Mystery output transformer help?

Discussion in 'The Workbench' started by Matthews Guitars, Mar 1, 2021.

  1. Matthews Guitars

    Matthews Guitars Well-Known Member

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    This is allegedly an OT from a 2203. I bought it cheaply enough. I sent it off to Mercury Magnetics to be tested, and if good, baked out, varnished, and re-leaded.

    It did test good and here it is.

    BUT...the wiring mystifies me. Or more specifically, it seems to have an extra winding I don't know what to do with.

    The primary side is simple enough. Red, white, brown. Easy to figure out which one is the center tap.

    The scondary side...I'm trying to figure out why they put cloth insulated heavy gauge solid core wire only on the green and brown taps coming out of the left side exit. The black and yellow coming out of the right side exit are connected to the green and brown taps. So there are my speaker taps. Black, green, brown, yellow. But the black wire is much smaller gauge, and the yellow wire isn't solid core or cloth insulated.

    And then there are the two mystery wires: Light gauge green and brown wires that form a circuit that my meter sees as having 2 ohms DC resistance. (My meter is not very usable at very low ohmages. ) I have no idea what that winding is for.
     

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  2. thetragichero

    thetragichero Well-Known Member

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    yikes
    so am i to assume the bell covers are full of dried varnish? would make it difficult to get a good look at the windings
    ain't ever seen a transformer meant for high voltage with such tiny wires

    for the record you can suss out a transformer with an ac power source (10vac makes the calculations easier. i just use 120vac but tend to not suggest it as there's danger there if you're not careful), a dvm, and a calculator
     
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  3. StingRay85

    StingRay85 Well-Known Member

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  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Actually I remembered now that we elected not to revarnish this one because Mercury Magnetic's guru said it was fine and didn't need baking or revarnish. I just remembered that. I looked under the end bells and everything is clean and accessible if needed.

    I have an inductance meter so I'll figure out what tap is which. But the mystery winding is still a mystery.
     
  5. Travis398

    Travis398 Well-Known Member

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    Are the extra 2 wires connected to the primary? or just by them selfs
     
  6. Matthews Guitars

    Matthews Guitars Well-Known Member

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    It's its own circuit. One coil, not connected to anything else.
     
  7. Travis398

    Travis398 Well-Known Member

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    I have seen ultra linear Hammond transformers that had an extra tap on the secondary. Yours isn't ultra linear but I think it may be similar on the secondary and as i recall there may be a couple different ways to use it or connect it.

    Let me see if I can find a pic and post it
     
  8. Travis398

    Travis398 Well-Known Member

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    Probably not helpful but I knew I saw one before.
    Screen Shot 2021-03-01 at 2.29.30 PM.png
     
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  9. XTRXTR

    XTRXTR Active Member

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    The picture looks like 6 wires on the secondary, bottom right yellow wire, black wire and a partially hidden green wire. Bottom left two brown and one green.
    I saw an old 100W Marshall schematic with a OT with a 100V, 16,8,4, and common, five leads from the secondary...not six. Usually see those types of transformers for PA system for large buildings like a mall, or car lot.
    The only other thing I can think two of those are on the Primary for ultra-linear amplifier connections.
    One more nonsense idea would be neg feedback leads, for optional tap? nonsense

    I think you're going to have to put a voltage on there and determine what its all about, turns ratio, reflected impedance etc.
     
    Last edited: Mar 1, 2021
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  10. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The two mystery wires, 22 gauge green and brown, are their own separate winding with an inductance of 24 mH as my meter reads it.
     
  11. neikeel

    neikeel Well-Known Member

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    Putting ac across the primary ends is the best way.
    The cloth wires would be common/4/8 and 16 as I’m sure you know. The others could be an electrostatic screen or part of a pa feed for 100v.
    I use 100v on a variac in as it makes calculations much easier and cleaner. If you do not have a reliable variac and have a Marshall 100w pt you can use the bias feed instead.
     
  12. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I have a variac. Seems to be a good stable one.

    That extra winding is very interesting. I am trying to figure out why you'd have an extra winding, on an OUTPUT transformer, on the secondary side that's isolated, all by itself. It'd only generate a signal when there's a signal going into the transformer. So it's a signal source, not a voltage source. Hmm...


    70 volt output? Don't know....yet.
     
  13. 2L man

    2L man Member

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    When you measure it keep that picture Travis posted in your mind!!!

    70V and 100V are standards which are used for PA which cables are long and using higher voltage and lower current the power loss comes smaller. There are other transformers which change it back high current/low voltage in cabinets or near them. There is also 600 ohm standard which is almost 100V standard.

    When you reply the voltages it is easier to think what it is.
     
    Last edited: Mar 3, 2021
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  14. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Mercury Magnetics says it's a separate winding for use for negative feedback connection. I can just dead end the wires and forget about them.
     
  15. XTRXTR

    XTRXTR Active Member

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    That was my third guess but scoffed at the idea...seems like nonsense. Nonetheless I am going to go through a thought process here, not disrepecting the OT just trying to reason it out in my mind.

    What could possibly be the reasoning for separate NFB windings? Assumptions: designer knows the output stage the OT was designed for cathodyne, long tail pair, or paraphase NFB loop, and obviously the reflected impedance/ tube types and total power transfer from Primary to Secondary, as well as speaker load windings. All those things and more aka design parameters.
    1) Thus an expected direct connection from the OT to the PI circuit with appropriate voltage and impedance at said circuit
    2) Provides possible two NFB values (one per NFB wire) to the circuit; similar to 4 or 16 ohm tap
    3) if direct connection; reduces wire in the amp layout and resistors needed for amp designed output distortion levels
    4) limited designed output variation from one amp to the next IOW they all sound the same.
    5) Headroom designed?

    Maybe someone else can explain why you would have separate NFB windings. A big expense for a simple circuit, wire or trace and a resistor. Or, Its kind of like the idea of ultra-linear but with a specific level of distortion built in. That OT could be very well made to specific headroom and distortion parameters using the NFB windings. Rather than dead ending them maybe see what they do in the circuit or contact MM for more information. Could be tone gold.
     
  16. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I can assure you that this extra winding is electrically separate from all others in this transformer. I've checked very carefully.

    Mercury Magnetics did not make this transformer, but they did test it and replace its cut-off short leads. The seller of it advertised it as one that was pulled from a 2203. Aside from the extra winding it conforms to 2203 output transformer specifications.
     
  17. neikeel

    neikeel Well-Known Member

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    @XTRXTR
    Your reasoning is fair
    I’m guessing that if you have a dedicated NFB that is fixed and not hooked up to the load it is less likely to be affected by the OT being heavily pushed when similarly driving a reactive load such as a speaker cab ?
    When a PP output stage is driven heavily into clipping most linear (and I guess non-linear) characteristics become irrelevant?
     

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