Output Transformer Placement

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by darkbluemurder, Jan 20, 2006.

  1. darkbluemurder

    darkbluemurder Member

    May 6, 2005
    Recently I was wondering which was the optimum spot to place the OT. I read that it should be far away from the power transformer. Yet in most amps they are quite close together with no apparent problem (e.g. Fenders). To put the OT as far as possible away from the PT to me is intuitively bad for 3 reasons (yet Soldano and other makers do not seem to have a problem with this arrangement):
    (a) it will be close to the input jack and may radiate hum into low level signal paths;
    (b) it will be further away from the power tubes, thus we end up with long wires from the plates to the primary winding and
    (c) it will also be further away from the speaker jack therefore necessitating long wires from the secondary wiring to the speaker jack.

    Of course I would not bother to relocate the OT unless I had problems but what is your view?

    Any input would be greatly appreciated.
    Thanks a lot
  2. donnyjaguar

    donnyjaguar Member

    Nov 4, 2005
    Its not so much the location of the 2 transformers, but the placement. That is to say, the windings must not be in the same plane. If they are, it'll hum even with no tubes installed. :)

    I wouldn't worry about long leads on the output transformer primary myself as there's no real current here. The secondary, however, is a different story and I like to keep these leads short.

  3. Matt H

    Matt H Guest

    there are a couple of things that should be done for "proper" OT placement.

    the biggest three are a) keeping it at right angles (or perpendicular along at least one axis), b) keep the wires, specifically of the secondary, twisted together (as should be the case for anything passing AC in the amp), c) the "headphone trick" is used.

    a search for "headphone trick OT" on ax84 should bring up details. basically, you hook the power transformer up to AC MAINS ALONE (don't wire the secondary, or maybe wire up the tube heaters or something). Then, with the output transformer not connected to anything, hook up a pair of headphones up to the secondary. now- move the OT around the chassis, playign with orientation and position until the hum is the most quiet.
  4. bob-i

    bob-i Member

    Oct 16, 2005
    Central NJ
    I've built amps with the xformers almost touching with no trouble as long as the windings are on a different plane. I've had interferance with transformers 10" apart when they're on the same plane.

    I agree that the OT secondaries should not be close to any low level signals. Short leads are not really an option, because effectivly the speaker wire is the secondary connection. You simply need to keep them away from the early stages.
  5. John Phillips

    John Phillips Member

    Mar 17, 2002
    I would guess that it doesn't matter where the OT is. Hum pickup is far more dependent on orientation and wiring layout.

    It's interesting to note that some of the highest-gain amps - eg Soldano and Mesa, and the newest Marshalls - have the OT at the end nearest the early gain stages, and don't have problems. Mesa do put the speaker jacks directly behind the transformer, Marshall and Soldano (I think) don't.

    There is one major advantage to putting the transformers at opposite ends of the chassis, which is that the amp balances better on the handle. No, I'm not being funny - given that it appears to make no difference electrically, you may as well go with other considerations.
  6. Shea

    Shea Member

    Sep 7, 2004
    It's no joke. That imbalance makes amps awkward to carry, and feel heavier than they really are. I have a hunch that might be why Hiwatt transformers are at opposite ends of the chassis, and why the very first JTMs had those offset heads.


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