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Thread: OT placement too

  1. #1
    Supporting Member eschertron's Avatar
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    OT placement too

    Many of the amps I've seen have the plate lugs of the power tubes right at the OT, making the primary side wires as short as possible. I've started to notice that it is not always the case. Aside from balancing the heavy iron on opposite ends of the chassis, what are some basic considerations for the layout? What do you feel is a priority?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  2. #2
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    Since no one had replied - I'll give my opinion (FWIW)

    Those connections between anodes of the output tubes and the Output tranny Primary connections carry the highest signal voltages and highest signal currents anywhere in the entire amp.

    So magnetic (from signal current) or capacitive (from signal voltage) coupling from these wires into other parts of the circuit are highest too which can cause stability problems.

    Keep those wires as short as possible. Do NOT run them past sensitive preamp circuits. If you have to run them past or over other connections (signal connections) then cross those other connections at right angles to minimise both magnetic and capacitive coupling.

    Magnetic coupling can be reduced by twisting the push pull sides primary wires together (like you do for heater runs). That may add some capacitance and knock of a little high frequency response but you often see caps deliberately wired from each primary side to ground or wired from push side to pull side primary connection anyway (to knock off some high frequency response) so would not be too fussed about that added capacitance by twisting the anode to OT primary feed wires.

    On an old Push Pull 807 Amp running +700v B+ I have actually seen shielded wire (screen connected to ground at output tube end ONLY) used for the anode to OT primary connections.

    For runs of more than say 6" (150mm in real units) I would recommend twisting them. 3 to 5 twists per inch (per 25mm), less is ineffective, more is a waste of time. That twisting info applies to AC heater supply runs too.

    Cheers,
    Ian
    Last edited by Gingertube; 11-22-2017 at 04:19 AM.

  3. #3
    Supporting Member eschertron's Avatar
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    Thanks for checking in, Ian.
    The high voltage/high current is why I posted. The practical (that is to say, the in practice) solutions you mention are good for me to know about. Cheers!

    edit: so on the application where the OT connections are shielded, do you see both conductors in the same shield?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  4. #4
    Senior Member Enzo's Avatar
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    Why not? We are not concerned one plate wire will pick something up from the other, we are concerned the plate wires will radiate signal into the preamp circuits.
    eschertron likes this.
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  5. #5
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    No clue about the elecronics end of it, but as far as balancing the chassis, just move the handle! Marshall did it way back when, and I do it too. And if my Bassman 100 handle ever breaks, I'll move that handle, too!

    Justin
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  6. #6
    Old Timer J M Fahey's Avatar
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    On own builds I often separate transformers, PT always goes to one end and OT may go to the middle (so not exactly "balancing" ) and sometimes fully to the other end ... close to input jacks and tubes

    BUT, in that case I run those wires on the *outside* of the chassis, and taped/glued/tied down to it .
    Obviously metallic and grounded chassis is an excellent shield

    "Ugly" ?
    Define ugly please ... I go for function first and only after that consider "beauty" .

    If pressed I can run those wires inside, but tucked along the back edge and covered in aluminum paper or if I feel VERY fancy, inside a strip of U channel aluminum.

    And I always shield input jacks inside a little aluminum paper cage, costs nothing and helps a lot..
    Juan Manuel Fahey

  7. #7
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Enzo View Post
    Why not? We are not concerned one plate wire will pick something up from the other, we are concerned the plate wires will radiate signal into the preamp circuits.
    Exactly the point I was trying to get to. Shielding, twisting, both was to collapse the radiant fields.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  8. #8
    Supporting Member eschertron's Avatar
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    So when the wires are not as short as possible we need contingencies, some more elaborate than others. It's good to know about the need for a solution, and also some example solutions.

    I also like the 'shielding around the jacks' idea. Is that something you came up with, Juan, or are there other manufacturers out the that do that?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  9. #9
    Senior Member Enzo's Avatar
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    Lots of amps have input jack shields, even Behringer.
    Education is what you're left with after you have forgotten what you have learned.

  10. #10
    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    img_0513.jpgimg_0512.jpg
    Here's a donor Ashdown amp i'm converting to probably a train wreck circuit with a turret board .What are your guys opinions on the OT being on opposite ends & will end up fairly close to V1 ? .
    I'm wondering if i should move it towards the PT like a marshall style amp .
    "UP here in the Canada we shoot things we don't understand"

  11. #11
    Supporting Member loudthud's Avatar
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    Trainwrecks have a strong tendency to oscillate. You will have a much easier time if the OT is moved over by the PT.
    copperheadroads likes this.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  12. #12
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    I had in a past the same problem into a marshall jcm900 converted project. The original was stable but changing OT start to oscillate even with the same wiring path. The trannies was placed at oposite ends of chasis and had a long bundled wiring (10-12") from OT to output tubes. I cured just passed primary OT wiring through a cooper instalation pipe bolted to chassis cause twisting was not helpfull enough in my case
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  13. #13
    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Quote Originally Posted by catalin gramada View Post
    I cured just passed primary OT wiring through a cooper installation pipe bolted to chassis cause twisting was not helpfull enough in my case
    Not sure if i understand ? You used a copper pipe from the OT at the end of the chassis with the wires within going to the power tubes ? & it didn't help ? one other question This conversion build has a 15vac tap on the PT for DC heaters on the preamp tubes to run 12vdc (I have the voltage rectifier circuit figured out ) So with Dc heaters .Is twisting them similar to what is done with AC wires necessary & does same rules apply ?
    "UP here in the Canada we shoot things we don't understand"

  14. #14
    ric
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    I believe catalin (and Juan in post #6) are describing how to deal with long wire runs that could be a problem. Gingertube/ Ian in post #2 gives a pretty clear description of why to keep wire runs short.
    I would suggest putting the OT so that the primary wires enter by your power tubes and the secondary wires enter near your speaker jack(s).
    If you have a choke it would go between the PT and OT with it's laminations 90 degrees to the OT (turned 1/2 way).
    With your laydown PT you automatically have it turned 90 degrees from the vertical trannys.
    Heater wire is twisted because of it's current, so I would twist ac or dc.
    copperheadroads likes this.

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