Marshall12xxx transformer wiring question

Discussion in 'Building the Classics' started by soundcharger, Feb 13, 2021.

  1. soundcharger

    soundcharger New Member

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    I recently got all the parts for a 12xxx series from valve storm and transformers from heyboer ( 1203-80-ML power , and c-1998, choke ) no problem wiring it up from the 67-68 Superlead layout but im stumped on some 1203-80-ml connection points. The 68-69 wiring layout shows to blue to standby that I get , a yellow ct , a yellow bias tap and a ground to chassis. my heyboer has a red yellow stripe that I take is the ct, and 2 red wires I understand are 178 v (im not using the152v taps). question is where is the ground? what red wires where ? Can someone help me figure this out please as no one seem to want to let me in on this ... heyboer says ask valvestorm .. valvestorrm wont tell.. I hoping someone kindly knowledgeable on these 67 - 68 Marshalls can Help? thanks.
     

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  2. william vogel

    william vogel Well-Known Member

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    The transformer you are using is designed for a full wave bridge rectifier. Typically (not this case), this type of transformer would not have a center tap. Because it has a center tap and the first capacitor filter arrangement is series connected for voltage capacity increase, you hook the center tap between the capacitor junction to ensure voltage balance between the capacitors. Many series connected capacitors use parallel resistors to balance (like your screen filtering capacitors). The center tap eliminates these resistors for the first node filtering. Because it’s a full wave bridge rectifier, the red wires (ac taps) go to the center junctions of the four diode rectifier. The top end of the rectifier is the rectified dc tap and the bottom is ground.
    upload_2021-2-13_20-5-32.gif
     
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  3. South Park

    South Park Well-Known Member

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    The ground is the red wire with the red stripe . It used as a floating ground on the filter cap . The bridge pack needs to be grounded . The main ground is the heater center tap . The floating ground is only hooked to the filter cap . No ether amp in the world has this set up you pick a good one to build . Take you time and get it right
     
  4. soundcharger

    soundcharger New Member

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    im confused
     
  5. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    The two PT blue wires are your bias tap, it appears you have incorrectly wired them as B+. The red, red/yellow stripe (center tap), red wires are your B+.
     
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  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    This PT is Metrospec, email your question to George Metropoulos if you’re confused.
     
  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    How do the two blue wires act as the bias tap? Isn’t that tap usually one wire? Where would they go?
     
  8. Travis398

    Travis398 Well-Known Member

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    One blue wire to ground the other to bias supply. I think coolidge56 is correct.

    (assuming picture in post 1 is correct)
     
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  9. Travis398

    Travis398 Well-Known Member

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    I think the white and black in layout is blue and blue on your transformer

    The blue and blue here is red and red on your transformer

    Screen Shot 2021-02-14 at 9.31.21 AM.png
     
    Last edited: Feb 14, 2021
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  10. adew1

    adew1 Active Member

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    I agree with coolidge56 and Travis398. I used the standup version of the 1203-80 PT in my 2203 build. Assuming that wire colours are the same for the ML version:
    Blue wires = bias, one to ground, one to bias supply as mentioned by Travis398
    2 x Red = B+ (higher voltage)
    Red/yellow = Centre tap, goes to first filter cap junction, as mentioned by William.
    2 x Red/blue = B+ (lower voltage)
     
  11. neikeel

    neikeel Well-Known Member

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    To summarise the above
    If you have a meter you will measure continuity between the two HT wires and their centre tap and the two low voltage wires (so a bundle of 5)
    You will get continunuity between the two bias tap wires (a bundle of 2).
    You will get continuity between the heater wires and centre tap (a bundle of 3).
    You will NOT get continuity between any of the other wires.

    So you will have one pair of wires (blue/blue?) one will be the ground and the other the feed.
    The two reds will probably be the HT and the centre tap will be the mid point of these (btw what colour are your low voltage wires? They are usually the same colour as the main wires but with a tracer).

    Can you post a pic of your transformer and the wires clearly shown?
     
  12. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    @Travis398 and @adew1 have it correct. We really must follow the data sheet and wire color codes on the data sheet that comes with the PT. I say we because I have gotten bit by looking at the amp layout wire colors instead of the data sheet. :realmad:

    Some PT's have a single wire for Bias, referencing back to a common center tap. Other PT's are wound differently. For example the Ceriatone PT used on their Dumble overdrive clone has these 10 wires...

    B+ 340/0/340
    Relays 6/0
    Bias 50/0
    Filaments 3.15/0/3/15
     
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  13. soundcharger

    soundcharger New Member

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    ok . here is my red /yellow stripe going to the first filter
    cap
     

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  14. soundcharger

    soundcharger New Member

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    heres the red going to the first bias resistor
     

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  15. soundcharger

    soundcharger New Member

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    the second red wire I have to ground
     

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  16. soundcharger

    soundcharger New Member

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    my transformer wiring diagram I go with transformer
     

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  17. soundcharger

    soundcharger New Member

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    layout for my reference
     

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  18. Travis398

    Travis398 Well-Known Member

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    According to the Transformer paper work Blue is bias feed not red.

    You have red and blue mixed up.

    Screen Shot 2021-02-14 at 1.00.06 PM.png
     
  19. Travis398

    Travis398 Well-Known Member

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    or pull the red wires and blue wires don't let them touch anything and safely turn the power on and measure ac volts of each pair. I am guessing blue and blue is around 95v
     
  20. soundcharger

    soundcharger New Member

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    So if the 2 blue are 95 vac one should go to chassis ground and the other blue to the first top left resistor on the main board ( the one resistor and diode), the red yellow ct to the 1st 100uf cap -, and the two red 178 v high voltage go to the 4 diodes? please correct me if I am wrong.
     
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