Biasing a JCM900 SLX model 2500

[Kuga]

Hi

It's a 50W EL34 model. A stock one no mods. Acording schematics you can see bias need to be at -41Vdc in R26/R27. But looks like it's a cold bias at -41Vdc because you get 25mV If you mesures voltage across a 1R resistor on pin1. I need about 35mV in a 1R resistor on pin1 to get a 60% dissipation on power valves.

How would be the best way to bias It?

Thanks

 

[Pete Farrington]

Acording schematics you can see bias need to be at -41Vdc in R26/R27. But looks like it's a cold bias at -41Vdc because you get 25mV If you mesures voltage across a 1R resistor on pin1

What's your mains voltage?

If that's not exactly what the designer was expecting, then all schematic voltages will be off to some degree.

Perhaps with the Marshall selected EL34 (Tesla?) used at that time, a nominal -41V bias provided suitable idle conditions?

Whereas a random matched pair of valves, even though matched to each other, may not idle at the level intended.

I need about 35mV in a 1R resistor on pin1 to get a 60% dissipation on power valves.

It's standard procedure surely? eg tweak VR2 until the desired idle condition is achieved.

https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm900_slx_50w_2500.pdf