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Power transformer in a 12 volt/25 volt radio? - Printable Version

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Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-27-2016

I've been trying to increase my understanding of power transformers lately, and in the process began looking at the schematics for a couple of radios I have around here with bad transformers in them. The biggest oddball I have is this Stewart Warner 01-SK1 set. It has the dreaded 6X5 tube as a rectifier and a set of 12 volt tubes in the rf/if stages, then a 25 volt tube as output.

The voltages at the rectifier are 160 volts at the plates and 110 volts at the heaters and cathode. The power transformer has three secondary windings, so that would mean one HC with a CT and a winding for the rectifier filaments (is that a 5 volt or what?). The third winding has a tap on it, so one lead in the third winding supplies 25 volts to the output, and the tap supplies...what about 12.5 volts to the rf/if tubes?

That seems really specific, doesn't it? What are the chances that I would ever find a PT with those specs?

I really love the cabinet for this table top radio. It has some inlay copper separating diagonal grained from horizontal grained panels in an almost blonde tone.

The Stewart Warner 01-SK1 schematic: http://www.nostalgiaair.org/pagesbymodel/838/M0019838.pdf

The data sheet for the 6X5GT: https://frank.pocnet.net/sheets/127/6/6X5.pdf
The data sheet for the 25L6GT: https://frank.pocnet.net/sheets/093/2/25L6GT.pdf

RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-27-2016

Your first link does not work.

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-27-2016

I've fixed the first link, now.

RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-27-2016

The Y (full winding) to GND is 25V, the X to GND (the tap) is 12V.

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-27-2016

So I would need a power transformer with three secondary windings: a 320 VCT, a 6.3 volt with no CT, and a 25 VCT? Like I said, that is pretty specific. Sounds like unobtainium to me. Could I get close to 320 VCT with a 6.3 volt winding and also run a second power transformer with a 25 volt, center-tapped secondary? Like this one (assuming the amperage is sufficient).

http://www.skycraftsurplus.com/xfmr25vct2amp.aspx?gclid=Cj0KEQiAperBBRDfuMf72sr56fIBEiQAPFXszU3GjWZdmqGu7HcTeFRBH3OZ6AxX2DSpWK_HFvw-WUcaAj588P8HAQ

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-27-2016

Someone else suggested that I could use a 6.3 filament winding and switch the 12 volt tubes for 6 volt equivalents? That would mean using a completely different output tube like a 6v6, right? How much changing of resistors and caps would that entail?

RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-27-2016

I was about to suggest to beef up 25V winding and use the 12V tubes filaments setialized in pairs, 12K7 pair and 12Sa7 and 12sq7. You will need 0.3+0.15+0.15= 0.6A @25V.
And of course 6V winding and the B+ one.

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-27-2016

So, one like this one?

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/166D25/?qs=KLj0QfLrw0H321WrIshQYw%3D%3D

It has a CT on the 25 volt secondary, and is rated at 1 amp.

Along with a HV transformer. How much leeway do I have in voltage on the HV secondary. 320 VCT with a 6.3 filament winding is also a bit hard to find. I could make a solid state rectifier and rig up some sort of voltage dropping resistor set up, I suppose, if I'm forced to use a higher voltage. Might actually be better than a 6X5.

I've had this radio for about two years, so its not a giant hurry. I got it originally because it had a phono input and my daughter wanted one that she could use an iPod on. I had a little stereo to mono adapter made up that used Radio Shack mini audio transformers...they don't make the transformers anymore).

RE: Power transformer in a 12 volt/25 volt radio? - Radioroslyn - 11-27-2016

Ok we can work with this.
Power transformer= 350 to 400vac ct @ 50ma., 5v@2a, 6.3v@ 1.5a or so.
Remove the 25L6 and replace it with a 12A6 (common war surplus tube).
Wire the 6.3v winding to the 6X5 and in series with the 5v winding. Phased properly you'll get 11.3vac. Connect that to the 12v heater tubes.
Wire the HV same as original.

RE: Power transformer in a 12 volt/25 volt radio? - Radioroslyn - 11-27-2016

(11-27-2016, 06:36 PM)ccomer1955 Wrote:  So, one like this one?

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/166D25/?qs=KLj0QfLrw0H321WrIshQYw%3D%3D

It has a CT on the 25 volt secondary, and is rated at 1 amp.

Along with a HV transformer. How much leeway do I have in voltage on the HV secondary. 320 VCT with a 6.3 filament winding is also a bit hard to find. I could make a solid state rectifier and rig up some sort of voltage dropping resistor set up, I suppose, if I'm forced to use a higher voltage.  Might actually be better than a 6X5.

I've had this radio for about two years, so its not a giant hurry. I got it originally because it had a phono input and my daughter wanted one that she could use an iPod on. I had a little stereo to mono adapter made up that used Radio Shack mini audio transformers...they don't make the transformers anymore).

it's rated at 100ma not an amp .1amp.

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-28-2016

Sorry, I have old man eyes. How about this one? A 166J25:

http://www.digikey.com/product-detail/en/hammond-manufacturing/166J25/HM553-ND/270154

There is a 166M25 version with a 3 amp rating which would be more what I need, right? In this radio, the chassis is fairly small and squarish. Inside the cabinet, there is tons of open space.

It would be more elegant (I guess that is the right word) to swap all of the tubes for their 6 volt varieties and change the output to a 6V6, but I know for a fact that my grip on theory isn't up to it. My tube manual says that the 6 volt versions of all of the 12 volt tubes are electronically identical, so (except for changing the heater voltage) I would not need to make any changes there, I suppose. It is changing the 25L6GT to a 6V6GT that worries me. The value of the resistor between the plate of the 12SQ7 and the 25L6GT is 100 ohms, so that needs to be more like 300 if I change it to a 6V6, right? I do actually have an output transformer and speaker matched to a 6V6 around here that came out of a Crosley 66Cs but the cone is a bit rough...I wonder if I could canabalize the OT and match it to the Stewart Warner FC and speaker? Probably not. But the changes to R13, 10, and C 27, C 28, C38, and C 59 A and B?  Plus anything else I'm missing from ignorance? It would be a great education for me, and force me to finally get better at calculating resistances and impedances.  

Like I said, I kind of love the cabinet, and the phono input is a plus, so I'd like to get the set working one day.

RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-28-2016

Charlie

Both transformers you have listed are 25V only. How are you going to obtain the high V output for the rectifier?

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-28-2016

(11-28-2016, 01:04 PM)morzh Wrote:  Charlie

Both transformers you have listed are 25V only. How are  you going to obtain the high V output for the  rectifier?

Well, that was the issue. I think I would have to run TWO transformers: one that supplied HV and 6.3 filament, and another that supplied 25VCT, so I could cover all filament voltages...that is unless I could stumble on the exact transformer that Stewart Warner had made for this set. 

I have a Philco 77 chassis that had a burned out HV secondary in the original but the filament secondary. The solution the technician from the 30s used was to bolt in a second power transformer from another Philco to provide the HV for the set and power it from the mains. I have no idea how well it worked. I used the chassis for parts.

That is why I've been thinking about the 6 volt solution.

RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-28-2016

Well, if you have space for two xfmrs...sure.
BTW a xfmr with a burned out winding may or may not work with the remaining one, depending on whether there is a short turn in the burned part.

RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-29-2016

It looks like every single tube has a 6 volt equivalent. The 6W6 is a close one, and the 6L6 is even in the same line. Both pin out the same it seems. So I could simply put in a PT with an HV winding around 160 volts and a 6.3 filament winding, change the tubes with 6 volt equivalent ones, and call it a day?

After figuring the amp ratings for the windings, that is. Am I missing something?

Maybe even make a diode rectifier to replace the 6X5?