Biasing a 6100/6101
This section deals with setting power tube bias by using the cathode resistor method.
The first thing you need to do is remove the chassis. Place the amps head case face down on a solid surface. You can now place the chassis onto the screen grates mounting flange and you have your instant Marshall chassis holding tool. The 6100's power supply has bleed off resistors installed so the power filter capacitors do not hold stored voltage.
What you'll need to do now is remove the black cathode lead connected to pin 1 and 8 of each tube socket and insert a one ohm 2 watt precision resistor. If you're unsure what to get i have them at a good price. When done you will want your tube sockets to look something like this where the bottom lead of the resistor is on pin 1 and 8 and the black cathode wire is connected to the opposite end of the resistor:
Once you have installed all four of the resistors you are now ready to set your amps bias.
!!!Caution!!! ALWAYS have either a speaker ( load ) or a dummy load connected to the amp when setting bias!!! Biasing is done with NO SIGNAL applied.
With all four power tubes removed, turn the power on and leave the amp on standby. Turn the bias adjust trim pot to obtain the highest NEGATIVE voltage reading at pin 5 with the multimeters black lead referenced to ground. The reason for this is so you will not accidentily red plate your new tubes.
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Power the amp down and install your new tubes. Turn the power on and allow the tubes to warm up for a bit and then take the amp off of standby. What you need to do now is measure the voltage at the anode ( pin 3 ) of the tube with your meter referenced to ground
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Write this voltage down. Now measure the voltage drop across the cathode resistor and write this down and do the same for the remaining tubes.
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Now you will use Ohm's law to figure out the tubes idle dissipation. Remember that one millivolt across one ohm is equal to one milliamp. So if your plate voltage is 475v and your voltage drop across the resistor is 21 millivolts you take the 21 millivolt reading move the decimal three places to the left and you now have 21 milliamps or .021A. Remember 1000Ma = 1A. Multiply your pin 3 voltage by the Ma number and that will give you your dissipation.
475 x .021 = 9.9 watts. 9.9 would be a tad low so what you will do is decrease the voltage at pin 5 ( control grid ) which will increase the current flow through the tube. As the current flow rises the anode voltage will also decrease in proportion.
So now you've lowered your bias voltage, you will again measure the voltage at pin 3 and write that down. Now again measure the voltage drop across the cathode resistor and write that down, do this for all 4 tubes.
So now say your pin 3 ( anode) voltage is 468 and your drop is 35Mv your dissipation would now be 468 x .035 = 16.38. This is much better for a 6L6GC tube. Remember the screen grid allows about 5Ma of current through that is included in the cathode measurement and will increase the end result slightly. If this is of a concern you can always subtract 5ma from the cathode reading.
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What you want is to have all four tubes relatively close in their dissipation while at the same time NOT exceeding the tubes current limits. It is generally considered proper to bias your tubes at NO MORE than 70% of the tubes maximum dissipation rating. For example a 6L6GC's max dissipation is 30 watts but you cant run it that hot. 70% of 30 watts would be 23 watts. This would be the MAXIMUM bias setting you would want if you want to get any amount of life from your tubes. Remember the colder you bias the longer your tubes will last. The rest is up to you where you choose the set your bias.
This is a list of some of the 70% ratings of common guitar amp tubes:
6V6 12 Watts Max.
6L6GC 23 Watts Max.
5881 Russian 24 Watts Max.
EL34 24 Watts Max.
KT66 24 Watts Max.
6550 27 Watts Max.
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Once you have set your bias and are comfortable with the setting let the tubes cook for 2 to 4 hours. Go back and check the bias readings on all four tubes periodically during this time to make sure none of them have drifted. Some drifting is normal and you can tweak the adjustments as the tubes settle in.
It's also a good idea to measure the line voltage coming into your house when setting bias. Some areas especially around dinner/dusk will see a substantial fall off in line voltage, enough so that if you are biasing your tubes at the upper end of their dissipation limits they could exceed that when the line returns to it's normal level.
That's about all there is to it. If after a few hours your tubes are holding steady you should be good to go. Play your guitar and enjoy the sound of new tubes.