Auto Biasing - 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 30th September 2004, 07:45 AM   #1
diyAudio Member
Wyatt Turk's Avatar
Join Date: Sep 2004
Location: Maianbar, NSW, Australia
Question Auto Biasing

Could one of you nice people be able to supply to me a schematic diagram for an auto biasing system. I was thinking of about the 100w el34 things.

If I could only hear a one hand clap!
  Reply With Quote
Old 30th September 2004, 11:31 AM   #2
SY is offline SY  United States
diyAudio Moderator
SY's Avatar
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 2
"Aotobiasing" has several meanings. The oldest and simplest is biasing via a (usually bypassed) cathode resistor.

One can also adapt so-called fixed bias schemes to compensate automatically for output tube idle current drift. For a class-A circuit, this can be done very simply with a diff amp that compares the voltage across a small cathode resistor with a reference voltage. For class AB amps, it can get quite a bit more complex- two examples that come to mind are the digital scheme used by Berning in the BA-150 from the late '70s and a circuit shown in Jones's "Valve Amplifiers" which uses several opamps configured as diff amps and clippers. I adapted a system originally published by Chater in "Audio Amateur" for biasing a N-channel mosfet stage. It worked, but as with the previous circuits, it was complex and really offered no significant advantage over properly-executed conventional bias arrangements.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
  Reply With Quote
Old 30th September 2004, 12:42 PM   #3
diyAudio Member
Geek's Avatar
Join Date: Sep 2004
I experimented once with an op-amp-driving-a-PNP autobias/auto balancer once. SY's right, no significant advantage. Just adds to the complexity and just adds creedence to the saying, "The more complex the plumbing, the easier it is to stop up the drain"

Maybe in stage gear where Xtreme punishment is the norm......
  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
car auto eq xplod1236 Car Audio 9 25th November 2005 02:13 PM
low-biasing or high biasing in Class-AB amps Workhorse Solid State 124 18th October 2005 04:45 AM
Auto-on nikita Parts 1 4th March 2005 09:19 AM

New To Site? Need Help?

All times are GMT. The time now is 08:10 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2