New build, some problems.

[soma_hero]

I'm guessing when you attach the OT to a speaker load, the magnetic flux creates some sort of hum produced through the speaker.

This would be a useful tool during part placement before you built the amp to determine if the transformer placement will hum or not. You could do it when you fire the amp up and move the OT around but do you really want to finish the amp and then wiggle around your transformers? Probably better to do this before you build.

Good tip!

[paulster]

What I don't understand is - if the OT is not hooked up - and the hum is generating from the PT in standby mode - why do you need to attach a speaker or headphones to the OT? I mean, if the placement of the OT is causing the hum in the PT in standby mode, why couldn't you just move the raw OT around on the chassis and listen to the PT?

What you do is hook up your PT and power it on.

On the OT, wire a pair of headphones to the highest impedance secondary tap (to get the highest voltage output) and leave the primary disconnected.

Then move the OT around the chassis and see how much 50/60Hz hum you hear through the headphones. That's inductive coupling of the magnetic field originating from the PT.

Once you've found a spot that's quiet with headphones it'll be really quiet when you've got a speaker attached instead as they're way less sensitive.

I also do the same thing with chokes, to find a quiet spot for them, using the whole winding connected to the headphones. The OT is the big ticket item though.

Paul

[bluegate]

Here are the photos of my new build

[img:800:526]http://www.bluegate.nl/Images/SW2/intern1.jpg[/img]

[img:800:558]http://www.bluegate.nl/Images/SW2/intern2.jpg[/img]

[img:800:487]http://www.bluegate.nl/Images/SW2/chassis.jpg[/img]

[img:800:457]http://www.bluegate.nl/Images/SW2/extern2.jpg[/img]

[img:800:431]http://www.bluegate.nl/Images/SW2/extern1.jpg[/img]

[rooster]

paulster - OK, that is interesting. And when you say 'big ticket item' you mean that the PT/filament supply could still be a sole source of the hum, right?

I guess you could try this method with an amp/build that is up and running, too? Put an amp in standby mode and move an external OT near it? I'm going to try this because this seems a very cool old school idea.

BTW, where did you get the idea for this?

Oh, BTW, nice pics! Now I see what you mean about the OT. Yeah, its hard to trouble shoot a one-off build. Paulster's may be your only hope then.

[paulster]

paulster - OK, that is interesting. And when you say 'big ticket item' you mean that the PT/filament supply could still be a sole source of the hum, right?

I guess you could try this method with an amp/build that is up and running, too? Put an amp in standby mode and move an external OT near it? I'm going to try this because this seems a very cool old school idea.

BTW, where did you get the idea for this?

Big ticket, as in the effect of the magnetic field from the PT being induced into the OT is more significant than the effect of the magnetic field being induced into the choke, if you have one. Any noise getting induced into the choke would be filtered by the caps, whereas noise induced into the OT hits the speakers directly.

Try it as you suggested with a separate OT connected to a pair of headphones so you get the idea of just how much change you can get by a small movement. Ideally you'd want to do it with the real iron though so you can see what the actual effect on your particular OT would be.

The noise you get is a low-level, static hum that has no bearing on whether the amp is in standby or play mode or the volume setting. It's different and completely independent from the hum you can get via a poor grounding layout, inadequate filtering or suchlike, but can be a real annoyance if you've built an amp that's otherwise electrically quiet!

I think it was a technique I picked up at the 18 Watt Forum a number of years ago. Can't remember for certain or whose technique it was, but the first time I tried it I immediately made lead consisting of an 1/8" stereo socket to a pair of alligator clips to be able to quickly test a new PT/OT combination on a chassis whenever I get a set in.

Paul