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Marshall DSL-TSL-JVM “How To” Bias Page
PLEASE READ THE TEXT BELOW THAT EXPLAINS THAT YOU DO NOT NEED TO USE A BIAS PROBE!!!!! This video is of a Marshall DSL100 and the bias procedure is the same on TSL100's and 122's and for the JVM but the test points and pots are on the inside of the amp on the JVM. For the DSL50 the procedure is the same but you would be setting the bias between 40 to 45 millivolts. The middle prong is your ground. The test points or prongs are shown in the video and in the pic's below. The most asked question we get is "do I need bias probes that are like the ones shown in this video to bias my amp." The answer is no. If you listen to the video I will explain that I'm showing two different methods of reading bias and showing the probes and meters so you can get a better visual  on how the bias works. All you need to bias one of these amps is a simple multimeter to do the job. The second most asked question is "what setting do I use on my multimeter" The answer we give in the video is Millivolts! The third most asked question is "do I use the black or negative lead for the middle test prong?" The answer is yes! The fourth most asked question is "do I bias the amp in standby?" The answer is no! The amp must be in the play mode to adjust the bias. The fifth most asked question is "do I have to have a speaker plugged in?" The answer is YES! You MUST have a speaker load on the amp. The sixth most asked question is "what do I bias KT77's at?" The answer is EL34's, E34L's, KT77's and the big bottle JJ 6CA7's all bias up to between 40 to 45mV in 50 watters and between 80 to 90mV in 100 watt DSL's, TSL's and JVM's.   The bias in the new JVM four channel Marshall's is the same as the DSL/TSL amps but Marshall inconveniently moved the test points and trim pots to the inside of the amp so you have to remove the amp from the chassis where you will find the same three prong test point setup with two trim pots. The bias for the TSL60 is NOT the same! It's actually even easier to adjust than the DSL/TSL50's and 100's which have two trim pots as explained above and shown in the video. The center tap on the TSL60 is ground and the left or right tap measures both the left tube and the right tube which is why there is only one trim pot. A setting between 70 to 80 millivolts is good because you are measuring both tubes at once. Just set your multimeter to DC millivolts and dial in the tubes. You guys with 201 and 401 amps have to take the amp apart to get the test points and trim pot. Here is a pic. Marshall recommends a setting of .675 volts for the 201 and 1.375 volts for the 401, I feel that the 1.375 is way too hot and recommend a setting of between .8 to 1 volt if you are using the test point and for the 201 to .400 to .500 is good. Here are some pic's of the 201 and 401 When biasing a Marshall 201 for the JJ EL844 tubes since they only dissipate 9 watts you would bias them between .300 and .375 volts. When biasing a Marshall 401 for the JJ EL844 tubes you would bias them between .600 and .750 volts.     We get a lot of email and phone calls asking "where are the test points on my 900? Or 800?" Sorry to say that there are none...  For these models and all older models you must use a bias probe.