Scott 130 power transformer fix revised.

stanko

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Scott 130 power transformer fix revised, rewinding transformer

The original power transformer was long gone, replaced by two that didn't fit. I have a Scott that is correct dimensionally but filament winding was for parallel, not series, like the 130. I was going to change the preamp to match the transformer but have instead decided to rewind the transformer to correct voltages to the preamp. I thought you might like to see a transformer come apart, and how to rewind one at home easily. I've done quite a few. Some pics of winders as well.
 

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Bob Carver was winding his own output transformers for a while,Wow!
 

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Did you use a new bobbin or were you able to re-use the old one?
Actually, the winding I need to change is the last one on the wind, so my life will be relatively easy. I'll post some more pics as I disassemble it showing how the wires are attached, etc.
 
Now that I have the outer paper removed, this is what you see or a variation of what you will see, depending on manufacturer. The paper is stiff from being dipped in varnish, but will pry off in little chunks if you are careful. Start away from the wires and work towards them so you can see what is going on. Take only the outer layer one at a time. The wires are typically stapled onto a piece of dielectric cardboard. If you look carefully at the picture, you can see the staples. You need not remove the wires from the cardboard, in fact I advise against it. Simply unsolder the transformer windings from the end of each wire and carefully unwrap them. Note with drawings and pictures where each wire goes or you'll have some work cut out for you identifying the leads. When all the core windings are unsoldered, the insulated cardboard can be pried off with all the wires still attached, ready to be reattached.
 

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Pic showing coil wires disconnected from lead in assembly which consists of lead ins stapled to a piece of dielectric cardboard. Now all that's left is a coil of different wires stacked on top each other. Since the core is rectangle, I will make some wood shims so the core is square, and then mount it to a winder to unwind the old windings and rewind it with the correct windings.
 

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Top paper cover removed, 6 volt filament winding removed, 6 volt rectifier winding is showing after removing insulated paper from under filament windings. It took 23 turns to develop 6 volts AC for filaments. Since we want about 21 volts DC working backwards makes for about 15 volts AC that we want. 6 into 15 is 2.5. Therefore we want 2.5 times the original winding, or 57 or 58 windings total in the new filament winding. The original winding only took half the total length available, so there is plenty of room to double the windings. I will use slightly smaller diameter wire because the current will be less than half the original design of this transformer.
 

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I wanted to show a close up of the rectifier filament windings. In this case the manufacturer wound a double wire in parallel, and then connected the ends of both windings so current is double what a single wire would carry. This is accepted practice in winding transformers, using two smaller wires instead of one large one results in a more compact winding with less air around the wires and it is easier to bend.
 

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:lurk:Watching,You make it look easy,Have you ever done an output transformer?,how bad is that,would the secondaries be on the outside also?
For instance if you didn't have 2 or 4 ohm taps,would the fix be similar to what you are doing?
 
:lurk:Watching,You make it look easy,Have you ever done an output transformer?,how bad is that,would the secondaries be on the outside also?
For instance if you didn't have 2 or 4 ohm taps,would the fix be similar to what you are doing?
I've done output transformers. The good ones are complicated in that the primary and secondary windings are interwoven. In other words, to get maximum saturation and information transfer, manufacturers of vintage audio first put a couple layers of primary, then a layer of secondary, then a couple layers of primary, then another layer of secondary. The thing that makes this difficult is you have to cut the wires from each set and label them so you can connect everything together when you are done. You might have 40 ends you have to figure out how to connect and then insulate, etc. Output transformers are a real pain in the ass.
 
Good output transformers are as much a product of art as they are design.

Stanko -- Thanks for taking the time to illustrate and explain your process in rewinding the transformer for your Scott. An interesting thread indeed!

My question is one of current rating. The current rating for the original filament winding of the Scott transformer you are rewinding could easily be determined by adding up the current draw of the various tubes and lamps connected to it in its original application (assuming that was known) -- this because I am assuming that based on your description, it also came from an application that employed an AC heater circuit.

In rewinding the filament winding for the new application, you are also converting from an AC heater system to a DC based one. Therefore, the current requirement of the various tubes being supplied by this winding is not the only current factor to consider, as the charging current of the filter system is also a significant consideration. I've always understood this to be a factor on the order of about 1.8 for the AC current drawn, versus the DC current supplied. As you have repurposed various transformers in a similar type scenarios, is this something you directly account for with your chosen wire size? Or has it not been particularly an issue in your experience?

Dave
 
Dave; Thanks for your kind words. You have provided me and this site with many insights and I am just trying to pitch in a bit with some of my acquired skills. In answer to your question, I am posting a picture of the 130 Power Supply. It consists of 8 tubes, one of which is a 6X4, the other 7 are 12AX7 types. Each channel used 3 tubes, the extra was for a center channel output. The original supply was DC supplied through a selenium rectifier. Approximately 21 volts, through a couple of resistors, at which potential was 15 volts, and then to each channel, where the filaments are wired in series groups of 3, and then paralleled together. The extra center channel tube is actually run off the 6X4 AC through a step down resistor that drops voltage about 1 volt. Let me try and find the picture. As far as the actual winding, I am reducing wire size slightly since I am drawing less than half of the original design of the transformer and to give me the space to put the extra windings on. Stan K
 

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I think I misunderstood Dave's comment and question. When I wind, I look at the total power consumed, not just of the tubes, but also any resistors that are in filter networks. , add a factor of safety and then see what I need. If I have more room than what I need, I will increase wire size to take up the space. I want as little of resistance heating in the transformer as possible, they get hot enough. I keep the interlayer paper as thin as possible, and keep the windings as close to the edge of the core as possible. Many times, doing it by hand results in a compact enough package to actually increase wire gauge by a size or so and still jam everything back into the E's and I's. I can't speak for manufacturers, but I try and keep wire size as large as possible, no matter what my simple calculations may say.
 
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