Marshall JCM 900 Model 4100 Biasing

[Carlosraj]

Hi

This amp had 3 of the 5881 output valves damaged. A friend of mine brought 4 of these new valves and asked me to replace them. I have downloaded the schematic, but unsure how to set the bias for this amp. On the schematic, it is noted to set the bias between -38V to -40V between R27 and R26, but how do I set the bias without any trimmer pot to adjust bias. Please assist.

Thanks

Carlos

 

[jazbo8]

You can change the overall bias voltage (PR1 Bias Preset on the schematic), but not for the individual tubes.

 

[Carlosraj]

OK, what should be the overall bias voltage and at which point this reading is to be taken.

Thanks

 

[jazbo8]

Like you said in post 1: -38V to -40V as shown on the schematic.

 

[JonSnell Electronic]

The bias is set by VR2. Start with maximum voltage (-50V) remove the cathode fuses of both pairs and measure the current that each pair draw in turn, set to between 40 and 60mA per pair. Re install the fuses. 40mA is cool and harsh, 60mA is more like a Fender sound.
The four 5881s must be matched as there is no way to set individual bias.

 

[jazbo8]

There seems to be two different schematics floating around, one is with -50V, the other -38V~-40V.

 

[Enzo]

The ones that say -50v are for the 5881 (6L6) version. The ones that say -38 to -48v are the EL34 versions.

The bias trimmer is PR1 on the older EL34 drawings, and VR2 on the newer 5881 drawings. Same circuit.


As much as we like matched tubes, they are not necessary, they were never matched from the factory on new amps. You set the overall bias for the average current of all the tubes. Of course matched tubes make that easier.

 

[Carlosraj]

Thanks to all for your advice.

Carlos