Adjusting BIAS - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 6th January 2007, 12:53 PM   #1
diyAudio Member
Join Date: Dec 2005
Location: Helsinki
Default Adjustig BIAS

I have VAC 80/80 KT88 amp. After adjusting bias so that plate current is exactly same, I can hear some hum from speakers. If I adjust bias by ear I can get that hum to disappear. But plate currents differ quite much then. 63 mA and 65 mA for other pair and 52 mA and 65 mA for another pair.

Which one is better? To adjust bias by ear to get minimum hum or to adjust by meter to get identical plate currents?

My amp is working in triode mode and is biased towards class A to get best sound quality.

Sincerely Yours, Sixtus Oest
  Reply With Quote
Old 10th January 2007, 11:07 AM   #2
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
Merlinb's Avatar
Join Date: May 2006
Location: Lancashire
When you discover something like that it indicates that having the valve off-balance is introducing a small hum signal that is cancelling the hum out. If you can't find the true source of the hum and beat it directly, I say go with your ears
  Reply With Quote
Old 10th January 2007, 12:18 PM   #3
diyAudio Moderator Emeritus
ray_moth's Avatar
Join Date: Jan 2004
Location: Jakarta
I am not familiar with the amp. One possibility is that you are measuring voltage across cathode resistors that are not accurately matched. Another is that the primary winding of your OPT is not tapped exactly in the center. Whatever the problem, it's usually best to balance by ear,
  Reply With Quote


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
Adjusting DC bias for Zen amplifier hlvan Pass Labs 8 9th January 2015 03:02 PM
Adjusting the BIAS for Threshold SA3 sonic22 Pass Labs 9 16th May 2011 12:38 PM
400A bias adjusting Mywing Pass Labs 7 25th May 2008 07:53 PM
Marantz pm 94 + Bias adjusting CederPM94 Solid State 0 10th May 2004 09:22 AM

New To Site? Need Help?

All times are GMT. The time now is 12:11 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2