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Originally Posted by American Viking
To get the plate voltage ground the neutral to the chassis of the amp, touch the positive to pin #3 on the power tube socket and have your multimeter set to 1000 DCV. Most multimeters only go to around 600 DCV but I think that will work too. Pin # 3 is the third pin clockwise from the guide pin of the tube.
Like Josh said, you don't need the most expensive gizmos to do this. If you have a newer Marshall the trim pots and bias pins are located on the back of the amp and you don't have to take it apart to check anything but the plate voltage. In older amps the trimpots are on the circuit board and the easiest way to read the bias on those is by using a probe since they don't have bias pins.
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+1.
As noted above this is exactly what Eurotubes recommends:
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You can measure the plate voltage on any amp with octal power tubes using a digital multimeter set to measure DC volts, in the 1000 volt scale. From the underside of one of the tube sockets, simply measure between pin #3 and chassis ground. You can see this demonstrated here: euro-m
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