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 Post subject: Silvertone Replacement Speaker Issue
PostPosted: Nov Sat 04, 2017 8:09 pm 
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Joined: Feb Fri 22, 2013 2:38 pm
Posts: 33
I hope this is the right place to post this. I have a Silvertone (6439 or 101.583 chassis?) floor model I am restoring and found the windings with the center tap was open from the center tap to each of the legs of that side of the transformer. The windings that go to the actual speaker measured 510 ohms. The cone was shot so I found a non-Silvertone 12' speaker replacement and notice that the "replacement" one only has a voice coil measurement of 210 ohms. So I am reluctant to hook it up and try it. My questions are: 1. Will the 210 ohms work? 2. If not, can I get away with adding a 300 ohm wire wound resistor in series with one of the voice coil leads. 3. If not what are my options? Does some one re-wind these transformers or is there a source for them anymore? Thanks in advance. Dutch


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 Post subject: Re: Silvertone Replacement Speaker Issue
PostPosted: Nov Sat 04, 2017 11:04 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7202
Location: Long Island
The answer is, more research is needed before you plug this thing in. The schematic for your set is available on Nostalgiaair.Org http://www.nostalgiaair.org/pagesbymodel/554/M0017554.pdf so looking at that, we see that there are five connections from the chassis to the speaker.

The two large pins of the speaker plug are used for the field coil of the speaker, and the schematic gives the resistance value as 600 ohms. Since the resistance of copper wire changes with temperature, 600 ohms is what you should get when it is fully warmed up. 510 ohms would be typical if the coil is cold.

The output transformer primary is connected to three of the small pins of the speaker plug (the fourth pin is not used). Unfortunately the schematic does not give the resistances of the windings, but you should get something like 100-200 ohms of DC resistance from either end of the winding to the center tap. DC resistance has nothing to do with the impedance of the transformer, BTW. The secondary of the output transformer is connected to the hum bucking winding and the speaker voice coil. Each of these should have a very low resistance, maybe a few ohms when measured. Since it is a complete circuit in itself, you'll have to disconnect one lead in order to test the coils. It is extremely unusual for voice coils or output transformer secondaries to be open in tube radios.

If the field coil is open, you can replace it with a 600-ohm resistor. The resistor will dissipate about 9 watts in operation, so a 15-watt or larger wirewound resistor would be called for, and it would have to be mounted somewhere where the heat would not cause problems. If your replacement speaker has a 210 ohm filed coil (figure about 250 ohms warm) you could put a 330-ohm resistor in series with it. It will still get hot and need careful mounting, but you'd be burning up about half the power in the field coil and the resistor need only be 10 watts.

Now if it is the output transformer that is open, yes you can buy new ones. The data sheet for the 6AC5 output tube recommends that the transformer should have a plate-to-plate impedance of 10,000 ohms when two of the tubes are used in a push-pull output circuit. Unfortunately, measuring the DC resistance of the transformer you have tells you nothing about its impedance on AC, so there is no way to know if the transformer you have is even close to what is needed. Tubes are very forgiving and the radio will work with just about any tube push-pull output transformer of the same physical size as the one that came with the radio or bigger, but if you can match the recommended impedance, you will get minimum distortion. In order to determine if a new transformer is the right impedance, you will need to know the impedance of your speaker voice coil. As a rough estimate, you can figure that the voice coil impedance will be about 1.25 times its DC resistance.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Silvertone Replacement Speaker Issue
PostPosted: Nov Fri 24, 2017 12:50 am 
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Joined: Feb Fri 22, 2013 2:38 pm
Posts: 33
Thank you Chris for the good advice. My original speaker was open across each of the windings to the C.T. on the output transformer primary but the secondary was fine. Voice coil on it was also fine. But I am not a transformer rewinding kind of guy so I bought another 12" vintage speaker it was the one that had a voice coil resistance of about 210 ohms. Taking your suggestion, I put a 270 ohm resistor in series with the voice coil and it works fantastic. I am trying to research the new speaker to see how far off I might be from the right impedances and will continue to look for a replacement output transformer for the original speaker. Again, thanks for your help. Dutch


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 Post subject: Re: Silvertone Replacement Speaker Issue
PostPosted: Nov Wed 29, 2017 11:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4872
Location: 253 Blanche St. Plymouth, MI USA
dutchmasters wrote:
Thank you Chris for the good advice. My original speaker was open across each of the windings to the C.T. on the output transformer primary but the secondary was fine. Voice coil on it was also fine. But I am not a transformer rewinding kind of guy so I bought another 12" vintage speaker it was the one that had a voice coil resistance of about 210 ohms. Taking your suggestion, I put a 270 ohm resistor in series with the voice coil and it works fantastic. I am trying to research the new speaker to see how far off I might be from the right impedances and will continue to look for a replacement output transformer for the original speaker. Again, thanks for your help. Dutch



you say "Voice Coil" but you must mean "Field Coil" as no VC is over 8 ohms DC resistance generally.
Output transformers are often called out by the type of tube they serve. So, if you have type 6F6 output tubes running "push pull" generally any transformer from any other unit that ran a 6F6 push pull will do. Or even one that ran the similar 6V6 tubes. So when I am scrapping out an old chassis,. I mark the output tube type on the xfmr. Its all I need to know in the future !
Mark Oppat


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