Bias Mod for Mark IV Widebody Head???

[Monsta-Tone]

Does anybody have an adjustable Bias Mod for a Mark IV head? I have been building & modifying amps for some years now and am very confident in my skills, so you don't have to worry about me killing myself.

I want to experiment with other brands of tubes.



I have several schematics of the Power & Preamp sections of this amp, but none of the Power Supply or Bias Supply.

The place that I got my schematics from is: http://www.tubefreak.com/mesa.htm

They don't have any Power Supply drawings posted.

Any help would be greatly appreciated.

 

[Monsta-Tone]

No takers I guess????

 

[Ken j]

From my limited experience I believe the bias voltage comes in at point "E". You would most like have to change resistance at the 220K resistors. I measure bias by the method found here: http://www.aikenamps.com/Biasing.html

 

[Boogiebabies]

The Mesa Bias section for the IV uses the same PT for the Heartbreaker.
If you cannot find that point, it is generally near the two Nichicon electrolytic 47uf 75V caps and the two resistors next to it. It is towards the back right of the board. I would like to help more, but I am too lazy to pull my Mk IV chassis out. Find those resistors and you can put trimmers in. I leave mine the way thery are. I have found what tubes to buy and where to get them to
bias pretty well out of the box. Another thing about Mesa's is that their idle bias even at 20 ma can draw up to 55ma during playing.

 

[Monsta-Tone]

That's perfect. Thanks.

I can easily pull my chassis out to take a look. I just wanted a little bit of direction before I operate. I'm just lazy enough to not want to spend 1/2 an hour looking for the Bias Resistors.