Major Hum Problem - 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 12th December 2008, 07:54 PM   #1
diyAudio Member
 
Join Date: Dec 2008
Default Major Hum Problem

Hi All,

I have just finished building myself an SE-5W tube amp and have a Major hum issue. I have wired it so the chassis earth is isolated from the ground. Yet when I have my fingers on the strings of my guitar and put my toe on the chassis of the amp the hum is greatly reduced. Where am i going wrong? I always thought it was best to isolate the ground from eath on a tube amp.


Any other ideas where the problem might be?
  Reply With Quote
Old 12th December 2008, 08:23 PM   #2
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Default Re: Major Hum Problem

Quote:
Originally posted by steve080
Where am i going wrong? I always thought it was best to isolate the ground from eath on a tube amp.
That's totally backwards. The chassis of a tube amplifier should be directly connected to earth ground by a third wire. This is for safety reasons. If something inside the amp, like a transformer, shorted to the core or case, ths chassis would become hot. And possibily deadly. Shockingly uncomfortable at best.

Edit: OK, if the chassis is earth grounded but the B- is kept isolated, try grounding it to chassis. That will probably stop the hum issue.

Victor
__________________
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell
  Reply With Quote
Old 12th December 2008, 10:21 PM   #3
Hi_Q is offline Hi_Q  England
diyAudio Member
 
Hi_Q's Avatar
 
Join Date: Oct 2007
Is it not the case that the B- and chassis come together at one place, normally at the audio input connector? The chassis should also be earthed at this same point. I am not sure if this is the standard convention but I have seen it on many amplifier circuits.
The older amplifiers that used non split bobbin mains transformers used to earth the chassis at the point where the inter winding copper screen of the transformer was also earthed.
I suppose the best guys to answer this question are the those that have had most experience at DIY design/construction and highlights the priceless value of Forums like this one.
Les
  Reply With Quote
Old 13th December 2008, 04:42 PM   #4
diyAudio Member
 
Join Date: Dec 2008
Yes, the chassis is earthed to the mains input, just the B- that is isolated. I understand that connecting a 100 ohm resistor between the chassis earth and B- can help reduced ground loop effects introduced by connecting B- to the chassis.

I will try this and see how it helps.

Thanks for yuor replies
  Reply With Quote
Old 15th December 2008, 01:37 AM   #5
diyAudio Member
 
Join Date: Dec 2008
Location: Victoria, BC
Default Re: Major Hum Problem

Quote:
Originally posted by steve080
Hi All,
Yet when I have my fingers on the strings of my guitar and put my toe on the chassis of the amp the hum is greatly reduced.
When the guitar cable is unplugged from the amp (ie input shorted- assuming you are using a shorting input jack) do you still have the hum problem?

Guitar pickups (especially single coil types) are great for picking up all kinds of 'hum' from things like dimmer switches and halogen light SMPS units.

Incidentally I do agree that you need to look at the safety issues other folks have raised.

Cheers
John
  Reply With Quote
Old 15th December 2008, 01:42 AM   #6
diyAudio Member
 
Join Date: Dec 2008
Location: Victoria, BC
If you haven't already found your way to Aiken Amps tech pages, they are worth a look, IMO.

Grounding

Cheers
John
  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
Gaps in my horn - a major problem? wixy Full Range 6 7th September 2007 12:18 AM
Having major problem with speakers Bull Multi-Way 23 18th February 2007 08:07 PM
GainClone troubleshooting, major problem phorensic Chip Amps 6 13th June 2006 02:00 PM
Major Vibration Phrozen06 Car Audio 4 18th August 2005 02:36 AM


New To Site? Need Help?

All times are GMT. The time now is 05:15 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