On a mass spectrometer I recently replaced an old-fashioned triode gauge tube and these are the same size glass as the 9-pin miniature radio tubes of the 1950's

On a mass spectrometer I recently replaced an old-fashioned triode gauge tube and these are the same size glass as the 9-pin miniature radio tubes of the 1950's. The base looks like an audio 12AX7 and fits the same size socket, the metal pins are supposed to be Kovar coming through the glass. But this electron tube is not sealed at the top, it is wide open with a Kovar metal precision adapter ring fused to the glass instead, and a Viton o-ring is used to lock it into the vacuum measurement port. As a vacuum tube, it "simply" behaves differently depending on residual trace gases present. It is connected to a circuit optimized to give calibrated vacuum readings. No doubt I could connect it to an audio circuit instead, and get some excellent output. It's a well-made tube, only a single triode though.