Marshall JCM2000 bias

[justbass]

Hi all, anyone know what the voltage should read on the little connector at the back of the amp when setting the bias? These usually have two adjuster pots so I assume that its one for each side of the amp rather than one for bias and one for ballance.
cheers Just

[melvin]

The three-prong connector's middle is ground. Each of the other two is connected to the top of a small value resistor in series with the cathode of one of the power tubes to ground.

So, measure the resistance from an outside pin to center pin.
Then, use that value to calculate the voltage for a particular bias current.

If you want to bias each tube at 30ma and the resistance you got was 1.5 ohms, the voltage you want to see across each outside pin to center is V= 0.03 X 1.5 = 45mV.

[Enzo]

It's a 1 ohm resistor. Most meters have a few tenths in the probe wires, so it is hard to measure it. Just consider millivolts at the test point equals milliamps through the tubes.

SInce you say they are at the back of the amp, I can rule out the DSL200 and DSL400. But I have no idea if you have a 50 or 100 watt, DSL or TSL. JCM2000 doesn;t specify an amp model any more than Ford specifies a car. If there are two power tubes, then each pin represents one tube. If there are four power tubes, then each pin represents two tubes. And that means if you get 60ma, that means 30ma per tube.

As melvin said, the center pin is ground. The adjuster is next to each outside pin. Just adjust the left one, then adjust the right one. They can be a little interactive, since the B+ will be dragged up and down, so go back and forth until you get them. That is why the amp makers tell you to do a balance instead of individual settings. They have you set one, then adjust the other so the sides are the same. They don;t care that the first one may move a bit in the process.


And in case you are troubleshooting, for some odd reason, looking at the amp, the pin on the left controls the tube/tubes on the right, and the pin on the right controls the tube/tubes on the left.

[Enzo]

Oh, and one I learned the hard way, some Marshall amps have a power mute pushbutton. it shorts the opposing grids together in the power amp to silence the amp. If you are trying to set the bias, having the two sides shorted together means both controls affect both sides. Took me a while to figure out the first time.