Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 17th February 2007, 05:12 PM   #1
bdeko is offline bdeko  Canada
diyAudio Member
 
Join Date: Feb 2007
Location: Ottawa, ON
Default setting bias for power FETs?

Hi everybody, I'm new to this forum.
I have an Electrovoice 81PMX power mixer that was blowing fuses. The previous owner had replaced all the power FETs and did a pretty crappy job. I put in new ones (IRF640 and IRF9640) and now it's OK, but there is a note on the power amp board saying I need to adjust the bias when replacing FETs. What exactly am I looking for when I adjust it?
I'm not used to high power stuff and my FET theory is shaky.
Can anyonelet me know how to adjust this properly?

Thanks!
  Reply With Quote
Old 18th February 2007, 03:58 AM   #2
diyAudio Member
 
unclejed613's Avatar
 
Join Date: Dec 2006
first, attach a dc voltmeter across the emitter resistors and bring up the amp on a variac. if the voltage across the emitter resistors goes over 100mV before you get to full line voltage, shut it down!!!!!

if you are able to get to full line voltage, adjust the bias pot until you get between 10 mV and 20 mV across the emitter resistors. the bias pot is usually near a transistor or diode "stack" (usually a red blob shaped thing in a clip that screws to the heat sink). you will also want to adjust the offset by attaching a voltmeter across the speaker terminals and adjusting the offset pot (usually near the amp board input cable) until you have 10mV or less across the speaker terminals.
__________________
Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net
spammer trap: spammers must die
  Reply With Quote
Old 18th February 2007, 02:05 PM   #3
diyAudio Member
 
XEAGLEKEEPER's Avatar
 
Join Date: Jan 2006
Location: central Iowa
Unclejed Dont you mean source resistors instead of emitter.
Dave
__________________
Ret. USAF... AKA- Avionic *** Solder slinger for hire...***
  Reply With Quote
Old 21st March 2007, 03:34 AM   #4
bdeko is offline bdeko  Canada
diyAudio Member
 
Join Date: Feb 2007
Location: Ottawa, ON
Thanks guys, but as I was adjusting the bias, I would see the voltage swing up to 15mV or so, and then right back to 0. I thought I needed to increase a bit more, but then true to Murphy's law, all four PNP FET's blew (these are double the price of the NPNs).
Before I try this again, I'd like to figure out why this happened. I measured across the source resistors (they are 0.33ohm, 2W). Could using a magnetic screwdriver affect it? I can't think of anything else. Help?
  Reply With Quote
Old 21st March 2007, 07:09 PM   #5
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
are the output devices set up as source followers
or
as CFPs?

I think the source follower requires more bias to reduce crossover distortion.

I wonder if the amp is actually a quasi complementary set-up to eliminate the need for expensive 9640?

How many pairs are fitted to each channel of the output stage?

Any chance of posting a schematic?
__________________
regards Andrew T.
  Reply With Quote
Old 22nd March 2007, 08:17 PM   #6
bdeko is offline bdeko  Canada
diyAudio Member
 
Join Date: Feb 2007
Location: Ottawa, ON
I don't have a schematic. There is an initial BJT NPN/PNP pair, then four pairs of 640/9640. I'm not used to the FET amps so much. Also, I was measuring the bias with no signal applied. Do I need a signal? I have a sine wave source.
  Reply With Quote
Old 23rd March 2007, 07:31 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
output bias and output offset are both measured and re-set, if necessary, with the input shorted to ground and with the output open circuit (=no load).

4pair of 120W devices should be able to give 150W to 200W of output power if designed to be reliable.

15mV on Vre is tiny if measured correctly.
There appears to be another fault.

You need a search out a schematic.
__________________
regards Andrew T.
  Reply With Quote
Old 23rd March 2007, 08:50 AM   #8
diyAudio Member
 
ifrythings's Avatar
 
Join Date: Oct 2005
Quote:
Originally posted by bdeko
Thanks guys, but as I was adjusting the bias, I would see the voltage swing up to 15mV or so, and then right back to 0. I thought I needed to increase a bit more, but then true to Murphy's law, all four PNP FET's blew (these are double the price of the NPNs).
Before I try this again, I'd like to figure out why this happened. Help?

And this is why I use a light bulb (60w's or so) in series with the mains when I first fire up or change settings. It's cheap current limiting and saves you many expensive parts. Also check or replace the bias pot, if it has a dead spot (aka open area) this could kill the amp.

Quote:
AndrewT

I wonder if the amp is actually a quasi complementary set-up to eliminate the need for expensive 9640?
That could be very possible.

Thats easy to check, if (-) rail goes to center leg of fet thens it's complementary, but if it goes to the source then it's quasi complementary (all N-channel Fets) (Gate will be a few 3-4 volts higher the the (-) rail)
  Reply With Quote
Old 5th April 2007, 01:34 AM   #9
bdeko is offline bdeko  Canada
diyAudio Member
 
Join Date: Feb 2007
Location: Ottawa, ON
It definitely is complementary.
What do you mean about the light bulb in series with the mains? Where exactly do you wire the bulb? I'm curious 'cause I really don't want another $40 going up in smoke.
The bias pot is OK; checked it with a Fluke multimeter and turned it slow - no dead spots.
  Reply With Quote
Old 5th April 2007, 05:47 AM   #10
diyAudio Member
 
ifrythings's Avatar
 
Join Date: Oct 2005
I have a light bulb with wires and holder straped onto a chunk of wood and on the wires are aligater clips.

As for were to put it, I usually pull out the fuse and put clip the leads onto the fuse holder, then the bulb is in series and stuff can't get hurt.

DO NOT use a high watage like, or it's usless, I use a 60w bulb.

you have to use a Incandescent light bulb too, the mini fourecents don't work.

Have you checked the bias transistor? for shorts? for leaky junctions? pull it out of circuit for testing.
  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
Nakamichi PA-7 & PA-5 Bias Setting? sourceoneaudio Solid State 6 8th March 2012 12:37 PM
Setting Bias kptseng Solid State 9 8th March 2008 01:56 AM
Bias setting for a Bryston 3B ? legarem Solid State 2 28th February 2008 08:02 PM
bias setting ben goh Solid State 8 14th August 2006 04:26 PM
power fets, bias, gain matching & other stuff. lowlevel Solid State 11 22nd January 2005 05:03 PM


New To Site? Need Help?

All times are GMT. The time now is 09:47 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