Well Joe, they were good for 1950'-1980's and maybe a little bit beyond? RAAF Aircraft intercoms which may be of note and were ultra reliable in Canberra and other multi crewed types where vibration was present but clear intercom was paramount.
I have nothing to show now but did make myself a few pre-amps and headphone amps a fair while back and found these near pinnacle of development valves to be an advance on many earlier B9A types.
A quick google for 6112 tubes /microphonics /pre-amplifiers will give you several hours of reading.
I found the Russian gold alloy grid 6n17B-V to be super exotic at a really good price
