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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:
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.
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.
I then leaned the pot forward slightly and rtv siliconed it against the adjacent orange drop cap for stability.
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
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:
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.
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.
I then leaned the pot forward slightly and rtv siliconed it against the adjacent orange drop cap for stability.
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
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