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 Post subject: You an audio transformer expert?
PostPosted: Feb Wed 26, 2020 8:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2100
Location: Monroe, NC 28112 USA
You an audio transformer expert?
This transformer is from a four tube battery powered German broadcast radio 1927/28.
A two winding unit from the plate of the grid leak detector to the grid of the first audio tube.
Turns ratio of 1:2.2
What has me stumped is an extra four turn winding on the core that is deliberately shorted.
What in the heck is that for???
Someone on a British group suggests that it would kill low frequency response and questions
why anyone would do such a thing.... Anyway, comments appreciated.
Robert


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 Post subject: Re: You an audio transformer expert?
PostPosted: Feb Thu 27, 2020 3:51 am 
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Joined: Aug Wed 04, 2010 5:40 pm
Posts: 418
Location: Tyngsborough, Massachusetts
I'm not an expert, but I have an Atwater Kent 318 chassis that includes a tone control switch. In the fourth position of the switch, it shorts a tertiary winding on the audio interstage transformer. I asked about it here several years ago in this thread:

viewtopic.php?f=6&t=258674

At the time no one seemed to have an answer. Unfortunately, I have not yet gotten to work on this chassis. It's still in my "to do" pile As I noted in the earlier post, one half of the push-pull secondary is open. It is my intention to try to rewind the secondary if possible, or to rewind all of it. If I can't rewind it, I'll at least figure out the configuration of that tertiary winding so I can try to recreate it.

Your post here is the first indication I've seen as to the possible function of that winding.

Jeff


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 Post subject: Re: You an audio transformer expert?
PostPosted: Feb Thu 27, 2020 4:40 am 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 3472
Location: Redding, CA
Regarding Jeff's situation:

In the fourth position the switch adds windings to the IF transformer resulting in overcoupling. Overcoupling an IF transformer will widen the IF bandpass characteristic allowing reproduction of higher audio frequencies.

Regarding Robert's question, the shorted windings will reduce magnetic flux necessary for signal transfer. This will reduce gain and I suspect this will reduce the lower level high frequency notes more than the low frequency notes. Is would be easy to test.

Norman

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 Post subject: Re: You an audio transformer expert?
PostPosted: Feb Thu 27, 2020 4:49 am 
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Joined: Aug Wed 04, 2010 5:40 pm
Posts: 418
Location: Tyngsborough, Massachusetts
It's not an IF transformer, it's the audio interstage transformer, it couples the single ended audio driver to the push-pull output tube grids.

Jeff


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 Post subject: Re: You an audio transformer expert?
PostPosted: Feb Thu 27, 2020 5:13 am 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 3472
Location: Redding, CA
Indeed, after the detector. In this case I suspect the additional windings provide negative feedback which reduces gain of mid-range audio frequencies accommodating a flatter audio bandpass resulting in more prevalent lows and highs.

Norman

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"No home is complete without music" - Capehart Sound Amplifying and Distributing Systems


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