
I've done this with a Zenith with some slight problems. It doesn't really matter if you have a field coil in series with the center tap of the transformer to ground or a wire wound resistor. I have found that the B+ often increases with a resistor and modern electrolytic capacitors. I often have to adjust the resistor values accordingly to get the proper B+ down line.
Any speaker with a field coil that is in the ball park will work. I have also adapted donor speakers with easy success. I have used a replacement speaker that had less than the anticipated resistance by adding wire wound resistors in series to take up the slack. It isn't all that critical to be absolutely spot on. Use the B+ values downstream as an indicator. Also, the resistance of a field coil increases slightly as the field coil warms up. Case in point, the radio I am currently working on drops 5+ volts from the B+ after it has been on for about half an hour. The field coil goes from cold to slightly warm to touch after that much time. Don't panic if you are 10 or so volts off as you experiment.
The output transformer does not have a center tap on the primary, so you'll need to take that into consideration when choosing a replacement. It doesn't matter if you are using a electromagnetic speaker or a permanent magnet speaker. The transformer should be matched to the output tube characteristics, and the voice coil impedance, if you're buying something new off the shelf.
Now I go from all smart and full of wonderful advice to totally stupid.
Are there pilot lights in that radio? If so where are the located on that print? (Rider, Fair-Morse 8-29) And where is the heater circuit. I've been at this a couple a three years now and I don't recall a print that does not indicate the heater/pilot light winding on the main transformer.
Good luck with your project
Tomie


There are 2 pilot light bulbs(make a pretty dial even better). I found that odd too though. But by ballpark, how many ohms higher or lower would work? If thats the case, could i just find a different speaker with a resistance in that range with an attached 10 watt transformer (was told 10 watt because of the 6L6G audio)


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Mon 14, 2017 2:26 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1383
Location: Cambridge MN USA
Anything +/- 20% easily. If you go higher you may need to add a bit of resistance in parallel to the coil, and like Tomie said, if you use one with less resistance you may need to add some in series with the coil.

Scott Todd


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Mon 14, 2017 4:11 am 
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Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
So correct wattage but at say 800 ohms, i'd just need to put in 200 ohms worth of resistors in series with the coil? Im guessing i could put these between the plug and the coil?


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Mon 14, 2017 5:39 pm 
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Joined: Dec Wed 02, 2015 9:42 pm
Posts: 100
Location: Santa Clara, CA 95051
I normally put those power resistors close to the rectifier under the chassis if there is room. I have mounted a terminal strip for just that purpose and then routed the B+ to and from that strip. I don't know your receiver and so I don't know how much room you have but normally most old radios have a ton of space under them.

As far as ball park, remember the goal is to sum out about what was asked for on the original design. The print says 1K ohm, so that should be the goal. If a new speaker is the proper physical size and will bolt into the cabinet, then you can chase the impedance together with wire wound resistors. I use clip leads and meters in these cases and so if I had a speaker that had a field coil that was say 850 ohms, I would not be scared to directly sub it for the 1K ohm field the print is asking for. But I would do it with clip leads and a meter attached so I could watch what was happening as I brought the radio up to power using my variac. That way I could be sure nothing was being driven too hard down stream, IE plate voltages, etc.

Ideally, the print asks for 1K ohms, so that is what you should shoot for.

Have you thought about putting a request in here, on the Radio Classified forum? You might be surprised at what folks have kicking around in their shops they would sell for a few bucks. It would sure be nice to have the exact replacement speaker.

So there are pilot lights? Just to satisfy my personal curiosity, do they feed off of a separate mains transformer winding? I'm just curious so please don't go out of your way.

Thanks

Tomie


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 Post subject: Re: Change from field coil to permanent magnent speaker
PostPosted: Aug Mon 14, 2017 6:06 pm 
Member

Joined: Nov Wed 04, 2015 4:04 am
Posts: 315
Location: Winfield, MO
Tomie wrote:
I normally put those power resistors close to the rectifier under the chassis if there is room. I have mounted a terminal strip for just that purpose and then routed the B+ to and from that strip. I don't know your receiver and so I don't know how much room you have but normally most old radios have a ton of space under them.

As far as ball park, remember the goal is to sum out about what was asked for on the original design. The print says 1K ohm, so that should be the goal. If a new speaker is the proper physical size and will bolt into the cabinet, then you can chase the impedance together with wire wound resistors. I use clip leads and meters in these cases and so if I had a speaker that had a field coil that was say 850 ohms, I would not be scared to directly sub it for the 1K ohm field the print is asking for. But I would do it with clip leads and a meter attached so I could watch what was happening as I brought the radio up to power using my variac. That way I could be sure nothing was being driven too hard down stream, IE plate voltages, etc.

Ideally, the print asks for 1K ohms, so that is what you should shoot for.

Have you thought about putting a request in here, on the Radio Classified forum? You might be surprised at what folks have kicking around in their shops they would sell for a few bucks. It would sure be nice to have the exact replacement speaker.

So there are pilot lights? Just to satisfy my personal curiosity, do they feed off of a separate mains transformer winding? I'm just curious so please don't go out of your way.

Thanks

Tomie


No cabinet (until i build one), and no one has had the correct speaker yet. And ill take a look the next time i pull it out of the closet


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