Marshall JCM2000 Stable Bias Mod


JCM2000 DSL100.The early (1997-2003) Marshall JCM2000 DSL50, DSL100, TSL100 en TSL122 amps are often troubled by an unstable bias (drifting bias). This can be difficult to diagnose correctly when the amp is on the repair bench as it can take half an hour or more of testing and/or playing before this bias drift manifests itself. Due to this drift the bias current of the power tubes will gradually increase, and keep increasing, until the power tubes will glow bright red/orange and (sometimes) the HT fuse will blow. When this phase has been reached the power tubes are usually defective and will need replacing. Often this happens with two of the four (DSL100, TSL100 en TSL122) or one of the two (DSL50) power tubes, on the right hand side (as viewed form the rear of the amp).

All sorts of wild stories about this phenomenon circulate on numerous guitar amp forums but none of these stories address this issue completely and accurately. Dr.Tube has thoroughly investigated this problem and has devised a simple and elegant but very reliable and affordable solution to this problem.

The cause of the problem.

The primary cause is to be found in the main printed circuit board (PCB). That is the big PCB with all the tubes on it. If the code on this PCB is either JCM2-60-00 or JCM2-60-02 then it is likely you allready 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 wil only stop due to the power tubes saturating at some very high current (whilst going into meltdown mode) or the HT fuse blowing.

Up to now this problem has only been ascertained on the JCM2-60-00 and the JCM2-60-02 PCBs. These PCBs are also often fitted with the incorrect control grid resistors (JCM2-60-00: R55, R57, R62 & R70, JCM2-60-02: R7, R10, R66, R70). These are then 220kΩ instead of the desired 5.6kΩ. This will further aggravate the drifting bias problem. The later TL10-60-02 PCBs aren't troubled by the drifting bias problem and also have the correct control grid resistors fitted.

The solution.

Dr.Tube JCM2000 Stable Bias kit.The solution for this problem is actually rather simple: remove all the bias components from the main PCB and place these on their own PCB isolated from the main PCB (and its high voltage). Pin 5 (the control grid) of the power tubes must then also be isolated from the main PCB. This is crucial!

Dr.Tube has designed a small PCB (and a complete kit with all necessary parts) which is easy to mount in these amps. This PCB is fitted with the new bias parts and is connected between the main PCB and the small bias pot PCB. The extra required cable is supplied in the kit. A couple of wires need to be connected between the main PCB and the new bias PCB. The main PCB needs to be slightly modified where all the pins 5 of the power tube sockets need to be freed from the main PCB. This can be done with a Dremel, but Dr.Tube can supply a very handy hollow drill making this part of the job a piece of cake!

Dr.Tube JCM2000 Stable Bias kit in the amp.Also situated on the new bias PCB are the new phase inverter (PI) coupling caps. The bias PCB has been dimensioned so that the original coupling caps (from the main PCB) fit on here (if you so desire), but also Wima MKP10, Orange Drops, MKT1813s, Mallory 150s or MKS caps fit on here too. Whatever suits you best. The bias PCB is normally fitted with two Wima MKP10 caps.

This kit can be ordered as a kit in the Dr.Tube Online Store. There is a version for the DSL50 and DSL100 amps and there is a version for the TSL100 and TSL122 amps. Installing this kit requires more than average skills. You need to be able to correctly disassemble and reassemble a JCM2000 amp. The main PCB needs to be modified with a Dremel or a hollow drill and you need to be able to solder. All together this isn't rocket science but if all this is new to you then it is probably wise to have this kit installed by an experienced tech. This kit can ofcourse also be installed by Dr.Tube. The installation costs are € 125,00.

Bare kits are available for repairshops and amp-modders with only the PCB, and the hard to find parts like the connectors and the extra cable: Bare Kit.