Marshall JCM800 2205 6l6 mod

[Vizier]

I'd like to try this. What do I have to do to get correct bias ?

[John Phillips]

Change the bias supply limiting resistor (R55 on the schematic for the late models, may be different on the earlier ones) from 220K to 150K. You can do this without lifting the board if you parallel the 220K with a 470K. (Be careful not to overheat the leads and damage the solder joints under the board, or you'll lose the bias altogether.)

That should get you into the right range to set the bias accurately with the trimmer.

[Vizier]

Mine is -85. Thanks John, I will check that out.