Toroidal PT Best practices?

Discussion in 'Shock Brother's DIY Amps' started by Huddy, May 11, 2021.

  1. Huddy

    Huddy Tele-Holic

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    I've got the ANTEK Toroidal PT Variety Pack (200/240/280) last week and a few blank chass-eye as well... I want to start putzing around with a micro build or two but want to make sure I don't create issues for myself soooo... a couple of questions.

    In a traditional PT transformer it's best to make sure the laminations are not on the same plane - are there similar considerations that need to be made? Proximity concerns regarding the OT? Any benefit/detriment for mounting inside vs outside the chassis? Anything else that I would only know to ask if I've had past experience?

    Thanks in advance!
     
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  2. 2L man

    2L man Tele-Holic

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    I have built many amps using toroids and mount them flat to chassis and there is rubber plate under it. Toroid magnetic stray field is strongest straight out of hole but it is said to be not too strong. This is 5F6A for 6V6 and has also toroid OPT. Brown box between toroids is 10uF 400V PIO which feeds the OPT. Cabin will have two 10" diaconally and that is the reason there is nothing between V2 and V3.

    PHOTO-2021-05-04-18-19-28.jpg
     
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  3. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I did the headphone trick test on my hifi amp with toroids.

    The transfer of hum from the PT to the OT is very small. Much lower than with the EI transformers.
    The only time I heard some hum was when my connecting cables (quite a mess :) ) came close to the hole.
     
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  4. Huddy

    Huddy Tele-Holic

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    what’s this “headphone trick test” you speak of?
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  6. Huddy

    Huddy Tele-Holic

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  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I *think* he's saying you don't need to do the headphone test (an EI thing) with toroids. With the windings in the round, the EMF is not directional in the plane of the OT.

    A big deal: I think the Antek *requires* a bridge rectifier.

    A small detail IIRC: On the Anteks is the wires emerge from the 'top' of the stack if you mount it with the label 'upright'; but for shorter wire runs, you just mount it with the label 'upside down' so the wires emerge next to the chassis.

    Scary things one hears about toroidals (things IDK):

    1. Potential for big inrush current? I *think* this is not a problem with the smaller toroidals, but as I say, no knowledge.
    2. Some need to isolate (the PT? the center bolt?) correctly from the chassis. I haven't found a simple explanation, but I'm sure it's well understood. Some discussion here:

    https://audiokarma.org/forums/index...ions-for-a-toroidal-power-transformer.723804/
     
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  8. Huddy

    Huddy Tele-Holic

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    - Ah gotcha on the headphone test...
    - Certainly understand the needed Bridge Rectifier aspect on non-center-tapped models (ex 0-200 vs 200-0-200) - no problem there.
    - If my chassis is laying down - like a marshal or something (how I plan on using it) - the writing is right side up with the leads closest to the chassis. My plan is to make two grommeted thru-holes for the leads. One for the primary right at the IEC receptacle and the other for the secondary. Was also planning on using one set of heaters for the pilot and the other for the two tubes. The plan is to tuck them in just past the straw-looking isolation covers where the leads are connected to the windings so they don't crimp or anything.
    - Inrush current would do what? cause mains fuse to blow easily? or is there something else I need to look for?
    - I've seen a lot about the provided steel bolt potentially causing a "1-turn" secondary to ground and to replace it with a nylon bolt. I've read that the rubber pads are sufficient. I've also read that it's more of a concern with a steel chassis than an aluminum for some reason - don't know why that matters. Also don't know why they'd send you a steel bolt of said steel bolt would prevent you from returning as a customer. ;-) But i'll definitely check out that link as well. Thanks!
     
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  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yeah, let's hope we'll hear more from folks who know toroidals. As I say, maybe we're not worried about inrush current on these smaller PTs? and I *think* (zap/gasp/thud) the metal bolt may be OK as long as you (maybe???) isolate it with rubber/nylon washers? And don't connect the top to the bottom with something metal? I imagine our EU friends @Tom and @2Lman can tell us.
     
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  10. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The bolt doesn't need to be isolated at all. What you can't do is create a complete loop through the inside of the toroid. In other words, the bolt cannot be connected on both sides. The top or bottom can connect, but not both. You can put a metal box all the way around it if you want, as long as what goes through the middle doesn't connect both sides.
     
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  11. Huddy

    Huddy Tele-Holic

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    I'm not sure what you mean by both sides - If I have a single 3/8" hole in the top of my chassis - that the bolt mounts to that's okay, correct? So I've got Bolt, Black metal spacer, Rubber Spacer, Toroid, Ruber Spacer, then the chassis with a hole that the bolt goes through the previously mentioned mounting hole secured by the supplied nut - that's all good to go right? I feel like I've read a thread or two that has confirmed this - pretty sure you were the one doing the confirmation as well ;-) but triple checking never hurt anybody I don't suppose. Thanks!
     
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  12. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    @Huddy you should be fine. What I was trying to explain is that you cannot create a complete loop from the inside around the outside and back to the inside of the toroid. The rubber, by the way, is only there to protect it from damage, and has nothing to do with isolating the transformer.

    This was my last build, with the toroid inside the chassis. If the hex bolt were sitting up high enough to touch the top of the chassis (the top plate of the chassis that screws on is obviously not in the picture), then that would be a problem.

    [​IMG]

    [​IMG]


    @King Fan mentioned that you have to use a full wave bridge. I believe this is true based on Antek's documentation. You can run the secondaries alone, in parallel, or in series with a bridge rectifier, but I don't believe you can create a center tap out of them and use a standard dual phase full wave rectifier. This also pretty much limits what you can do for a fixed bias supply. I'm assuming a capacitor coupled supply would be okay. I'm about to find out on my next build soon, because that's exactly what I'm going to try.
     
  13. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I assume this is to keep the DC off the OT. I have never really thought about doing one. How in world do you do that, and separate the AC load, since the DC load normally flows through the OT?
     
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  14. Huddy

    Huddy Tele-Holic

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    Okay - I had a feeling that's what you were getting at but I couldn't think of a scenario when that was possible but the protruding bolt head could make that connection when attaching the cover or something like that. So if I'm mounting top side I definitely shouldn't have an issue.

    Also in another thread you mentioned that capacitor you've got across the PT primary. Can you explain the purpose and/or point me in direction of some info on it?

    Last thing... the purple lead (or whatever color the shield is) where should that go? w/ the safety ground? Power Filter Section Ground? Somewhere else?

    Your pictured build has inspired me to build something out of some cable boxes that verizon didn't want back ;-)
     
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  15. Huddy

    Huddy Tele-Holic

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    Curious about this to - But don't understand keeping DC off the OT? I assume meaning off of the secondary? I'm probably quite a few rungs below you in understanding this concept ;-)
     
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  16. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    It's something I haven't at all looked into really, and I think my thinking is wrong. I know there is an issue with DC, but a quick search and I find discussions about DC imbalance. I don't know. Hopefully @2L man will chime in and tell us what he did.
     
    Last edited: May 12, 2021
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  17. 2L man

    2L man Tele-Holic

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    No that PIO is just last filter capacitor before OPT and circuit is normal PP but I have also balance pot on bias circuit so that DC impalance is possible to null better. Toroid OPTs are said to saturate easier if there is imbalance between PP tubes and there comes DC component.

    I have used russian paper in oil coupling capacitors on few builds and they make amp sound different in a good way so I wanted to use more PIOs. It is only 10uF as I hope to achieve more sag-effect and thats why there is also a series resistor before it from previous elecyrolyt.
     
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  18. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I’m watching this thread since I’m using a toroidal PT on my 4x5” Micro Bassman build... and I just glued this pine bee box together as it’s cab :)
     
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  19. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    To provide some crude RF filtering of the mains supply. It was something that I just copied from a solid state bass amp with a toroidal power transformer. Since it's just a line-to-line (hot to neutral) and a line-to-ground (neutral to ground) the RC filter is dependent on the source (the mains) impedance. So, it's probably only really filtering RF frequencies above 1MHz. There are better alternatives, EMI/RFI filter modules you can buy. I don't really know much about those things though. I may look into it for my upcoming project.

    On my build, it's soldered to the same chassis mounted solder lug as the mains safety ground. If you're using a ring terminal and a dedicated mount point for the chassis safety ground, and you want to keep it dedicated, then I'd just mount the transformer shield close by. I don't remember the exact UL standards and they vary depending on whether or the power cord is detachable, i.e., an IEC inlet ... but, that's a discussion for another thread.

    I've seen the argument made that the shield should connect to the chassis with the secondary center tap, but there is no center tap in this case. There is no direct connection of the secondary to ground.
     
    Last edited: May 12, 2021
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