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Transformers: minimizing coupling

Discussion in 'Shock Brother's DIY Amps' started by chas.wahl, Mar 8, 2015.

  1. chas.wahl

    chas.wahl Tele-Meister

    Dec 8, 2012
    In doing research for a fairly simple amp build, I've come across something that puzzles me. Please take a look at the following 4 items:

    From Merlin Blencowe's Designing Power Supplies for Tube Amplifiers
    (no longer in print, but available electronically through B&N, and in bits and pieces on http://www.valvewizard.co.uk/ )

    From Designing Vacuum Tube Amplifiers and Related Topics by Charles Crouch
    (an online publication available here )

    A typical Fender Tweed amp chassis with laydown PT (Harvard 5F10)

    A Hammond laydown PT, with long side on left, and short side on right (having copper shielding band exposed)

    Now, though Blencowe and Crouch differ about diagramming where the source of the coupling might originate (Blencowe leakage from the iron core, Crouch fields around the windings), they agree about the optimum orientation: keep the zones of influence of these separated by keeping their major planes oriented to different axes (X, Y, Z).

    What mystifies me is that the most common orientation for a laydown PT (that I’ve observed) is to have the long side of the core across the width of the chassis, facing the OT, as in the photo of the Harvard chassis. That's really like Crouch's "best" diagram, but with the PT rotated 90 degrees on the Y axis, so that the field emanating around the upper part of the windings couples with the adjacent field of the OT he shows there. There may be a practical reason for this practice, such as preserving real estate for separation of the transformers and the preamp. However, it seems to me that, in theory, the least leaky side of the transformer is the short side, the one where you can see the joints between the E and I leaves of the core on my photo of the Hammond laydown PT. The central rib of the core (middle stroke of the E) thus runs perpendicular to the axis between the PT and OT, and the maximum leakage (Blencowe) and direction of the field loop around the windings (Crouch) both are oriented across the width of the chassis, paralleling the axis between the PT and OT. So, again in theory, it seems to me that it would be better to have the laydown PT's short side face the OT.

    I haven’t tested this out yet using the “headphone procedure,” and of course I will (though I haven't found much difference when doing this in the past), but I just would like to have a reality check from those more experienced than I about this hypothesis. Comments?
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I agree in theory it would be better to have the short end of the lay-down power transformer toward the output transformer and I believe you are right in suggesting it's real estate or perhaps just tradition (that's the way Leo did it so that's how I'll do it) that the long side of the PT is oriented to face the OT.
  3. mojo2001

    mojo2001 Tele-Holic

    Feb 5, 2008
    The best thing to do is determine orientation empirically with a scope or high Z headphones.

    One difficult issue to theorize is that air gaps in chokes and SE transformers distort the radiation pattern, which due to reciprocity, suggests that they will have distorted pickup patterns as well.

    I think that existing designs might represent successful prototype layouts more than any specific theoretical cookbooks.

    Space/distance helps a lot, if you have it to spare.
  4. bparnell57

    bparnell57 Poster Extraordinaire

    Feb 10, 2014
    Philadelphia, PA
    I always liked amps that put the choke and output transformer inside the chassis. This makes for a very quiet amp in that regard. Most of the time they're under 20 watt amps but it's still a good way to design it. I even have some compact radio chassis that are packed with the output transformers inside.
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