Major output transformer wiring

Discussion in 'The Workbench' started by pete mac, Dec 10, 2019.

  1. pete mac

    pete mac New Member

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    The output taps on my '72 go to a 'christmas tree' (?) termination to very thin screened cables to the impedance selector.My '74 output wires go direct to the selector.I found a comment somewhere online about the major output wires needed routing outside the chassis to stop interference/feedback.I'm restoring and making more reliable a couple of majors.The thin screened cable doesn't look capable for the potential current especially on the 4 ohm winding.Was the screening system/christmas tree dropped later as not needed/caused transformer failure ? A bit of a long shot after many years but someone might remember !
    Thanks
    Pete
     
  2. South Park

    South Park Well-Known Member

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    The feed back noice is all in the grids mainly in the preamp gain stage . Not the ot . Does the input have the grid stopper
     
  3. pete mac

    pete mac New Member

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    Thanks for that.Yes they have.My question is to do with the output transformer secondary windings routing and when/why the termination method changed , any idea ?!
     
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    They all have grid stoppers, do they not?
    I mean unless someone removes them all. :scratch:
     
  5. pete mac

    pete mac New Member

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    I'll post some pictures later about what i mean ! : )
     
  6. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Once again South Park gives out stupid and unqualified info

    Take no notice of what he says Pete, he is just as others here say a Googler, just reads and thinks he is an expert.

    Sorry for the rant guys.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Ken's had all he can take...:realmad:
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    "I found a comment somewhere online about the major output wires needed routing outside the chassis to stop interference/feedback."

    I don't think you need to run the wires outside the chassis.
    As long as the amp is working right you won't need to. The question is: does the amp work right to start with?

    "The thin screened cable doesn't look capable for the potential current especially on the 4 ohm winding."

    If the factory built it that way then it is capable.
    Can you post a lot of clear pictures of the insides?
     
  9. MickeyJ

    MickeyJ Active Member

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    I can't see any other reason marshall would have routed them externally, I wouldn't mess with the routing.
    Note the higher power of these things would have meant ( and still does ) a different / stronger field around the wires carrying signal/current, I can easily see this causing interference.
    The failures, if you believe the internet were not due to insufficient thickness windings or leads, but rather the windings were shorting with each-other due to poor insulation ( cheap probably) and they found ' a guy' who was repairing them by re-winding o/t with better wire.
     
  10. pete mac

    pete mac New Member

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    Hi , here are some pictures which might explain what i'm going on about ! My '72 has the insulated and screened termination (stock) and very small wires for 200w rms into 4R and my '74 hasn't ! So just wondering if the instability was cured by '74 and shorts in the wiring to the impedance switch have caused transformer failure.These amps are both stock in that area. 72A.jpg 72B.jpg 74A.jpg 74B.jpg
     
  11. South Park

    South Park Well-Known Member

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    That is a funky set up . Try to run the speaker straight off the transformer .
     
  12. pete mac

    pete mac New Member

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    I'm thinking of wiring direct with thicker wire , don't want the sotw effect : )
    Just was wondering if anyone had any info from the time , it was a while ago !
    I wonder how many are left of the 1200 ?
    Pete
     
  13. South Park

    South Park Well-Known Member

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    Thicker wire is good run the ground straight to the center tap .
     
  14. MickeyJ

    MickeyJ Active Member

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    I see your concern, My goodness that does look shitty.
    I'd check those wires that are up against the tube base, if they've worked this long without charring, I'd leave them alone, maybe try and pull them away from the base 1/8 of an inch or so.
    This is a job for someone who already has a major to compare.
     
  15. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The stock wire size is fine. Don't mess with success. Into a 16 ohm load the voltage and amperage required to achieve 200 watts is 56.5 volts at 3.5 amps.

    22 gauge wire is good for 7 amps in this low voltage application.
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I believe they only routed the OT secondary outside for convenience and a cleaner layout due to where the components and impedance selector was located. I do not think it had nothing to do with feedback or interference.

    1968 [​IMG]
    1969 [​IMG]
    1971 [​IMG]
    1972 [​IMG]
     
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  17. MickeyJ

    MickeyJ Active Member

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    i wasn't sure but the photos confirm it, that shielded wire is stock, I thought so because why else use a bus rail like that on that little board, the presence of shielded wire proves they had an oscillation/coupling issue, and mounting the wires externally mitigated it.
    Just run the wires as the 1972 photo shows and it's all good thereafter.
     
  18. South Park

    South Park Well-Known Member

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    If all else fails insulate the stand offs . The chassis is one big ground. Tubes can do weird things with the magnetic fields.
     
  19. 66 Kicks

    66 Kicks New Member

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    I thought the OT had multiple secondary windings and the Christmas tree was a break-out box for those windings.
     
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  20. MickeyJ

    MickeyJ Active Member

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    The secondaries are going to the impedance switch, the ones hugging the tube base, the impedance switch is underneath that tubes socket , it's the plate wires that are being shielded, well, that's what it looks like to me.
    You'd want to not change a thing, just route wires as they were stock.
    An interesting way to get around the need for shielded plate wires ( I've never seen that before, they may not be shielded plate wires at all) is to use top cap anode output tubes, that way the plate wires would be up high, but that's a bit dumb really isn't it, just thinking out loud. I'm sure a major dude will come here and clear the mystery up.
     

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