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Measuring the cathode current can help you determine if its weak...measure voltage drop across 1000 ohm resistor in the cathode circuit (400 volts both sides of resistor.) Measure with the set's brightness turned all the way down.
Nominal current is 1 milliamp, (1000 microamps,) equaling 1 volt drop across resistor.
If the 6BK4 is weak you may only get around 300-500 microamps (.3-.5 volts)
Symptoms of a bad tube are usually a picture that changes size when the brightness is changed.
I don't think there is a way to really test it in a tester as you really need high voltage to get it to conduct.
It is recommended that the latest version of the tube be used as it has the best x-ray protection...I think the latest one is 6BK4C/6EL4A.
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