Announcement

Collapse
No announcement yet.

Mesa Boogie F-50 adjustable bias mod

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Mesa Boogie F-50 adjustable bias mod

    I thought I'd post this as it may be helpful to some folks in the future. I couldn't find a post or pics from anyone on the web who had performed this mod before on this particular amp. My photos were taken looking over the amp with the front panel toward the top of the frame.

    WARNING: If your amp is still under warranty and if you are not experienced in amplifier repair work and safety DO NOT EVEN OPEN YOUR AMP! There are lethal voltages stored in your amplifier! THEY CAN KILL YOU!! Take it to a professional technician!!

    This amp came to me with a dead output tube. I tried 3 different sets of output tubes including a set of Mesa 6L6s and they all biased WAY cold, around 11-12 watts static dissipation. So, either finding the correct value bias resistor or installing an adjustable bias pot was a necessity and I opted for the latter. According to the schematic the resistor to be replaced is a 15k(22k in export models). I searched high and low and couldn't find the location of this resistor as there were no 15k or 22ks to be found! So I lifted the circuit board and followed the circuit trace from the bias supply. I found the resistor located below and to the left of the V4 phase inverter tube socket. This amp has a 33k stock resistor! Interesting:
    Click image for larger version

Name:	BIAS MOD PIC1.jpg
Views:	1
Size:	930.4 KB
ID:	865313
    This is the value they chose at the factory! It's even printed under the resistor "15k(22k)" but I guess they used this 33k value to achieve proper bias with the original output tubes. The amp had aftermarket JJs in it when it came to me.
    Click image for larger version

Name:	BIAS MOD PIC2.jpg
Views:	1
Size:	973.2 KB
ID:	865314
    So I removed the stock 33k bias resistor and replaced it with a 15k and a cool little 25k mini pot I found at my local surplus shop. Soldered the center wiper and the left leg of the pot together and soldered one end of the 15k resistor to the connected legs and soldered the other end to the left board eyelet. I then soldered the remaining pot leg to the right eyelet. I also soldered a bare wire across the existing connections on the right side to reinforce the weak circuit board traces thus it may look like just a blob of solder but it is not.
    Click image for larger version

Name:	BIAS MOD PIC3.jpg
Views:	1
Size:	994.2 KB
ID:	865315
    I then leaned the pot forward slightly and rtv siliconed it against the adjacent orange drop cap for stability.
    Click image for larger version

Name:	BIAS MOD PIC4.jpg
Views:	1
Size:	936.7 KB
ID:	865316
    Here's the full Schematics PDF for the F-series:
    https://docs.google.com/open?id=1xuZ...FdKZDL46FXikZl
    The mod works great and I have plenty of range for correct bias now.
    Cheers!

    Jason
    Last edited by jasonguitar; 04-18-2012, 03:20 AM. Reason: typo

  • #2
    Good call securing the added trimmer. You sure don't wanna leave anything in the bias circuit subject to breaking loose from vibration and lose bias.
    The farmer takes a wife, the barber takes a pole....

    Comment


    • #3
      Hey Jason, ancient thread I know but I need info and I'm striking out everywhere!!! I just bought an F50 and I want to add bias pot. Which resistor is it? Do you have any pics? I know you said below and left of v4, I think I may see it but I need to be sure. Thanks

      Comment


      • #4
        Hey men, do you find the resistor to change to add bias pot? I need it too

        Comment


        • #5
          Hope this helps.

          Comment


          • #6
            Originally posted by Pedro Vecino View Post
            Hope this helps.

            Thanks man! You saved me! But can I ask why did you replace even the resistors under the V4 with cords (violet, red and yellow cords)? Thanks!

            Comment


            • #7
              It's a master volume post PI. No relation with bias circuit.

              Comment

              Working...
              X