Marshall 6100LM Anniversary Series super quick Bias repair

     This is kind of like fixing a tube amp with both amps tied behind your back.  The problem as it seems can only be described as output tube meltdown when power is applied.  Since this is an undesirable effect, the amp was brought to me to see if anything could be done with a minimum of modification.  Im lost on the minimum of modification part, but that was the constraint. And so we begin......


     The amp comes with a standby switch, allowing for complete heat-up of the unit, this will give an instant current reading when the switch is thrown.  Kris from cornerstone music joins me for this evenings event. Funny thing.    You would think that there would be no plate voltage in standby and yet, that is not the case...... it was actually almost 57 volts.  This should already be a sign that there might be power supply issues, but yet i proceed.

     My standard inline 6V6 current meter will be employed when standby is shutoff, the amount of current was incredible!!! About 106ma.   I decided to turn the bias pot to the very end but all I could get was -41vdc.  Even at the end of the POT the lowest I could get was 46ma, waaay too high.

     In my mind I can already visualize a leaky capacitor pulling the bias voltage positive, yeah thats confusing, negative voltage and all.  The problem is I would have to remove the caps to test and replace them, and that option did not exist. Shady caps....

C209 and C210 sit across the output of the rectifier and ground, would have been my next area to test....

Saving grace I did find R213 dropping 30volts across it, no doubt its intent was not to be bridged, but when you need bias, well then.  This allowed for dialing 35ma on the output tube with the bias pot at mid position.

Once properly biased, kris entertains us with some guitar playin.


The movie is much more entertaining than this essay....