Design Tips to reduce flux leakage/Stray Magnetic Field

[guitarmike2107]

Short Question: What are the design/build tips for reducing stray magnetic flux from a power transformer other than fitting a flux band and running the core at a lower B

Long Version: I have two Tweed Deluxe power transformers (guitar amp), same lamination size and core size, one with M400 core running at 10000 gauss and one with M6 core running at 14000 gauss ( thicker wire and less turns for running 6L6's)

The induced hum in the output transformer is higher with the M6 core and is almost non existent with the M400 core. The tweed deluxe OT is mounted close to power transformer, It could be moved but don't really want to do that every time.

So I was wondering if I have missed something when winding the power transformers that could be increasing the stray magnetic field?

Also does anyone know of a good place to buy copper strip for flux bands, I can find copper tape easily but getting strip is more difficult.

Thanks

 

[EthanSpaulding]

Can you add more laminations?

Can you experiment with OPT placement angle? There is a technique where you connect the output of the OPT to a set of headphones and listening for least hum.

 

[guitarmike2107]

Thanks Ethan, I am aware of those things to reduce this issue, I was more just trying to make sure I had ticked all the boxes on the design side of the PT to reduce the induced hum.

 

[EthanSpaulding]

Make your stack bigger, make your stack tighter, shield it in a permeable material to keep leakage in (and make it more efficient). I'd add more laminations if possible. I've noticed that over time I can get more laminations back into a rewind and that makes the core tighter as well as lowering the core's flux density, because the flux is distributed through more laminations. Supposedly winding the heaters directly over the primary winding can lower noise due to the heaters usually being referenced to ground with a low impedance path.

I pot in wax and while its hot and slippery; I pull the transformer out, square it up, and fit 3-5% more laminations in.

My guess is you are more experienced at this than I. I'm just spitballing here!

 

[guitarmike2107]

Are you using paper bobbins, no way I could get 3 - 5% mor lams in a plastic bobbin.

That is interesting about the heaters on the outside. Unfortunately on this one the heaters are on the outside.

Thanks

 

[EthanSpaulding]

I'm using plastic bobbins. I was surprised at how much more dense I could get the stack once the friction was gone.

The idea of using the heater winding as a shield requires that the heaters are wound over the Primary. Heater windings are often a single layer wind, are low impedance, and are (usually) referenced to Ground. Electro-static coupling of noise is coupled to Ground. There may be a good argument to put the Heater winding on the outside, its just that I remember reading about winding it directly over the Primary for noise reduction.