Headphone trick and OT orientation

[mobisimo]

Hi all,

I'm using a CE chassis for my new build but the bracket on OT I'm using didn't fit the mounting holes in the chassis. Since I had to drill new holes anyway I figured I would use the headphone trick to inform the new OT placement.
I found a very clear difference between the traditional placement, with the laminations running from the back of the chassis toward the front, and mounting the OT with the laminations running side-to-side. Back-to-front orientation induced a considerable hum in the OT secondary whereas side-to-side was dead quiet.
Has anyone come across this in a build? If so, what would be the reason for mounting the OT in the traditional way, given that the headphone experiment shows it is much noisier--at least in terms of 60-cycle hum?

Best,
Aaron

[martin manning]

This may not always be the case depending on the transformers. Did you have the choke in place? It will shield the OT to some extent.

[mobisimo]

Hi Martin, I didn't think about that. I did not have the choke mounted when I did the experiment. The only things attached to the chassis were the PT, and OT, and the IEC connector.
I wonder if it could also be that the PT's proximity to the output tubes makes the other orientation viable...

[xtian]

I found that placing a choke between PT and OT can influence hum in the OT. By rotating the choke, I could increase and decrease hum in the OT (the choke was not connected! just the coil and core were deflecting the field generated by the PT!).

[mobisimo]

Thanks for the replies, gents. I've got a little more work to do on the chassis, but before I start to populate it I will do another trial with the choke in place and post results.
Thanks again!

[mobisimo]

I just tried the experiment again with the choke in place and the results were virtually identical to those from my first trial. I can post pictures of the final placement if anyone would find it helpful. I'll also run a build thread if folks would like to see the progress. It's a 100W high-plate skyliner HRM with EL34s and a standard PI.

Best,
Aaron

[Ang3lus]

I would like to see the pics of the final placement, I too am using an OT that is not standard and had to drill new holes and currently struggling with traditional vs logical.

[mobisimo]

Hi Ang3lus,

I attached a couple pictures I just took of the chassis: 1) marked up with the new placement, and 2) with the OT mounted as it will be on the finished amp.

Even though the results from both trials were clear, I'm still incredulous... At any rate once I get the thing built I'll post the results.

Good luck with your build! Are you getting 60 Hz hum on a finished amp?

[martin manning]

I'm guessing the bobbin axis of that OT is horizontal, 90 degrees from the vertical axis of a Fender Twin type OT, and you are having to rotate it 90 degrees to compensate for that difference.

[Ang3lus]

Haven't hooked up anything yet, i'm still waiting on the PT, but the orientation (which is the same as yours) works better for my OT, so i was just wondering.