Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 13th May 2008, 06:48 PM   #1
Mywing is offline Mywing  Italy
diyAudio Member
 
Join Date: Mar 2007
Default 400A bias adjusting

Hi all,
as recommended me by Nelson I would adjust the 400A bias, but I don't know how.
Could someone that has already done with success, suggest me the procedure to follow?
Thank you so much.
  Reply With Quote
Old 13th May 2008, 08:28 PM   #2
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Each channel has a bias potentiometer which is used to set
the bias. The bias should be set so that after one hour operation,
with the cover on, each channel should run about 50 deg C. in
a 25 deg C. room. This means that a ordinary room temperature
the heat sinks should be at the temperature where you can put
your hand on them for about 10 seconds.

The critical thing is not to accidentally find yourself biased at too
high a current figure, otherwise you risk blowing the channel up.

You must monitor the current draw of the channel, best done
by reading the voltage across one of the .33 ohm emitter
resistors on the output stage. Since it takes time for the amp
to warm up and the bias is somewhat temperature dependent,
you want to adjust the value of the bias very slowly, and then
wait while monitoring the current draw.

If the channel is running cold, then you increase the bias above
the existing value by a little bit. Then you wait.

Did I mention that there are hazardous voltages and stuff
inside the amp? If you don't know to do this safely, then don't
try it - find a technician and buy him a 6 pack.
  Reply With Quote
Old 14th May 2008, 07:24 AM   #3
Mywing is offline Mywing  Italy
diyAudio Member
 
Join Date: Mar 2007
Thaks boss. I'll try that.
A last question: where is exactly situated the bias potentiometer?
Cheers
  Reply With Quote
Old 25th May 2008, 03:36 PM   #4
Mywing is offline Mywing  Italy
diyAudio Member
 
Join Date: Mar 2007
Hi guys,
I've noticed, after about 4 hours of work, that the handles and the front panel become very warm, at least how much the heatsinks.
Is it normal?
Cheers
  Reply With Quote
Old 25th May 2008, 06:13 PM   #5
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
Originally posted by Mywing
Hi guys,
I've noticed, after about 4 hours of work, that the handles and the front panel become very warm, at least how much the heatsinks.
Is it normal?
Cheers



read again what you wrote ......


and tell me - are you really sure that you don't know answer ?

tip:

make that front plate and handles of wood .......... then they will be just slightly warm

__________________
my Papa is smarter than your Nelson !
tnx to clean thread ; Cook Book ; PSM LS Cook Book ; Baby Diyaudio FORUM ; Mighty ZM's Bloggg;I'm dumb
  Reply With Quote
Old 25th May 2008, 06:15 PM   #6
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
Originally posted by Mywing
Thaks boss. I'll try that.
A last question: where is exactly situated the bias potentiometer?
Cheers

gimme a good pic of pcb and I'll tell you
__________________
my Papa is smarter than your Nelson !
tnx to clean thread ; Cook Book ; PSM LS Cook Book ; Baby Diyaudio FORUM ; Mighty ZM's Bloggg;I'm dumb
  Reply With Quote
Old 25th May 2008, 07:41 PM   #7
Mywing is offline Mywing  Italy
diyAudio Member
 
Join Date: Mar 2007
Quote:
Originally posted by Zen Mod



gimme a good pic of pcb and I'll tell you
I've already found them by myself.
Thanks anyway.

About the warmth of handles and front panel, it sounds strange to me that it is so high, I own other amps including a valvestate with eight 6550 and handless and panel remain cold.
Unfortunately, this is my first experience with a class A: sorry if I annoy you with silly questions, I will try to avoid them in future.
Greets,
Nino
  Reply With Quote
Old 25th May 2008, 07:53 PM   #8
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Quote:
Originally posted by Mywing


I've already found them by myself.
Thanks anyway.

About the warmth of handles and front panel, it sounds strange to me that it is so high, I own other amps including a valvestate with eight 6550 and handless and panel remain cold.
Unfortunately, this is my first experience with a class A: sorry if I annoy you with silly questions, I will try to avoid them in future.
Greets,
Nino

number of smilies can tell you that I'm not annoyed .......

there is unlimited number of questions one can ask , under condition that he show doing own home work ;

__________________
my Papa is smarter than your Nelson !
tnx to clean thread ; Cook Book ; PSM LS Cook Book ; Baby Diyaudio FORUM ; Mighty ZM's Bloggg;I'm dumb
  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
Adjusting the BIAS for Threshold SA3 sonic22 Pass Labs 9 16th May 2011 12:38 PM
Adjusting BIAS barmesan Tubes / Valves 2 10th January 2007 12:18 PM
Marantz pm 94 + Bias adjusting CederPM94 Solid State 0 10th May 2004 09:22 AM
Adjusting DC bias for Zen amplifier hlvan Pass Labs 6 5th November 2001 07:43 PM


New To Site? Need Help?

All times are GMT. The time now is 05:07 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