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Thread: OT arrangement on chassis

  1. #1
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    OT arrangement on chassis

    Is there a better or preferred facing of OT with relation to preamp tubes or would it not matter much?

    For instance, in the pic I have OT primary facing preamp tubes. Would flipping 180 degrees so secondary faces pre tubes be any better or worse, or a different orientation all together ?
    img_2023.jpg

  2. #2
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    Hi. Just my thoughts : don.t bore to much with tubes. the most parasitical noise can get comes from OT coupling which acts as antenna. (You can get PT noise into the speakers even without tubes connected). So much concern regards PT / OT placement. Use headphones to get the best orientation.
    " I know I'm stupid but get courage when look around "

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    This is pretty interesting. I knew they should be orthogonal but didn't know that flipped by 90 degrees and 90 degrees in two axis is even better

    https://www.youtube.com/watch?v=Ublo6uSBwfw

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    Senior Member Enzo's Avatar
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    The windings are covered by the steel end bells. But you are thinking of the wires coming out when you say primary side and secondary side. But the windings go all the way around it, they are wrapped on the core. There are no "sides".

    Orthogonal is too big a word for me, I thought that was a kind of shoe... 90 degrees is 90 degrees.

    But serious now, keep the PT and OT at right angles. The tubes really don;t care. I agree with catalin.
    Education is what you're left with after you have forgotten what you have learned.

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    i would flip the transformer around so the sec leads are near the preamp tubes just in case.

    Fender keeps the OPT far away from the first preamp tube by having it in the middle of the chassis.

    Marshall does do this not but then again, most Marshalls are more unstable and noisier than Fenders, jus sayin.

    those end bells provide some electrostatic shielding by forming a Faraday cage around the pri winds, but the leads are not shielded and contain high AC voltage and frequencies many times that of 60 Hz which can add to their ability to propagate into circuits that are in close proximity to the leads. To experiment with this, do not trim the pri leads, inject 1 K Hz signal if you can, and move wires around with insulated pliers into the neighborhood of the first control grid of the 12AX7a.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by nsubulysses View Post
    Is there a better or preferred facing of OT with relation to preamp tubes or would it not matter much?
    My primary concern would be to keep the primary side wire entry as close to the power tube plates as possible. Short wires, limited exposure inside the chassis, less radiated noise from the unshielded high AC voltage. I agree with the others: outside the chassis, the issue is 60Hz from the PT leaking into the OT secondary.
    J M Fahey likes this.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Quote Originally Posted by cjenrick View Post
    i would flip the transformer around so the sec leads are near the preamp tubes just in case.

    Fender keeps the OPT far away from the first preamp tube by having it in the middle of the chassis.

    Marshall does do this not but then again, most Marshalls are more unstable and noisier than Fenders, jus sayin.

    those end bells provide some electrostatic shielding by forming a Faraday cage around the pri winds, but the leads are not shielded and contain high AC voltage and frequencies many times that of 60 Hz which can add to their ability to propagate into circuits that are in close proximity to the leads. To experiment with this, do not trim the pri leads, inject 1 K Hz signal if you can, and move wires around with insulated pliers into the neighborhood of the first control grid of the 12AX7a.
    What always blew my mind was Soldano. Such an expensive and coveted amp brand, that seems to perform well other than hiss, but it's high gain so it makes sense. It has DC heaters so the hum is minimal. But the chassis depth is really shallow, maybe 7.5" or 8" I'd guess, and the OT butts up right against the first pre tube
    soldano-avenger.jpg
    avenger-closeup.jpg

  8. #8
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    Perhaps it is a bad idea to have the OT mounted as I do. This is what Im' workin with. feel free to criticise my layout. Even thought the OT is a standup style I did try it standing up and laydown and did not have any change in noise floor. I did not try it in the orientation of having primary and secondary reversed however

    my-amp-chassis.jpg
    PT on right OT on left

    my-amp-inside-chassis.jpg
    PT on left OT on right

    my-amp-ot.jpg
    I put secondary of OT right by the impedance selector switch. Maybe this should be switched to keep B+ wires as far away from V1 preamp tube as possible

    my-amp-pt.jpg
    Close up of PT side. primary winding enters chassis from right under the main filter cap and goes right to on/off switch. secondary side is on the right. I put a tag strip since heater winding branches out there to go to hum balance, power tubes then preamp tubes, and to the pilot light which is a LED as per Merlin's suggestion. I guess I got it slightly wrong though since the pilot light does not come at the end of my chain. Not sure it that would be problematic. I never tried it both ways
    heater12.jpg

  9. #9
    Senior Member trobbins's Avatar
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    I can't see any obvious problem with layout in those photos - the basic layout looks as good as one can get. You'd probably have to get the amp working, and selectively short the signal inputs right at the valve bases, and use a spectrum analyser to assess how and where your residual hum and noise levels were coming through.

    You have electrostatic screens on the preamp valves above the chassis. The only areas of concern I can see underneath, if you did have noticeable hum/noise, would be:
    - are the valve base spigots of the preamp valves connected to local ground?
    - the input socket could be replaced by a screened version.
    - the chassis will mount to a metal base plate?
    - some internal metal divider walls could be added to further screen the input stage from the the OT wiring.

  10. #10
    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Comment on this img_0512.jpg
    As you can see the original has the OT at the opposite of the chassis & this new conversion being basically a marshall/trainwreck express style build ... Which will incorporate either a traditional Fender style eyelet board or a Marshall style turret board . I won't be using the middle holes that the tubes were in & I will fab a cover for that area as it will be where the board will be mounted . the question I have is .leaving the OT where it is & being close the V1 will it cause any extra noise ? ( keep in mind all preamp tubes will be powered with DC 12 VOLT heater & not the 6.3vac like on the Power tubes .
    "UP here in the Canada we shoot things we don't understand"

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