Marshall DSL Biasing

The Marshall DSL and JCM2000 series of amps are fairly easy to bias as they have bias prongs on the back of the chassis that allow you to measure cathode current without removing the chassis from the head box.

The first thing you’ll have to do is remove the back metal grille. Once you’ve removed the grille, you’ll see the bias prongs.

Connect your amp’s speaker jack to a proper load, turn it on, take it off of standby and start biasing!

The 2nd step is to measure the plate voltage. The plate voltage can vary a lot from amp to amp. I’ve seen values range from 430vdc to 500vdc. That’s a wide range and it will affect how you set the bias.

Measuring the plate voltage on an amp like the DSL is done safely with a plate voltage probe, like the one pictured below.

With the plate voltage, you can now set the bias.

In this example, the plate voltage is 463vdc. I find that setting the bias based on a 65% plate dissipation is best on these amps. Typically, people use a value of 70% as a max value.

The target bias setting, using the cathode current method as we will be doing here is as follows, per tube: (0.65 * 25/463) - Screen Current.

The screen curent on this amp with the pictured Tung Sol EL34B tubes is approx 1.3ma. This can vary depending on the tubes. Some say that screen current can go up to 5ma. The Electro Harmonix EL34s I have have a screen current of 3.3ma.

The resulting target bias current we’re looking for is 33.79ma. At 70% dissipation, you would set the bias @ 37ma. This is a per tube value. On a 100 watt amp, the value would be double that: 67.58ma @ 65% dissipation.

Before setting the bias, always set it to the minimum value, then insert the tubes. If you’re re-biasing existing tubes, you don’t need to reset to the min value.

Set your Digital Multimeter to the mv setting and connect the reference to the middle pin and the positive lead to one of the outer prongs. The prong on the left sets the bias for the tubes on the right and vice versa. With a jewller’s screwdriver, slowly turn the trim pots until you’ve reached the desired bias setting

When the bias settings are set, leave the amp on for another 30minutes. The bias will tend to move up a little. If it has moved up, back it off a little until you reach the desired value.

Don’t overthink the whole bias procedure and don’t look for absolute precision.

Good luck!