Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Thu 02, 2021 11:11 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 8 posts ] 
Author Message
 Post subject: Speaker Field coil resistance
PostPosted: Sep Wed 29, 2021 9:32 pm 
Member

Joined: Jun Wed 28, 2017 5:32 pm
Posts: 113
Location: Hampton Roads, VA
I had to rewind a field coil in a jukebox speaker that I'm restoring as it was open. Calipers indicated that the wire was 34awg, 794g of wire weight, and the schematics note that the field coil is a 5kohm coil. I printed a new bobbin and wound the thing, but I'm sitting at a smidge over 3.5kohm. (Maybe I should have used 36awg) Looking at the schematic, it looks like the field coil is being used as a reference to ground. I'm trying to decide whether I need to add a high wattage 1.5k resistor to make up for the lost resistance or whether it should be ok as it is. If anyone has any ideas, it would be much appreciated.

Attachment:
1422schematic.PNG
1422schematic.PNG [ 383.29 KiB | Viewed 236 times ]


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Wed 29, 2021 9:53 pm 
Member
User avatar

Joined: Jul Tue 15, 2008 6:13 pm
Posts: 9646
Location: Gretna, Nebraska
For the weight of wire you took off, 35 gauge calculates closer to 5000 ohms than 34 gauge, although it still puts you at 5,900 ohms for single build insulation..

I found my calibers were less accurate than my micrometers for measuring wire o.d.

_________________
http://www.vintagerestorationservices.com
Paul
...... how hard can it be?


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Wed 29, 2021 11:02 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17002
Location: S. Dartmouth MA 02748-1225 USA
The field is powered in shunt with the B+. A PM speaker could have been used if a 5% approx increase in voltage could be tolerated. :roll:

_________________
List' & I will Enchant Thine Ear


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Wed 29, 2021 11:09 pm 
Member
User avatar

Joined: Jul Fri 18, 2008 10:02 am
Posts: 2172
Location: near ST Louis Mo 62002
Most likely you will be fine the way it is , just bring the amp up on a variac and monitor how warm the coil gets .The resistance is not as critical as if it was in line with the B+ .

_________________
Mark
(Conti the brain damaged robot )


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Wed 29, 2021 11:56 pm 
Member

Joined: Nov Tue 14, 2017 5:09 am
Posts: 4103
Location: Austin, Texas
Since the field coil is across the B+, a reduction from 5K to 3.5K is a significant increase in the B+ supply loading. I would put the 1.5K resistor in series with the field coil. If you have problems with the speaker operation, you may need to rewind with higher AWG wire. Going high on resistance is not as much problem as going low.

Jay


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Thu 30, 2021 12:12 am 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 4786
Location: Lexington, KY USA
The schematic shows the field coil connected to the same B+ source as the output tube screens. The lower resistance field will pull this voltage lower, reducing the output tube plate current. It may be a wash for total B+ current drain.

You want to watch the filter choke and 1000 ohm resistor, though, as they will have to carry the extra field coil current.

A resistor in series with the speaker field coil, or an increase in the 1000 ohm B+ dropping resistor will get the current back to normal, if need be.

I kinda think I would add any extra resistance to the existing 1000 ohm B+ dropper, in order to reduce the output stage current a little. This should help the old parts last longer.

I should add that the 5000 ohm figure for the old coil was probably taken with the coil at operating temperature. This resistance will be higher than at room temperature.

Ted


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Thu 30, 2021 12:17 am 
Member
User avatar

Joined: Sep Mon 16, 2013 2:42 am
Posts: 5183
Location: Tucson, Arizona U.S.A.
If the bobbin isn't full, you could just splice on more wire and keep winding til you got the right resistance.

_________________
Jim Mueller

Who's that swimming in the punch bowl?
It's Walter Wart, the freaky frog!


Top
 Profile  
 
 Post subject: Re: Speaker Field coil resistance
PostPosted: Sep Thu 30, 2021 12:24 am 
Member

Joined: Jun Wed 28, 2017 5:32 pm
Posts: 113
Location: Hampton Roads, VA
The bobbin is pretty much full, I'd be lucky to get much more on there. I do recall somewhere that the winding measurement was taken hot, I just can't remember where. Though the hot temperature isn't more than like what, 10% raise in resistance?

I guess the good thing is that the amplifier isn't gonna be seeing voltage when there isn't a record being played, so it won't be on for a long period anyways.


Top
 Profile  
 
Post New Topic Post Reply  [ 8 posts ]  Moderators: Norm Leal, Marcc

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 45 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB