• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

MC-40 bias current?


This is my first thread on here so bear with me. I have a pair of MC-40's that I have been using for about 15 years now. I am generally pleased with the sound. When I first got them, I changed out the caps and put in the schottky diodes in the power supply. However, until recently while scanning ebay I noticed re-cap kits which include a pot to set the bias current, as changing the original selenium diodes to silicon will change the bias current, and it will need to be adjusted. I never considered this aspect until I read the listing. I have not seen any information anywhere as to where or what I am supposed to be looking for as far as voltage/current values, other than the power supply voltages along the voltage divider resistors. Does anyone have any info as to what I should be looking for? Also, at the time, I replaced all the Bumblebee caps with Sprague Orange Drop 751 series caps. Was this a bad idea? Are there any other caps that won't break the bank that would be recommended? Should I replace the old resistors? Any info would be greatly appreciated.:)


  • P6150124.JPG
    865.8 KB · Views: 36