.
The demands for these tubes today are primarily in vintage and recent design audio amplifiers and preamplifiers of the high-fidelity type, up to and including the most expensive and esoteric.
See: 'More' - 'Manufacturers Index' - 'Siemens'.
See: 'More' - 'Instruments' - 'More'.
Hopefully everything is still alright! Most have flat gray plates.
These are often requested, but rarely found.
Typically, the lifespan of these devices ranges from about 500 to 1000 hours.
But it had too low a mu to be really suitable for pass-tube use.