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 30th October 2009, 10:33 PM   #1
martyh is offline martyh  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wisconsin
Default Magnetic shielding with aluminum

I have been digging around in the archives looking for some information on magnetic shielding with non ferrous materials. I saw varying opinions about its effectiveness. Looking for info on the internets in general I came across this paper

http://www.chomerics.com/products/do...ory_of_emi.pdf

Reading through it seems that aluminum can be an effective magnetic shield if it is thick enough. Playing around with the nomographs at the end it looks to me like .25 of aluminum is about as absorptive as .009 of Mu Metal (about 6dB of absorption losses at 60 Hz) the reflection losses look to be about 20 dB but I gather these are to be ignored if you are only worried about the magnetic portion of the wave.

Does anyone have practical experience or a better grasp of the math that confirms what I think I see?

Thanks,
Marty
  Reply With Quote
Old 30th October 2009, 11:46 PM   #2
diyAudio Member
 
Speedskater's Avatar
 
Join Date: Sep 2002
Location: Lakewood, Ohio
The John Curl "Blowtorch" tread spent some time on the machined aluminum chassis about a year ago.
__________________
Kevin
  Reply With Quote
Old 31st October 2009, 12:05 AM   #3
diyAudio Member
 
tomtt's Avatar
 
Join Date: Aug 2002
Location: kansas city mo, and on occasion, around the world ...
Blog Entries: 15
Quote:
Originally Posted by martyh View Post

Does anyone have practical experience or a better grasp of the math that confirms what I think I see?
not much (none)

looks like your on the right track ....







blowtorch photo -


Click the image to open in full size.


John Curl's Blowtorch preamplifier




transformers mounted on one piece chassis -


Click the image to open in full size.

http://page11.auctions.yahoo.co.jp/jp/auction/n82058654

Last edited by tomtt; 31st October 2009 at 12:08 AM.
  Reply With Quote
Old 31st October 2009, 03:13 PM   #4
Sheldon is offline Sheldon  United States
diyAudio Member
 
Join Date: Dec 2003
Location: San Diego
Quote:
Originally Posted by martyh View Post
Does anyone have practical experience or a better grasp of the math that confirms what I think I see?
No grasp of the math, but I can confirm that thick aluminum does provide some shielding. I've used a piece between the power transformer and output transformer in tube projects. Noticeable reduction in mains hum. In my case, thickness was about 8mm. As good as 5mm mild steel.

Sheldon
  Reply With Quote
Old 31st October 2009, 03:55 PM   #5
pointy is offline pointy  United Kingdom
diyAudio Member
 
pointy's Avatar
 
Join Date: Mar 2009
you can turn transformers through 90 degrees to each other which helps stop the fields of the transformers effecting each other.
otherwise the metal work of the chassis is used mainly to block out RF and airborne hash from the small component connecting wires and pcb copper tracks.
there are few other components that are effected by low level magnetic fields such as (j type) wire wound resistors and some say valves and caps.
with the valves all you normally need to do is space them well and caps can be put below and spaced 1"+ from each other to stop them from over heating each other.
  Reply With Quote
Old 31st October 2009, 05:43 PM   #6
diyAudio Member
 
Join Date: Apr 2006
Location: Minnesota
Aluminum will not provide magnetic shielding, since it's permeability is one; the same as air.
  Reply With Quote
Old 31st October 2009, 06:11 PM   #7
martyh is offline martyh  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wisconsin
Quote:
Originally Posted by sawreyrw View Post
Aluminum will not provide magnetic shielding, since it's permeability is one; the same as air.
Well if I am following the paper I linked to it seems that it will just not as well as ferrous metals for a given thickness.

I have read others who say it will not work at all but I always had this nagging question. If highly conductive metals have no effect on a magnetic circuit why are they used as shorting rings in loudspeakers?
  Reply With Quote
Old 31st October 2009, 11:01 PM   #8
Sheldon is offline Sheldon  United States
diyAudio Member
 
Join Date: Dec 2003
Location: San Diego
Quote:
Originally Posted by sawreyrw View Post
Aluminum will not provide magnetic shielding, since it's permeability is one; the same as air.
My personal experience is that it does work. The difference is clear with the plate between the transformers. It can be heard as the plate is moved into position, so it's not imaginary.

The material used was not pure aluminum. I believe it was T6.

Sheldon

Last edited by Sheldon; 31st October 2009 at 11:04 PM.
  Reply With Quote
Old 31st October 2009, 11:56 PM   #9
diyAudio Moderator Emeritus
 
Bas Horneman's Avatar
 
Join Date: Apr 2002
Location: The Netherlands
Blog Entries: 18
Your personal experience would be correct. I did a google and found out that there is such a thing as lossy magnetic shielding.

Magnetic Shielding Solutions

"Lossy magnetic shielding depends on the eddy-current losses that occur within highly conductive materials (i.e., copper, aluminum, iron, steel, silicon-iron, etc.). When a conductive material is subjected to a time-varying (60 hertz) magnetic field, currents are induced within the material that flow in closed circular paths - perpendicular to the inducing field. According to Lenz's Law, these eddy-currents oppose the changes in the inducing field, so the magnetic fields produced by the circulating eddy- currents attempt to cancel the larger external inducing magnetic fields near the conductive surface, thereby generating a shielding effect."

I read somewhere else on the bottlehead forum that this was only about 5-6dB maximum (according to John Swenson)
http://www.bottlehead.com/smf/index....90.html#msg290


Nothing close to mu-metals but it still works a little.
  Reply With Quote
Old 1st November 2009, 12:54 AM   #10
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
If you are at the lower limits of hum, it might provide the extra little bit you need to knock it out.
  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
Magnetic Shielding Foils VHF man Solid State 2 27th November 2008 10:46 AM
Magnetic shielding jnb Power Supplies 15 9th June 2007 11:12 AM
DIY Magnetic Shielding Flippit Multi-Way 32 6th September 2004 09:11 PM
magnetic shielding slackerbob Multi-Way 21 1st July 2003 07:15 AM


New To Site? Need Help?

All times are GMT. The time now is 11:07 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