Why did Mesa criss-cross leads of the 50/50 OTs?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Humble Texan Fan, Aug 23, 2018.

  1. Humble Texan Fan

    Humble Texan Fan Member

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    In an effort to revitalise my Mesa Fifty Fifty it's going to receive a new set of filter caps, bias-pot and OTs with more iron to them. As seen in the photo below the bundles of wires from each OT are cable-tied separately. Then for the 8 ohm outs the orange wires cross over to the yellow wire of the opposite OT.

    Any particular reason Mesa would do this, you think? Those 2 x 4 ohm outs below (mostly obscured by the yellow and orange) are a single green without any interference from the opposite transformer.
    [​IMG]
     
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  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Per the schematic of the 50/50, there is NO interconnection between channels. The OT secondary taps are wired to two 4 ohm jacks, an 8 ohm jack and then a negative feedback connection back to it's phase inverter cathode stack.
     
  3. Humble Texan Fan

    Humble Texan Fan Member

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    Cheers mate. Yes, I didn't find any evidence of such wiring on the schematic either. And it would appear as if all versions have this.

    That orange wire could be somehow connected to the feedback circuit then. There seems to be some red wires running from the presence controls towards the OTs, which I have yet to remove, and I suppose that these could somehow be interconnected.
     
  4. JD13

    JD13 Member

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    I wish you could take a look at the early 90s 50/50 I bought years ago. It had one channel red plating so i replaced all tubes and it still red plated that channel. I played only on the good channel safely and it made a noise and stopped working all together. The pre amp tubes dont light up but when tested with a multimeter they all give off a continuity signal. Its been sitting unused for so long and i payed a good chunk of cash for it. Too bad you are not in Texas so you could take a look at my amp. The two techs around here have some not so good reviews that kept me from going there for a repair.
     
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  5. Humble Texan Fan

    Humble Texan Fan Member

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    I honestly don't see why they would be so hesitant to get their hands dirty with the later Fifty/ Fifty, being quite well laid out for a mainly PCB construction. And there are schematics readily available for it as well.
     
  6. JD13

    JD13 Member

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    Im the one hesitant to take it to them. Their customer service is bad from some reviews i read online (holding customers gear hostage,taking months to work on items). I dont have much of a choice so im probably stuck taking it to them. Do your preamp tubes light up?
     
  7. sockeye

    sockeye Supporting Member

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    The 50/50’s Presence control used negative feedback like most — but it was configured unusually. I suspect that’s what’s going on here & it makes sense the wiring would be reversed if phase-reversal was the aim.
     
    Last edited: Oct 26, 2019
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