Hivac four-pin (SM4)

Hivac SM4 has four contacts arranged as shown in the bottom view. Contact 2 is larger than the other three.

The diameter difference is a mechanical identification feature, not a universal assignment of electrical functions. Contact numbers must be read in the direction of the illustrated view.

The full designation Hivac SM4 must be retained. SM alone, or the pin count alone, does not define every subminiature construction. Other Hivac families and AVO adapters with similar designations remain distinct.

Hivac miniature bases were introduced in the 1930s for valves designed for hearing aids. Miniaturisation thus met a specific application. No more precise introduction date for SM4 is established here.

SM4 and SM5 have different contact counts: SM5's pinout cannot fill in SM4's. On the four-contact construction, the enlarged pin 2 is an identifying feature to be read in the correct view.

4662 is an example of the family. Verified terminal diameter, spacing and projection are not available here. Miniature does not define a metric scale or make this base equivalent to other four-wire constructions.