After finding the base of the triode, we can determine the conductivity type of the tube according to the direction of the PN junction between the base and the other two electrodes. Touch the black test lead of the multimeter to the base and the red test lead to any one of the other two electrodes. If the deflection angle of the meter pointer is large, it means that the transistor under test is an NPN tube; if the deflection angle of the meter pointer is small, Then the tube under test is PNP type.