How to bias a JCM2000 series amp.

Here's the Marshall Service Bulletin on biasing the JCM2000 series amps:


1. Make sure the amp is connected to a load with the proper impedance selected.  

2. Power up amp on Standby, and let the circuit stabilize for a couple of minutes.

3. Locate the male three pin molex connector (CON2) with the two mini pots (PR1 and PR2) (located on each side of the molex connector) on both ends found at the bottom of tube bay.  

4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.  

5. Take amp off Standby with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to mV voltage that is listed below.

6. Repeat steps 4 & 5 for pin3 mV and until pin1 & pin3 mV readings are the same.  

Bias Settings:

DSL50 45 mV     TSL 601&602 80 mV

DSL100 90 mV    DSL 401 1.375V

TSL100/122 90mV    DSL201 .675V

DSL201 0.675 V
DSL401 1.375 V
DSL50 45 mV
DSL100 90 mV
TSL60 80 mV
TSL100/122 90 mv

Here's a quick bias lesson:

What you want to do is measure the plate voltage (B+) from pin3 of one of the power tubes. It should measure between 450-500V. Now the max plate dissapation (Pa Max) for EL34s is about 25W. To calculate the correct bias, you want to:

(Pa Max/B+)(.7) = 38-45mA depending on the exact plate voltage of your amp. Check and adjust via the molex connector and pots from above.