Magnetic shields for transformers - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st October 2007, 08:13 AM   #1
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Question Magnetic shields for transformers

After a long and painful search for hum, I finally found that I am getting magnetic coupling between a power transformer and some output transformers. It is very low, so low that you can't really hear it during the day, but a night it is there. I found that moving the power transformer back about 3 inches is enough to eliminate the hum -- problem is that 3 inches is off the chassis. Everything is already at right angles, etc.

So, either I need to find some clever method of shielding, or I need to build a new chassis which I definitely do not want to do. So, is there some method to add a magnetic shield to the power and/or output transformers? Maybe some mu metal bell ends, or a way to pot them? Thoughts?
  Reply With Quote
Old 21st October 2007, 02:19 PM   #2
Sherman is offline Sherman  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Chicago area
You might try picking up a metal basket at The Container Store. They have various sizes. Choose one made of steel of course. The ones I've seen there are painted, usually silver or black.

You can mount them to the chassis using plastic wire clips, the kind that form a loop and have a hole for a screw to go through. I used them in one project. I decided to use them for cosmetic reasons, open frame OPTs look ugly to me and these would mostly hide them while also preventing fingers from touching the B+ connection on the trafos.

I initially mounted the OPTs much closer to the PTX and had hum in the left channel. Dropping the basket over both OPTs "cured" the hum. I was surprised since they are baskets with 1/8" openings between the steel weave. I eventually moved the OPTs about 1.5 inch farther from the PTX and the hum is gone even with the basket off. The basket is still used for aesthetics though.
__________________
--Sherman
  Reply With Quote
Old 21st October 2007, 07:17 PM   #3
BudP is offline BudP  United States
diyAudio Member
 
BudP's Avatar
 
Join Date: Feb 2007
Location: upper left crust, united snakes
dsavitsk,

With a picture of your amp and a mention of the chassis material I can begin to help.

Just for starters, did you orient the OPT 90 degrees from the power, in three axis? If so, then airborne EMF coupling can almost be ruled out. If your chassis is non ferrous and the transformers are decoupled in three axis, then it is very likely due to antenna effects from the wiring. Perhaps the power rectification circuit is coupling with either the OPT leads or coil orientation.

Bud
  Reply With Quote
Old 21st October 2007, 08:03 PM   #4
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
The amp can be seen at http://www.ecp.cc/semha.html

As you can see, transformers are 90 degrees, but slightly off axis -- to be fair, I was warned that this could be problematic, but there really isn't anywhere else to put them. After redoing nearly every part of the amp, I removed the power transformer, placed it far away and ran some long wires to it. This solved the problem completely. I also oriented it as if flush mounted and put it right behind the chassis and again silent. However, maintaining the flush mounting orientation but setting it right where it is, there is still some hum -- maybe a little less, but it is still there. Maybe this transformer radiates more than others (I have a Hammond with the same specs to try to see if this is correct), but I think I either need a shield, or a bigger chassis.
  Reply With Quote
Old 21st October 2007, 08:19 PM   #5
tvi is offline tvi  Australia
diyAudio Member
 
tvi's Avatar
 
Join Date: Apr 2001
Location: Moonee Ponds, Vic, Australia
I think what Bud means by 90 in 3 axis would put you power transformer like the one in the attached pic.

Regards
James
Attached Images
File Type: jpg bl-80m monopower triode amplifier.jpg (20.6 KB, 363 views)
__________________
The small part of ignorance that we arrange and classify we give the name of knowledge.
Ambrose Bierce
  Reply With Quote
Old 21st October 2007, 08:30 PM   #6
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
That is what I mean by flush mount orientation, which didn't help. The measured hum, if you can trust my DMM, is ~300uV. Low, but not quite low enough here.
  Reply With Quote
Old 21st October 2007, 08:31 PM   #7
BudP is offline BudP  United States
diyAudio Member
 
BudP's Avatar
 
Join Date: Feb 2007
Location: upper left crust, united snakes
dsavitsk,

What you have is only a single axis of rotation. If you laid the transformer on it's side and pointed the short side at the OPT's that would be very close, with two axis of rotation. Then rotation of the OPT's from vertical to horizontal mounting, still with mounting end bells, would be the last test for compatibility.

You can shield the core of the power transformer. You need at least grain oriented M6 applied over a layer of thin, two sticky side tape, with the overlap on top, if the chassis plate is steel and down if it is aluminum. Ideally the material width you use should extend beyond the core stack edges by about .050" on either side.

You can also wrap a copper shield around the coil and up over the laminations and core field suppression band. This is a shunted shorted turn and likely should be grounded at the power ground point.

If you look for the hum with a scope attached, look at power lead outs and surface of endbells and core with a sniffer coil. Look for spikes in the sine wave. A leading edge spike comes from the coil and a trailing edge spike from the core. You may also be able to find these at the OPT.

I do have some M6 shield stock already cut to size for specific transformers. If we have similar sizes you may be able to use it. If not, Lamination Specialties in Chicago and Yates Electro-Magnetics on the East coast, will usually cut to order. Also investigate the conetic alloys of Mu Metal that are available. You can purchase a "samples" set of sheet stock, but I no longer remember from whom. The cut to size strips of M6 will likely be cheaper.

Another avenue is a 0.060 thick CRS box, that has been heat treated to marstenite/osteonite temperature, to render it dead soft after forming the box. This will provide 100% EMF containment, even with constant voltage transformers.

Bud
  Reply With Quote
Old 21st October 2007, 08:49 PM   #8
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Quote:
Originally posted by BudP
What you have is only a single axis of rotation. If you laid the transformer on it's side and pointed the short side at the OPT's that would be very close, with two axis of rotation. Then rotation of the OPT's from vertical to horizontal mounting, still with mounting end bells, would be the last test for compatibility.
Okay, I see. I'll give this a try and see if it helps.

Quote:
Originally posted by BudP
You can shield the core of the power transformer. You need at least grain oriented M6 applied over a layer of thin, two sticky side tape, with the overlap on top, if the chassis plate is steel and down if it is aluminum. Ideally the material width you use should extend beyond the core stack edges by about .050" on either side.

You can also wrap a copper shield around the coil and up over the laminations and core field suppression band. This is a shunted shorted turn and likely should be grounded at the power ground point.
The transformer is custom made, so I will call the maker on Monday and see if he can do some, or all of these things to this transformer. It is certainly the easiest option from my perspective, as realignment will require a new top plate, and otherwise a new chassis.

Quote:
Originally posted by BudP
Another avenue is a 0.060 thick CRS box, that has been heat treated to marstenite/osteonite temperature, to render it dead soft after forming the box. This will provide 100% EMF containment, even with constant voltage transformers.
Any idea where to get such a box?
  Reply With Quote
Old 21st October 2007, 10:13 PM   #9
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Quote:
Originally posted by dsavitsk
Okay, I see. I'll give this a try and see if it helps.
Nope. The hum might be slightly lower, but not appreciably.
  Reply With Quote
Old 21st October 2007, 10:56 PM   #10
tvi is offline tvi  Australia
diyAudio Member
 
tvi's Avatar
 
Join Date: Apr 2001
Location: Moonee Ponds, Vic, Australia
Less EMF have a range of "Magnetic Field Shielding" materials often sold by the foot, might be of some help?

Regards
James
__________________
The small part of ignorance that we arrange and classify we give the name of knowledge.
Ambrose Bierce
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
transformers with electrostatic shields rick57 Parts 16 27th March 2013 09:37 PM
MTX 202 power on shields jol50 Car Audio 4 1st November 2008 08:21 AM
Tube Shields korneluk Tubes / Valves 1 17th December 2007 04:31 PM
magnetic Pbassred Solid State 6 26th May 2004 12:14 PM
Electrostatic Shields Gumboots Tubes / Valves 5 21st August 2002 02:48 AM


New To Site? Need Help?

All times are GMT. The time now is 03:48 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2