MESA STRATEGY 400 - 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 4th November 2012, 11:49 AM   #1
diyAudio Member
Join Date: Dec 2011

Hi everyone looking for some help in adding adjustable bias to this amp , as i would prefer to have a choice of brand tubes that can be used , any help with this will be greatly appreciated ,many thanks Black arab.
  Reply With Quote
Old 5th November 2012, 12:03 AM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
kevinkr's Avatar
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
You'd need to at minimum give us a (partial) schematic of the amplifier circuit. A hand drawn schematic of the bias circuitry as it currently exists would be sufficient.

**Please do not link to unauthorized online repositories of copyrighted material.**
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
  Reply With Quote
Old 7th November 2012, 03:48 PM   #3
diyAudio Member
Join Date: Dec 2011
Hi Kevinkr i enclose a schematic of the circuit for you to look at many thanks Black arab.
Click the image to open in full size.
  Reply With Quote
Old 7th November 2012, 04:09 PM   #4
diyAudio Member
Join Date: Aug 2008
You mean tube types, right? Because if it was designed for STR brand 6l6GC's, it will work fine with any 6L6 gc or other similar variants. @Special Biasing" is 90% guitar tech myth.

Consider this. If the designer thought bias is an important enough issue, why did s/he not provide any means of adjusting it individually per tube?

Last edited by costis_n; 7th November 2012 at 04:12 PM.
  Reply With Quote
Old 14th November 2012, 06:33 PM   #5
diyAudio Member
Join Date: Dec 2011
what would be suitable trim pot value and resistor value to exchange for the fixed resistor ,many thanks black arab.
  Reply With Quote
Old 14th November 2012, 06:34 PM   #6
diyAudio Member
Join Date: Dec 2011
thanks Costis_n for your advice.
  Reply With Quote
Old 14th November 2012, 08:26 PM   #7
diyAudio Member
Chris Hornbeck's Avatar
Join Date: May 2011
Location: Little Rock
You'd probably have a better response to this question in the musical instrument amp section, but, in general, maybe choose a pot and a fixed resistor that together add up to the same as the sum of the two un-labeled resistors in the bias circuit. Make the pot about half of the total resistance and put it on "top". Add a small resistor in the output valves' cathodes to ground as a current sample point.

All good fortune,
  Reply With Quote
Old 15th November 2012, 02:14 PM   #8
diyAudio Member
Join Date: Jun 2011
well the philosophy of the method the amp bias was engineer is this:

the bias is set up for the first 40-60% of the tube life. this allows tube swapping at the musicians level without worrying about bias. And to change the tube before a failure arises.

The spec on the output tubes is matching Rp. JJ Tesla matches them for them at the factory and labels them Mesa Boogie for the OEM solution. The phase splitter tube is of course matching in gain as well as rp.

so why change it? 98% of the sound is before the output stage.
  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
Mesa boogie strategy 110 to 230 volt conversion blackarab Power Supplies 0 20th August 2012 07:36 AM
mesa boogie strategy 400 amp blackarab Swap Meet 0 12th August 2012 02:44 PM
Mesa/Boogie Bass 400+ mod question fenderjazz Instruments and Amps 5 10th October 2011 07:50 AM
Mesa/Boogie Bass 400 psu Greg3333 Instruments and Amps 10 21st February 2010 08:08 PM
Mesa/Boogie Bass 400 clone PSU specs Greg3333 Power Supplies 1 26th October 2006 02:13 PM

New To Site? Need Help?

All times are GMT. The time now is 10:07 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