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Thread: PT and OT transformer core orientation

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    PT and OT transformer core orientation

    Hi All
    I got the sorta kinda general idea, but not what to do in practice. I have an amp project, a Supro Thunderbolt S6420 in a JTM45 chassis. The two transformers, PT and OT are both stand up. So the cores will both be perpendicular to the chassis. Not sure I'm using the correct term, thought "core" meant the steel laminations that the windings are around.
    Read through a few posts, but didn't get the conclusion, mostly because the amp was a different shape. The PT was a laydown, so its laminations were parallel to the chassis, and the OT was standup, so the laminations were naturally 90 degrees (assuming that they didn't rotate the OT).
    With both cores standing upright. what is the best relative orientation of the two? And do they need to be some distance apart, or can I mount them closer together?
    I sort of understand that in a general sense, you don't want the varying B field of one transformer to induce into the other one. This PT has end bells but the OT does not.
    Thanks
    MP

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    Oh and the other comments Ive seen talk about really expensive Mumetal guard between the two, but Ive never seen a commercial amp like that.

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    Supporting Member The Dude's Avatar
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    You generally want the laminations perpendicular and not parallel- and transformers as far apart as possible, within reason. It's not a bad idea to think about weight balance when you carry the thing either.

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    Awesome, thanks El Dudarino!

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    The design of the E-I lamination and coils makes the magnetic field run along the length of the center leg of the E, then return through the outer legs of the E. Think of the field orientation as a long stick running through a hole drilled along the length of the center leg of the E.

    Magnetic field pickup is worst if the orientation of the center legs on two transformers is parallel. It's least if the center legs are at right angles. There are three possible orientations for "right angles" in our three dimensional space. So the classic way to orient transformers in a tube amp is with one (generally the PT) lying flat with its center leg direction parallel to the chassis plane. The OT can either stand up, with its center leg vertical relative to the plane of the chassis, or lie flat on the chassis, but with its center leg pointing at right angles to the center leg of the PT. The choke takes the other orientation not used by the PT and OT.

    That's good as far as it goes. It neglects the other items for M-field, conduction and the inverse square law. The inverse square law says that the M-field will drop inversely with the square of distance. So if you double the distance between the PT and OT, the field radiated to the OT from the PT will drop by a factor of four. Distance is much cheaper than mu-metal.

    But iron "conducts" a magnetic field much better than free space does. So if you use a steel chassis, the chassis itself can "conduct" hum over to the OT. Steel is a "lower resistance" path than free space for M-fields by a factor of 5,000 to 20,000, depending on alloy. Aluminum conducts M-field the same as free space. I've heard that aluminum chassis amps >may be< a little less hum-my than steel ones. Of course that only matters if all the grosser hum generators are already eliminated.

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    Plug on the PT and Use a headphone to determine OT best position.

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    Check out pages 5-13 of this preview

    https://books.google.com/books?id=Mw...page&q&f=false

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    Quote Originally Posted by R.G. View Post
    The design of the E-I lamination and coils makes the magnetic field run along the length of the center leg of the E, then return through the outer legs of the E. Think of the field orientation as a long stick running through a hole drilled along the length of the center leg of the E.

    Magnetic field pickup is worst if the orientation of the center legs on two transformers is parallel. It's least if the center legs are at right angles. There are three possible orientations for "right angles" in our three dimensional space. So the classic way to orient transformers in a tube amp is with one (generally the PT) lying flat with its center leg direction parallel to the chassis plane. The OT can either stand up, with its center leg vertical relative to the plane of the chassis, or lie flat on the chassis, but with its center leg pointing at right angles to the center leg of the PT. The choke takes the other orientation not used by the PT and OT.

    That's good as far as it goes. It neglects the other items for M-field, conduction and the inverse square law. The inverse square law says that the M-field will drop inversely with the square of distance. So if you double the distance between the PT and OT, the field radiated to the OT from the PT will drop by a factor of four. Distance is much cheaper than mu-metal.

    But iron "conducts" a magnetic field much better than free space does. So if you use a steel chassis, the chassis itself can "conduct" hum over to the OT. Steel is a "lower resistance" path than free space for M-fields by a factor of 5,000 to 20,000, depending on alloy. Aluminum conducts M-field the same as free space. I've heard that aluminum chassis amps >may be< a little less hum-my than steel ones. Of course that only matters if all the grosser hum generators are already eliminated.
    Ah, OK, so that's why the "laydown" design marshall and fender: the PT center leg is parallel to the chassis, and at right angle with respect to the OT which stand up. So, both transformers for this thing have feet that situate the cores upright. The OT I can see the center leg, since I can see how the windings are in there. I don't have the OT yet, still being mfr'd, I suppose I can pop one of the end bells off to see how its oriented, I imagine the same as the OT, windings around the center leg which sticks up out of the chassis. The chasiss is aluminum, which by your description sounds like a good thing.

    Since I can't lay down the PT without great pain and misery, (I've seen a few home made PT holes made with a dremel and steadier hands than I have), then I should put the OT as far away as possible. I can situate the OT so that the secondary wiring drops right over the output jacks. Then I only need to get the 3 primary OT wires to the right place without causing undue problems. The center tap comes from the dirty end of the power supply.

    In the build instructions I have for a JTM45 kit I never finished (will get right on that after this build is done) the OT pri and sec wiring run together and between the output tubes. But this places the OT fairly close to the PT. So, wen't around in a circle.

    Im assuming they didn't spin the OT 90 degrees so that the OT secondary hole was on the phase inverter side of the first power tube? It would then run closer to the phase inverter. <frown>

    So, it looks, by the way the wiring needs to be routed, the OT has to be about as close to the PT as it is in standard Fender and Marshall designs?

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    Quote Originally Posted by nsubulysses View Post
    Check out pages 5-13 of this preview

    https://books.google.com/books?id=Mw...page&q&f=false
    I absolutely love that book. his chapter on metal work and tooling is spot on.

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    Metalworking for Poets

    oh yeah

    My favorite is how he talks about doing chassis and headcase design with the idea of creating a convection current in mind so warm air dissipates from the amp well as it can without using a fan.

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    Quote Originally Posted by nsubulysses View Post
    Metalworking for Poets
    Yes! There is so much great insight into the process. I think one of the most important points I was able to take away from that particular chapter is the value in investing in high quality tools for a particular job. There is absolutely a difference.
    I remember a point early on, and I was at a point in my build where I needed to make a large diameter hole (I don't remember what it was for). But it was holding up the process that night. I had some good momentum up to that point and I wanted to keep going. The only thing I had the money for was the Harbor Freight step drill, so I rushed over to get there before they closed. I think they had a set of 3 for like, I don't know.. $15 or something. Maybe $30.
    But I grab them off the rack and looked over the drills and I just couldn't do it. The machined cutting edges and nitride finish were total dog shit. Even though I wanted to get it done, I knew I would really regret it I compromised what i really wanted in the finished product.
    Don't get me wrong, Harbor has their place, but I'm really glad I decided to wait and find a quality tool that will hold up under 30 or more years of reliable use

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    There's an awesome thing he says where it's something along the lines of, "most people dislike doing the metalworking or chassis fabrication aspect of an amp build. However, the real reason most people don't like doing it is because they have very poor or incorrect tools for the job and it makes it a huge pain in the ass"

    but imagine he phrases it not like that, but like a British person instead

    edit:Oh here it is
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    Last edited by nsubulysses; 08-21-2019 at 06:15 AM.

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    Quote Originally Posted by nsubulysses View Post
    There's an awesome thing he says where it's something along the lines of, "most people dislike doing the metalworking or chassis fabrication aspect of an amp build. However, the real reason most people don't like doing it is because they have very poor or incorrect tools for the job and it makes it a huge pain in the ass"

    but imagine he phrases it not like that, but like a British person instead

    edit:Oh here it is
    Click image for larger version. 

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    Ooo, the Jones book looks awesome. Looking fora used copy. Re the comment: spot on! Ive done very little metalworking, but know that you need REALLY good tools. Box brake, with enough oompf to bend 18ga steel, good punches, drill press, power shear etc. A CNC would be nice. I don't have the room in the 6' x 12' room I use for music, electronics, guitar, computer and general time wasting.

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    The classictone OT arrived two days ago. Just waiting for the note that the PT is done. Haven't been able to get in contact with Heyboer in a while, maybe he's on vacation.

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    I also vote for using headphones to orient transformers. I use a connector block to safely isolate the secondaries and another block to power up the PT. I then use clips to connect the phones to the OT secondary. That way you're not leaving anything to chance. Guitar amps are a little more forgiving with background hum, but in my hifi builds its essential to get the lowest level of quiescent hum. Sometimes the best positioning isn't exactly how you think it ought to be.

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    I also vote for using headphones to orient transformers
    Yeah, that's what I did 40 years ago. I used a cheap "crystal" piezo earphone connected across the OT primary. Worked great. Meanwhile I have a scope.

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    Quote Originally Posted by nsubulysses View Post
    Check out pages 5-13 of this preview

    https://books.google.com/books?id=Mw...page&q&f=false
    Can't praise Morgan Jones' books enough.

    The other volume, about tube amp theory, is probably one of the best tube amp texts you can buy.

    Because of this particular book I started building my own mixed aluminum + brass chassis.

    I needed good electrical conductivity for my star grounding with as little as possible magnetic conductivity.

    This is what I came up with:

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    The mid section is 2 to 3mm thick brass or mixed aluminum with a thin sheet of brass under it (note on side photo below). (Pure 3mm brass is too expensive so for my experiments I used aluminum + brass.) I could then easily solder electrical terminals to the brass layer that faced the circuit.

    The structure was made out of L shaped aluminum profiles (aluminum chassis assembly idea copied straight out of Morgan Jones book) as such:

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    My transformers would sit on the brass or aluminum middle section.

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    Zero magnetic noise. (And electrical conductivity for proper grounding don't get much better than brass.)

    Out of respect for square law m-field, I always set the transformer laminations at 90 degrees regardless of how far:

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    And electrical conductivity for proper grounding don't get much better than brass.
    Copper and aluminum are better electrical conductors than brass.

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    Quote Originally Posted by Helmholtz View Post
    Copper and aluminum are better electrical conductors than brass.
    Most amplifier chassis are made out of steel. My experiment was an attempt to improve on that.

    The above amplifier is dead quiet which is what I wanted to achieve.

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    Quote Originally Posted by Helmholtz View Post
    Copper and aluminum are better electrical conductors than brass.
    Sometimes I used this kind of brass pipe (3 mm dia.) for long grounding bus (meant double surface conduction for same section). I did not notice any diferences in term of noise in respect with cooper bus wire. One remark think should be done regards physical contacts between cooper/brass and aluminium as both tends to protect by auto oxidation and in contact each with another develop thin layer of oxide which maximise residual resistance.

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    Last edited by catalin gramada; 08-24-2019 at 09:59 PM.
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    Quote Originally Posted by catalin gramada View Post
    Sometimes I used this kind of brass pipe (3 mm dia.) for long grounding bus (meant double surface for same section). I did not notice any diferences in term of noise in respect with cooper bus wire. One remark think should be done regards physical contacts between cooper/brass and aluminium as both tends to protect by auto oxidation and in contact each with another develop thin layer of oxide which maximise residual resistance.

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    You shouldn't have to solder brass/copper and aluminum.

    The main point here is that the transformer will always induce some tiny current on a steel chassis but not on aluminum or brass.

    The tiny electrical conductivity differences between metals in chassis-distances won't generate perceivable voltages with signal-level currents. The metal conductivity was picked from my reply but it was definitely not the main point. Brass is acceptable, that's the point.

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    I did not notice any diferences in term of noise in respect with cooper bus wire. One remark think should be done regards physical contacts between cooper/brass and aluminium as both tends to protect by auto oxidation and in contact each with another develop thin layer of oxide which maximise residual resistance.
    For electrical resistance only cross section and specific conductivity matter - not surface. Low electrical resistance only matters with high ground currents. In most cases a heavy gauge copper bus is the best solution.

    Only aluminum is known for noteworthy auto-oxidation, that's why it isn't easy to solder.

    Especially if you have a ground loop also ground path inductivity matters.

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