Hey have any of you guys seen these before and what are they used for . They are actually miniature metal valves.

Got this from Wikipedia.

The nuvistor is a type of vacuum tube announced by RCA in 1959. Most nuvistors are basically thimble-shaped, but somewhat smaller than a thimble, and much smaller than conventional tubes of the day. Triodes and a few tetrodes were made. The tube is made entirely of metal and ceramic. Making nuvistors requires special equipment, since there is no intubation to pump gases out of the envelope. Instead, the entire structure is assembled, inserted into its metal envelope, sealed and processed in a large vacuum chamber with simple robotic devices.

Nuvistors are among the highest performing small signal receiving tubes. They feature excellent VHF and UHF performance plus low noise figures, and were widely used throughout the 1960s in television sets (beginning with RCA's "New Vista" line of colour sets in 1961 with the CTC-11 chassis) and radio equipment and high-fidelity equipment, primarily in RF sections. They competed with the solid state revolution, and along with GE's Compactron, probably held it at bay for a few years. RCA discontinued their use in television tuners for its product line in late 1971. One famous application was in the Ampex MR-70, a costly studio tape recorder whose entire electronics section was based on nuvistors. Another limited application of this very small tube was in studio-grade microphones from that era, the AKG/Norelco C12a, which employed the 7586, being a good example. It was also later found that, with minor circuit modification, the nuvistor made a sufficient replacement for the obsolete Telefunken VF14 tube, used in the famed Neumann U 47 studio microphone.

I have just one of them in my collection. I would expect them to be used where high sensitivity is required in a small space.

Never seen one in my life I must admit.

I've seen them in TV tuners, and in ham radio VHF circuits. And as mentioned above, they were used in microphone amps, but I haven;t seen such, but then again I never worked in audio recording. Only playback. The physical small size works well for VHF and UHF work. Back in the late 50's and 60's, low cost transistors were not able to handle VHF and UHF.

Had a look in the collection, and I was quite wrong. In fact, I have 3 of them, all of type 6CW4.