The C3m is a loctal socket metal-shielded pentode use by the German Post for audio repeater amplifiers in long distance telephone lines. They are extremely linear and were rated for 10,000 hours of use.
The use of this tube was popularized by Thorsten Loesch in his 'Legacy' 300B amplifier. There are also a few commercial pieces of gear (eg Ayon Audio has a full function preamp) that use this tube. I have experimented with these connected as a triode and my experiences reflect this use.
Breaking a defunct one open and removing the mesh shield, I found a black plate inside with beefy construction. There are no side supports on the mica, which leads to one of many issues with these tubes - microphonics. They are extremely microphonic - reminding me of an old 71A globe linestage I built many moons ago. The switch bounce from my Goldpoint stepped volume control was easily picked up and amplified by the C3m. I tried many brands of C3m tubes (Valvo, Siemens, Lorenz, Philips) and got varying results but some microphonics always remained. So if you do use this tube, I strongly suggest using an extremely heavy case.
Another issue is the loctal socket and 20V/125mA filament requirements. This puts them in the realm of DIY use. I used a 24V transformer with an regulator board using a 7820. The more rare (read expensive) C3o uses 6.3V filaments but is an equivalent.
Another concern is fragility. When making my linestage, I once fired it up for 2-3 minutes to take some voltage readings. I pulled the C3ms out, made some changes and put in the C3ms back in. The movement of the heated tube was enough to make one stop working - ie, it would no longer draw current. I don't suggest ever 'hot-switching' tubes, but it is something to consider if doing some work on a C3m equipped unit. Let them cool down for a few minutes before moving.
Ok - how do they sound? Connected as a triode and used in a linestage, I found them to be very neutral. No glare or grain, they are even-handed from top to bottom. They reminded me more of a really good 6SN7 'type of sound' than (let's say) a 6DJ8.
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