[N2IXK]

The 4422 is one of RCA's "mystery" types in the 4000 series. Many of these had little or no published data available, being modified or selected versions of standard types, usually for sale to a single customer.

I have access to a fair number of internal RCA tube division documents (including a bunch of the 4000 series types) and a shelf full of tube databooks, but nothing on the 4422. B&K themselves suggested the use of the 931A as a replacement type, rather than offering the 4422 as a replacement part. This would lead me to believe that it may have been an "off-spec" 931A that was "good enough" for the intended application (which isn't all that demanding compared to most PMT applications in scientific instruments and such) , and was sold to B&K at a discount. Not all "selected" tubes were "better" than their prototypes, and this MAY be one of those cases.

Only a former engineer from RCA or B&K would be able to give the definitive answer on this one, I'm afraid...