Fender Vibrolux Update: Mod 10: Install an adjustable Bias WHY: Optimize your preferred power tubes rather than relying on the stock fixed bias. HOW: Use a 10k linear pot. Mount the pot through the bottom of the chassis below and to the pilot-light side of the tremolo pots. Alternately, you can use some adhesive and bond the back of the potentiometer to the chassis. In this case, I didn’t want to modify the chassis, so it was bonded. Remove R59. Solder a length of wire to the PCB where the resistor was and a 10KOhm resistor to the other spot where the resistor was. Solder the wire to the middle tab of the pot. Solder the 10KOhm resistor to the tab as shown in the picture. For resistor values, I recommend starting with a 10k. . Depending on the particular B+ voltage of your amp and the particular 6L6GC tubes you are using, you may have to change the value of the 10 KOhm fixed resistor down to 8 KOhms or slightly less. What youve just done is replaced the 18k bias set resistor with a 10k resistor in series with a 10k pot. Where before you had a fixed 18k, you now have 10k-20k range. Button things back up and check your idle current. You will want to see anywhere from 30-40mA per tube. If you can’t get the idle current high enough, replace the 10k resistor as mentioned above and test again. Again, you are aiming for somewhere between 30 and 40 mA per tube (as measured across the 10 ohm current monitoring resistors previously added in Mod 9). I’ll post some more screen shots of this as I proceed to checkout. The exact amount of idle bias current will depend upon the B+ in each particular amp, each particular pair of 6L6GC output tubes, and desired tone and tube life. Remember, if run them right on the edge of their maximum dissipation ratings, don’t expect to get a real long life out of them! R59 is located on the vibrato speed and intensity front panel daughter board. You can also mount the new 10K potentiometer to a terminal post which is in turn mounted to the power transformer, if you so wish. Guess what? Its checkout time a little later tonight.....like maybe after the neighbor that likes to start his Harley and rev it up early every Sunday morning tries to go to bed!
Posted on: Sat, 07 Jun 2014 22:10:47 +0000
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