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 Post subject: How did they use a field coil as an output transformer?
PostPosted: Feb Sun 10, 2019 10:49 pm 
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Attached is the schematic for my Zenith 5D2611. There is no output transformer...instead, the field coil is connected like an output transformer, with one lead going to plate of output and other to B+. There are two leads coming out of the coil that go directly to the speaker.

How does that work? Is there a second winding in the coil that acts as the output transformer?

The coil and speaker are fine, btw....I just want to get a better understanding of it


Attachments:
zenith_radio_corp_5d2611_pg13-1.jpg
zenith_radio_corp_5d2611_pg13-1.jpg [ 223.15 KiB | Viewed 469 times ]

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Sun 10, 2019 10:55 pm 
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Works just like an iron core transformer.


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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Sun 10, 2019 10:57 pm 
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That is not working for me---show us a picture of the speaker and transformer.

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Sun 10, 2019 11:06 pm 
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I tried to take a pic, too hard to capture it. The speaker leads come out of the coil right next to the IF can (almost against it). On the other side are the two primary wires

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Sun 10, 2019 11:08 pm 
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So by carefully calculating the number of turns, did Zenith manage to create a field that also has the inductance of an output transformer?

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Sun 10, 2019 11:39 pm 
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I found a tidbit that describes this:
Attachment:
zenith_spkr_FC-OPT-combo.jpg
zenith_spkr_FC-OPT-combo.jpg [ 22.21 KiB | Viewed 454 times ]

When doing the search, this image links to an RM page for the Zenith 5D610:
https://www.radiomuseum.org/r/zenith_5d ... early.html

I think the secret to this (Mickey-Mouse) trick is that the small speaker has no bass response and this does not need much of an OPT.

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 12:01 am 
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Quite an interesting scheme, never have seen anything like that.

The speaker must be quite mechanically efficient to be able to function with so little input energy from the output tube. Also, although I did not see voltages on the schematic, the plate voltage & current must be quite a bit higher than typical, maybe maxxed out. The transformer impedance seems pretty low at 400 DC ohms.

I'm surprised that a lot of the low cost manufacturers didn't use this approach, unless, perhaps Zenith had it patented.

Thanks for posting.

Tim


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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 12:01 am 
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Very interesting! Thanks, Mark

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 12:04 am 
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I'm sticking with "Mickey Mouse"----YMMV....:)

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 12:06 am 
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This has been discussed before, and it works. They used a 75Ω cathode resistor on the 50L6, about half what we would normally see, to bias it a little hotter so it draws more current through the field coil/output transformer. So it saved them a few cents per radio, might have been an experiment they decided not to continue with, maybe related to perceived upcoming material shortages in 1942?.

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 12:08 am 
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As an addendum, I also bought a parts radio of this same model. At some point, someone had replaced the field coil speaker with a conventional PM speaker and output transformer. I did not try the radio out, as it was pretty rough, but I'm assuming it worked to at least some extent, although no doubt not optimally

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 2:06 am 
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Greetings to the Forum:

I don't see why the above scheme wouldn't work. Remember what a speaker does.... it translates changes in electrical current to changes in position of a cone diaphragm which in turn imparts compression / rarefaction waves to the air. The change in current causes a change in the magnetic field which transforms the electric current change into a mechanical position change. In this case, you have two changing magnetic fields in series aiding.... the "field" coil magnetic field and the voice coil magnetic field. The transformer / field coil would have to be carefully designed so that these fields add over the expected frequency range of the audio to be reproduced, but I don't see a problem with it.

We know that hum can be introduced into the speaker sound by a field coil that is connected normally, especially with minimal B+ filtering. That is why one often finds additional "hum bucking" coils to cancel out the hum caused by the varying DC in the field coil. In this case, let the "hum" introduced by the field be the audio signal.... and so the additional audio signal from the voice coil simply adds.

I suspect the reason that it is not found more frequently is simply that the speaker would be more expensive to construct than a simpler output transformer and PM speaker combination.

Regards,

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 2:40 am 
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tubes4life wrote:
As an addendum, I also bought a parts radio of this same model. At some point, someone had replaced the field coil speaker with a conventional PM speaker and output transformer. I did not try the radio out, as it was pretty rough, but I'm assuming it worked to at least some extent, although no doubt not optimally

Why not? If the original setup is "Mickey Mouse", a conventional xformer & speaker has to be better. No doubt should have R6 & R7 values be increased.

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 3:49 am 
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From your description, it looks like a regular combination of an output transformer and a PM speaker.
Likely that the draftsman was an old timer who was used to printing "field coil". Drafting misteaks do happen.

That reminds me of the time I was looking for something. My wife told me that it was on the kitchen table. After a long search, she pointed to it on the kitchen counter. Same thing to her!

Bob

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 6:11 am 
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I don't see Zenith going through the trouble of designing a pm speaker where the output transformer looks exactly like a field coil. Aside from the secondary winding going to the speaker, it looks just like any other field coil

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 5:24 pm 
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I have a feeling they did that to save copper for the build-up to WW-2, not to shave a few cents off the bill of materials. They would have had to wind custom field coils for those speakers, and custom almost always costs more. If they wanted to be cheap they could have ditched the pilot light and the external antenna connections too.

That speaker is a clever idea and it probably worked okay in a small table radio for a bedroom or a study where it would not be played at loud volumes. The plate current in a class-A pentode output stage varies about the quiescent value so the field current would never go to zero. It probably doesn't have much inductance so the bass response rolls off before distortion becomes a problem.

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 5:32 pm 
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That's an interesting take on it, I never thought about the war implications.

Truth be told, it doesn't sound any different from other AA5 table radios from the time. Of course, they didn't have much bass either

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 5:48 pm 
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What l see is a typo. Somebody labeled a run of the mill output transformer as field coil. The B+ goes through the resistors and through that coil--which is the sole source of plate current to the 50L6. Craig


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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Mon 11, 2019 7:14 pm 
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analog.tv wrote:
What l see is a typo. Somebody labeled a run of the mill output transformer as field coil. The B+ goes through the resistors and through that coil--which is the sole source of plate current to the 50L6. Craig


An output transformer that looks exactly like a field coil? If it's a pm speaker, where is the magnet? There's no question here that it's doing double duty

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 Post subject: Re: How did they use a field coil as an output transformer?
PostPosted: Feb Tue 12, 2019 2:47 am 
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With the 50L6 running at half of it's normal bias, I'll bet you could fry an egg on it! One running in a standard circuit runs plenty hot as it is.

Zenith was trying various cost cutting schemes at the time; remember, that was the era of the Bakelite chassis.

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