Transformer coupling

[EFK]

I found this in an old post by jrfrond:

"Transformer orientation becomes critical because they can couple inductively through a steel chassis. You can get audible hum at the output even with no tubes in the circuit, just powered up. This is the PT coupling to the OT through the chassis."

Gibson GA42RVT has a very subtle low level hum that seems to be originating with the PT but can be heard through the speakers. Tubes in or out, standby or not, doesn't matter. This is a steel chassis, PT and OT directly bolted to chassis. They are on opposite sides of the chassis but they are oriented exactly the same. W/ the chassis removed, if I flex it a little so one side 'twists' slightly (and it is very slight, but a little flex is there, and thius the transformers fall slightly out of exact alignment) the hum goes away. Does this seem like a case of the aforementioned coupling? Would placing some fiber or non-conductive washers b/t the transformers and the chassis diminish the hum? Or maybe, washers under one side of the PT or OT to slightly change the angle? Just curious, it's nothing that's really bothering me but since I've been messing with the amp I'm getting nit-picky.

[dai h.]

I skimmed over an interesting japanese page recently where the author was experimenting, building his own mu-metal shield for a small transformer. he tried that but in the end found distance plus relative orientation to the other transformer(s?) had more effect with regard to hum reduction than his home made mu-metal shield. Don't remember the particulars exactly but there should be some threads on this in the archives.

[ETR]

Chances are youre messing with a ground connection when youre flexing the chassis rather than a slight misalignment, but I suppose it's possible with a very flexible chassis and close proximity.

[Boogdish]

You said that the transformers were oriented the same way. I've been taught the rule of thumb before that the laminates of your transformers should be perpendicular. If one of the transformers is a stand up type and the leads are long enough, you might try unmounting it and rotating it 90 degrees.

[Sir Cuitous]

Perpendicular orientation will reduce the interaction between the transformers, but they are still mounted to the chassis. A steel chassis will pick up flux leakage from a transformer and pass it to your circuit - in particular the ground circuit which is really a signal path though we don't always think of it that way.

Fortunately it only taks a small gap to prevent this. I use a piece of 1/8" phenolic between the transformers and the chassis to decouple it magnetically from the chassis. I got the idea from "Building Valve Amplifiers" by Morgan Jones. A great reference for actual construction principles rather than theory and schematics.

[ETR]

That's interesting. Do you need special washers or screws for isolation as well?

[EFK]

I don't think the transformers can be re-oriented. No big square hole, just small holes on one side of each trans. and the wires are bundled and pretty tight. I think the isolation idea is interesting. I imagine you'd have to have isolating washers b/t the transformers and chassis, and also between the head of the attaching bolts and the transformer flanges. Otherwise the bolts themselves would act as conduits to the chassis, correct?

[stokes]

I don't think the transformers can be re-oriented. No big square hole, just small holes on one side of each trans. and the wires are bundled and pretty tight. I think the isolation idea is interesting. I imagine you'd have to have isolating washers b/t the transformers and chassis, and also between the head of the attaching bolts and the transformer flanges. Otherwise the bolts themselves would act as conduits to the chassis, correct?

You would have to drill new holes,but re-orienting the laminates is the fix for this problem.I dont like the idea of insulating the transformer from the chassis,the transformers casing acts like a shield and needs to be grounded.Some transformers actually have a copper band around them that is grounded for just that reason.

[Sir Cuitous]

I cut a small piece of phenolic to the mounting shape of the transformer. I have used this technique for vertical and horizontal mounting trannys. It does not isolate the electrical ground since I use the same nuts and bolts to fasten them to the chassis. It's just a 1/8" gasket-like piece of material that prevents the magnetic flux from leaking into the steel chassis.

Of course many amps do not feature this technique. Transformer orientation is way more important to an amp design than this technique. Transformers that are close and not oriented well will couple inductively with each other even if they were mounted on wood or aluminum (which does not couple magnetically). But the original post was about transformers coupling through a steel chassis, so my response is about providing a small gap between the transformer and chassis to help prevent that.

[cminor9]

Something I have been wondering about for my next build:

In general, is it better to have transformers (OT and PT) placed close together and with the laminates turned 90 degrees from one another, or would it be better to move them to separate ends of the chassis? Consider that moving them to separate ends of the chassis would result in the OT being right above the input section. Putting them next to one another means that you keep the iron away from the sensitive input section.

I have seen production amps built both ways. Forgive me if the answer is "it depends", but the root of it is I wonder if proximity or orientation is the more important factor in avoiding transformer coupling or other undesirable noise.

Of course, I could experiment, but I don't really feel that comfortable grasping the transformer on a live amp and moving it around. I'd be interested in thoughts on this.

[Sir Cuitous]

Both orientation and distance will affect transformer coupling. Transformers leak so much more flux along the axis of the coils making orientation much more important than distance. Flux leakage is practically zero at 90 degrees from the axis. Coupling decreases with the cube of the distance. Short distances will not make a big difference, but increasing the distance will quickly reduce coupling.

I have placed the OT at the "input" end of the chassis without any problems. The OT was "outside" of the chassis and oriented away from the input wiring. I suppose it was somewhat risky, but it did not affect the tone.

I believe it is possible to connect test equipment and actually see the improvement while orienting the transformer on a live amp.

[clyde1]

Actually it doesn't take much in the way of test equipment at all. Before you mount your transformers, hook up the PT to your AC supply, being careful to isolate (tape off) all secondary wires. Hook up an ordinary set of headphones to the secondary (fairly sure, might be primary) of your OPT. Now place the transformers on the chassis, manipulating them to minimize hum in your 'phones. It's been awhile since I've done this, hence the confusion on the attachment to the OPT. I can't take credit for this one, I got it from Bruce.