Bias Measuring points, pros cons - diyAudio
Go Back   Home > Forums > General Interest > Everything Else
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

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 28th January 2007, 06:50 PM   #1
diyAudio Member
Zero Cool's Avatar
Join Date: Sep 2004
Location: MN
Default Bias Measuring points, pros cons

I am curious about the Pro's and cons of various Bias measuring methods.

I am finally finishing the rebuild of an amplifier and i need to set the final bias once it is all done. It appears that the factory method of setting bias is by measuring across the emitter resistors. This is how i set initial bias. But noticed that if i was not very careful, that the amp would oscillate due to my meter leads.

So i am curious how i can better my method. I have a very good Fluke 189 multimeter that i have been using. Would shielded leads be better then the stock leads for the meter?

And what about measuring Bias by putting the meter inline with one of the power supply leads? the amp would be less prone to oscillation that way, but is that a better method? it is definitly harder to set up as this amp does not have B+/- fuses. and using this method i understand i need to add the total of all the devices for the bias measurement. but am i adding ALL devices. for example if i have 4 pairs (8 devices 4N & 4P) and they need to be set to sat 50ma each, would i then be looking for 200ma, or 400ma? I would assume 400 ma correct?

advice please.

  Reply With Quote
Old 17th November 2011, 09:34 PM   #2
ChrisA is offline ChrisA  United States
diyAudio Member
Join Date: Jan 2008
Originally Posted by Zero Cool View Post
I am curious about the Pro's and cons of various Bias measuring methods.
A common trick with tube amp building is to place a 1 Ohm resistor in series. Then to measure bias current you measure the voltage across this resistor (and divide by one). You can choose the location or the resistor. I think the best place is such that one of its leads in grounded. Now when you measure, one of your meter's leads is at ground and the other has a 1 ohm pure resistive impedance to ground. You are NOT going to pick up noise. You test leads are in effect shorted by a 1R resistor and one is on the ground rail

In my case, physically the resistor gets soldered across two banana jacks so I can measure the bias current without opening up the amp. But that is because tubes tend to get replaced years after the amp is built and the bias needs to be set again so a screwdriver trim pot get mounted right next to the jacks.

You can skip the externally accessible jack and the trim pot but keep the 1R resistor
  Reply With Quote
Old 17th November 2011, 11:57 PM   #3
Enzo is offline Enzo  United States
diyAudio Member
Join Date: Sep 2004
Location: Lansing, Michigan
Since he is talking emitter resistors, I have to suspect there are not a lot of tubes in there.

COnsider thoseme meitter resistors are connected directly to the output of the amp, as hot a signal as there is in the amp, and when you conect your meter probes to the resistor, the probe wires become antennas broadcasting that signal to the earlier stages of the amp. Try taking the reading with your meter and probe wires leaving the amp area as quick as possible and as far from the earlier stages as possible.

Try setting bias by watching crossover distortion on a sine wave. Setting by emitter current is at best an approximation.
  Reply With Quote
Old 18th November 2011, 12:28 AM   #4
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA
I'm surprised you're seeing oscillation, since that should be a low impedance point that's immune to such things. Since your meter is high impedance, you might try measuring through a couple 2 kohm resistors, one right at each probe tip. That should isolate the leads and not cause much error.

I've seen amps like the Marantz 250 where they set the bias by looking at the AC current draw. They start with the bias as low as it goes, measure the draw on the mains cord, then increase the bias until the mains draw goes up by a specified amount for each channel. I'm not a fan of the method but it probably works OK.
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 18th November 2011, 06:49 AM   #5
Enzo is offline Enzo  United States
diyAudio Member
Join Date: Sep 2004
Location: Lansing, Michigan
COnrad, I don;t think the probes are making the amp unstable by themselves, I think they are coupling the output signal into the earlier stages and causing feedback.
  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
pros/cons of a 845 VS 211 jarthel Tubes / Valves 2 18th June 2007 12:57 PM
TL vs MLTL, pros and cons Tomac Multi-Way 5 29th December 2004 12:55 PM
Filterless DAC - pros and cons? mr_push_pull Digital Source 1 11th August 2004 02:56 PM
Pros and Cons of this technique? G Tubes / Valves 35 23rd October 2003 05:37 PM
tweeter offset: pros and cons gary f Multi-Way 6 15th August 2003 07:57 PM

New To Site? Need Help?

All times are GMT. The time now is 12:52 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