At some point, there should be a Primer started in the stickies about the Nuvistor.
In brief: the mid-Nineteensixties saw the first wide-scale commercial application of the transistor. Those companies who were not ready to radically redesign their audio equipment to the new kid on the block were relieved when RCA introduced a sub-miniature, metal enclosed tube they named the Nuvistor (very clever double entendre!), which was to compete head-on with the transistor.
This tube was made available in penthode, triode, and specific models were designed for pre- and line amp amplification gain.
AKG and Neumann, realizing rather few changes were necessary for the adaptation of their existing models to these new tubes, jumped on it, and revamped the entire lines of their previously AC701 (or VF14) equipped mics, in the hope to fend off the impending transistor revolution.
What they did not realize early enough was that Nuvistors had some nasty habits which made them objectively inferior to tubes in their role as impedance converters:
1. Almost always audible ringing of the closely spaced filaments inside the tube (i.e.microphonics)
2. Almost impossible noise selection- few samples were quiet enough, and lasted long enough, to eventually avoid the wrath of the consumer.
3. Many of the mics' original transformers (optimized for specific tube types) were not altered to accommodate the different impedances of the Nuvistor.
4. AKG specifically tried to circumvent some of the inherent problems of the Nuvistor by re-designing their processors as cathode followers, and copied the circuit from the recently introduced sub-miniature C60 (AC701.)
The mating of Nuvistors with a cathode follower has not been particularly successful for the preservation of timbre or personality of tube mics.
In consideration of the above, one if not the main reason why a Neumann U47 Nuvistor will fetch considerably less in the collector's market can be explained with the three issues I cited above.
The Nuvistor's impedance mismatch in particular renders the formerly full low end of the U47 mic anemic, constricted and nasal.
A slightly off-topic P.S.:
I was for many years the pround owner of two Ampex MR 70 Nuvistor-equipped master tape recorders, which were later equipped and modified by Paul Stubblebine Mastering with Tim De Paravici 1/2" two-track heads.
This was one of those magical matings of machinery and tubes: I have never heard a finer sounding tape recorder before or after.
(Except for the constant pain of maintenance, especially, noisy and frequently failing Nuvistors.....! )