The early (1997-2003) Marshall JCM2000 DSL50, DSL100, TSL100 and TSL122 amps are often troubled by an unstable bias (drifting bias).
These are nice amps and I have one myself
This can be difficult to diagnose correctly, as it can take half an hour or more of testing and/or playing before this Marshall JCM200 bias drift shows up. . Due to this drift the bias current of the output valves will gradually increase, and keep increasing, until the EL34 glow bright red/orange ( “red plating” and this can also be bad enough to blow the HT fuse. When this phase has been reached the EL34’s are usually defective and are now ready to go in the trash can..
The Tung Sol El34B is a very good replacement. Here are our kits
The cause of the problem.
The primary cause of the Marshall JCM2000 Bias Drift is to be found in the main printed circuit board (PCB). That is the big PCB where all the valves are.
If the code on this PCB is either JCM2-60-00 or JCM2-60-02, then it is likely you already have been, or soon will be, confronted with this problem. Due to the heat produced by the tubes the PCB material becomes ever so slightly conducting. The specific layout of this PCB is such that some circuit tracks at the HT potential, (usually around 450V or so in these amps) run right next to the tracks for the negative grid (bias) voltage.
At one location, the distance between these two tracks is less than 1mm! The HT voltage will “pull up” the negative grid voltage, thus increasing the bias current. This is a runaway process and it will only stop due to the EL34’s liiting the current and getting cooked in the process
This issue is seen on the JCM2-60-00 and the JCM2-60-02 PCBs. These PCBs are also often fitted with the incorrect control grid resistors .
On the board numbered JCM2-60-00: R55, R57, R62 & R70.
and board numbered , JCM2-60-02: R7, R10, R66, R70
often have 220kΩ resistors fitted while they should be 5.6kΩ.
It is worth checking your board, to see if it has the correct resistors were fitted during manufacture. Wrong resistors will magnify the drifting bias problem. The later TL10-60-02 PCBs have not shown the drifting bias problems as yet, and they also have the correct control grid resistors fitted.
Marshall offers a new complete main circuit board as a “solution” for this problem. Not exactly a cheap option, and we believe that these new circuit boards are troubled by the same problem! Not really surprising, as the PCB layout of the problem areas is the same as the old boards.
We have a kit which solves this Marshall JCM2000 Bias Drift problem permanently.
This is a little kit which replaces the bias circuitry. It is small board that is mounted on the main pcb ( mountings supplied).
You do have to remove the main board in the amp. It is not for the inexperienced user. You will need to drill out one pin on each EL34 socket and be able to solder onto the main board. We supply the drill bit and all connectors with the kit. The kit is assembled by us here in the UK.
The kit comes with printed instructions and everything needed to fit the board.
You can order the kit on our website. From only £39.99 including delivery in the UK. ( price correct on 19th January,2019)
We export these kits all over the world and the website is multi-currency. At checkout you can select the delivery option for shipment to your country.
We will have stock of our UK assembled DSL Boards by 27th January, 2019. We are accepting back orders.