Today I thought I’d run through how to change the valves and re bias a Marshall JCM2000 DSL 100. It’s important to say up front that if you’re a young un or haven’t had too much experience with electrical equipment then it’s probably not a good idea for you to tackle this job. The voltages that are running around an amp are dangerously high and could kill you if you touch the wrong thing. That said, the beauty with the JCM2000 is that everything you need to get at to change the tubes is visible and everything else is hidden away under panels.
First thing to do is remove the back panel and gain access to the tubes you wish to replace. This could be a simple two in two out job (you should always replace tubes in pairs) or the full re-valve that I’m doing today. As you can see there are random tubes in the back of this amp that don’t appear to be matched which could have some bearing on the fact that the amp wasn’t very happy.
Once you’ve done this then you should install your new tubes. I’ve chosen the JJ Electronics Valve pack from Voc Rock Guitars. It contained the ECC83 pre amp tubes and EL34 power amp tubes, which are a classic combination for the JCM2000.
As you can see I was wearing gloves when installing the tubes. This was to avoid any oils or muck from my fingers getting onto the tubes. It’s not necessary but I like to do it.
When installing the tubes there are very obvious ways to install them. On the preamp tubes there is a gap in the pins on the bottom of the tube. Line that up with the gap on the circuit board and they’ll slot in nicely.
With the power amp tubes, there’s a ridge on the centre pole underneath the tubes that lines up with a notch on the chassis. You shouldn’t have to force the tubes in, so if you are not getting them to sit properly, chances are you don’t have the ridge and notch lined up.
Here you can see all the tubes fully installed and ready to be biased, which is the next part of the process and a very important thing to do. A cool thing to note is that the lettering on the tubes all faces forward, which is another way to check that the tubes are correctly installed.
How to Bias
This is the bias test point, it is a set of three pins that stick out of the amp underneath the power tube section on the right hand side. You will be using a digital multimeter to test the voltages you are getting in a moment but the first thing to do is plug in your speaker cab and turn the amp on. You should then let it warm up for 15 minutes and switch it off stand by. DO NOT TURN YOUR AMP ON WITHOUT THE SPEAKER ATTACHED! You could blow tubes or damage the amp if you don’t have it attached.
Bias in pairs
If you’re wondering why there are three pins, it because on these amps you bias the tubes in pairs. The centre pin is the neutral and the ones to the right and left are for the two pairs of power tubes.
Now you get your digital multimeter and VERY CAREFULLY touch the black probe to the centre pin and the red probe to ONE of the outer pins. The meter should be set to 2V DC current and you should get a reading of anything from 0.02 to 0.09. I was informed that .072 was a good number to go for as you get all the breakup and it doesn’t knacker your tubes as quickly.
Marshall recommend a value of 0.09 for these types of amps, which means the tubes will be running hotter and could wear out more quickly. In order to change this reading you’ll need a small flat head screwdriver that you slot into one of the mini pots that you find on either side of the test pins. You only need to move it a little to make quite a big change in the reading so go carefully and be prepared to make an adjustment, take a reading and then make another adjustment until you get the desired reading. Do this for both pairs of tubes and then all that’s left to do is rock out!