OT orientation?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Leonc, Feb 15, 2012.

  1. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Just curious to hear if anyone has knowledge of any sonic/performance effects resulting from the orientation of the OT relative to other parts of the circuit, such as the power tubes, output jack, B+, what have you. Anyone every study this?
     
  2. neteraser

    neteraser Member

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    The knoweledge is very simple. 2cm left or right = a slighly different sound. :)
     
  3. eddy999

    eddy999 Member

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    The positioning and orientation of the OT certainly has an effect on the amount of audible hum induced from the PT. The best method I know for determining the OT position/orientation is to wire up the secondaries to some headphones, with the primaries disconnected from the circuit and insulated off. If you then move the OT around whilst the power transformer is running you'll be able to find the position with minimal hum (which you can then mark off ready for drilling etc).

    I don't believe the position relative to b+ wiring will make a difference, firstly as the AC ripple of the DC should be very low and won't make a difference to the high OT current, plus b+ runs through the OT anyway on it's way to the plate(s).

    I haven't tested the effect of positioning the power tubes relative to the OT however.
     
  4. Zerksies

    Zerksies Member

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    I would say wire it up and move the transformer around for the least amount of hum and when you find it mount it there
     
  5. twotoneguitars

    twotoneguitars Member

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    I have always heard that having the OT iron oriented at 90 degrees from the PT cuts down on the magnetic interactions between the two. Mainly for noise and hum issues, but it can have an affect on tone. Physical distance may play a part in some circuits as well. YMMV.

    I have built amps with parallel iron due to space limitations and it was dead quiet. And then others that were perpendicular and quiet. It can be both ways. Each amp and circuit is unique in it's own quirks.
     
  6. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Very interesting idea eddy (and a bit scary!) but I don't think I understand your suggestion yet. You wire up the amp, w/o mounting the OT. You disconnect the OT's primaries from the circuit--including it's center tap to B+, right? -- then it's not connected to anything. So with headphones connected to the OT secondaries...what are you hearing?
     
  7. Ronsonic

    Ronsonic Member

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    The main interaction is between the power tranny and the output. You want to position the output tranny for minimum coupling between the two.

    The second best position for the OPT is at a distance from the PT and rotated 90 degrees. The best position is at some small weird angle that you find by putting headphones or better a 'scope on the output and moving it around for best results. You'll notice that almost all manufacturers use the second best approach. It works well and doesn't invite questions like "how come my transformer's crooked?"

    Other interactions to be aware of: Primary leads, the center tap lead is effectively DC and not critical, the other two are carrying huge signal and need to stay away from small signal circuits where they may induce oscillations and other issues. If you run them together the field around them cancels and they are safer, but still should be kept from preamp circuits.

    Layout is full of priorities and compromises. It's a major part of a successful build so take your time read everything you can and don't drill holes until you find something you can live with. As with all things amp design related check Randall Aiken's site, IIRC he has good stuff on the subject.
     
  8. guitarcapo

    guitarcapo Senior Member

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    My guess is that the power transformer is giving off electromagnetic flux noise that the OT is picking up. You don't need the OT connected to anything because the noise is being introduced from radiation outside the circuit.

    The signal isn't going to be very strong of course, which is why you use headphones.
     
  9. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Ronsonic; great suggestions thanks.

    GC - ah. Got it, thanks!
     
  10. Washburnmemphis

    Washburnmemphis Member

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    You don't need to wire up the amp, you just need to wire the primary of the PT to your 120V (and of course tape off your secondary so there are no nasty shocks!). As eddy999 mentioned what you are interested in is the amount of audible hum induced from the PT.

    All of this can be done pre-build when you are determining the layout of the amp.
     
  11. GrungeMan

    GrungeMan Supporting Member

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    I found out the hard way that one should keep the PT and OT at the same end of the chassis, there was interaction between the OT/OT wiring with the preamp circuit, until I found the problem it damn near drove me nuts! :huh :omg

    Yes OT 90 degrees to the PT, take a look at any Tweed Chassis.

    Grungy :D
     
  12. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Thanks guys. Great info.

    Eddy / Washburn - I'll give this a try later today.
     
  13. Structo

    Structo Member

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    Just to clarify what others have said.

    When we say move the OT 90 degrees to the Power Transformer, we are talking about the core laminations.

    So say the power transformer is oriented like this | then you would turn the OT like this __

    That way there is less lines of force from both transformers coupling inductively.

    Each core radiates these lines of force like the old magnet and iron filings experiment most of us did in science class.

    They tend to radiate out on each side of the core | so when we turn the OT 90 degrees, it lessens this coupling effect so there is less hum and noise.

    As mentioned sometimes the best position is not a true 90 degrees but it doesn't look symmetrical and people would wonder why.
     
  14. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Just tried this. Fascinating!!

    It was very, very easy to hear the difference that the OTs location/orientation made in terms of hum. I could completely eliminate the hum by putting the OT about 5" away at an angle that was just a bit off 90 degrees from the PT windings.

    I'm sure other factors (i.e.,. grounding scheme, etc.) come into play, but do I take it that this noise level will indeed be reflected the noise level in the completed amp?
     
  15. eddy999

    eddy999 Member

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    Glad you gave it a try. I've also found before that the angle can be off 90, especially if you have other inductors on board i.e. a choke. Minimising the induced hum will help to bring the overall noise floor down to give you a great basis on which to build a quiet amp.
     
  16. guitarcapo

    guitarcapo Senior Member

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    Why is it that you just don't want to install the OT as far away from the PT as possible? I'm thinking that these amps with the OT mounted on the speaker might not have been so silly after all.
     
  17. Ronsonic

    Ronsonic Member

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    Then it ends up near the preamp section.

    That speaker mounted tranny worked for small amps for a long time.
     
  18. guitarcapo

    guitarcapo Senior Member

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    I have a Magnatone M10 amp where the power section (power transformer, diode rectifier and filter caps) is at the base of a 3 foot cabinet. The rest of the amp is on top with the speaker and reverb tank in the middle. Odd design but it's deathly quiet. It uses a cable with about 12 wires in it communicating between the two. I would imagine two of them are high voltage because the power switch is up on top. The filament supply and pilot light supply run up that way too.
     
  19. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Well yeah, but while the M10 is really quiet (a lot of Magnatones are) it's probably the very last design approach any of us would want to adopt, LOL :p!! Next to the M15, that's gotta be the most fragile, most non-gig-worthy amps ever built (or one of them at any rate). I think the engineers at Magnatone knew a lot about designing great circuits, and they did seem to spend some time figuring out how to make them quiet--and I suspect there are A LOT of ways to skin that cat. But for every rule about what works in reducing hum, there are probably some exceptions and this may be one of them.

    I've also noted that in my 213s and derivatives, there was almost no twisting of the filament leads--and this is often cited as a "must do" in terms of building a hum-free amp. While the guys at Matnatone/Estey did a good job of keeping things quiet, I'm inclined to NOT follow their example with respect to filament lead dress...
     
  20. neteraser

    neteraser Member

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    That's right. I said the same. There's no other knowledge needed. :)
     

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