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

 

[Purgasound]

I've had better luck with current probes for the 900 series. Since the cathodes go through some fuses and travel through the PCB to a ground through some diodes there's a bigger discrepancy when using the 1ohm resistor method. You'll read a couple ohms from cathode to ground without a 1ohm resistor even though there should be continuity. Could be off by 10mV or so.

 

[Pete Farrington]

Since the cathodes go through some fuses and travel through the PCB to a ground through some diodes there's a bigger discrepancy when using the 1ohm resistor method. You'll read a couple ohms from cathode to ground without a 1ohm resistor even though there should be continuity. Could be off by 10mV or so.

Just measure across the resistor legs, ignore the chassis.
The V DC across a 1R resistor in series with pin 8 will still sample cathode current.

Whatever, there should be no current flowing via the hum loop breaking network D6//D7//C1//R36, ie between circuit common and chassis safety earth.

1R resistor on pin1

Pin 8 is the cathode. Pin 1 is the suppressor grid.

 

[Kuga]

What's your mains voltage?

If I use a variac I can adjust bias at 240Vac. The mains voltage on JCM900 it's 240Vac but it's not a big difference at 230Vac.

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

Could be. I use a matched NOS EL34 SVETLANA.

You'll read a couple ohms from cathode to ground without a 1ohm resistor even though there should be continuity. Could be off by 10mV or so.

On this amp I read 23 Ohms.

Just measure across the resistor legs, ignore the chassis.
The V DC across a 1R resistor in series with pin 8 will still sample cathode current.

I did It. If on R26/R27 I have -41Vdc then I have 25mVdc across 1R resistor in series with pin8 / pin1.

Pin 8 is the cathode. Pin 1 is the suppressor grid.

Pin 8 and pin 1 are joined.