The question of how to take electrode measurements in a tube amp often comes up. Below are some photos of how I take measurements from the milliAmp. Note, though it appears my hand could be resting on the chassis, it is not. In general, you want no electrical connection between your person and the unit under test. Even touching the chassis ground may be unsafe, especially in a circuit that is malfunctioning or improperly constructed. Exercise extreme caution when measuring live tube circuits. Always keep one hand in your back pocket and pay close attention to where you working hand is at and what you are doing.
Click on the images for a closer view.
At the junction of C2 and R8:
At the junction of R4 and V1-pin-1:
At the junction of R2 and V1-pin-6: