I have a quartet of Siemens C3Gs, two bought NOS around 2 years ago and used about 200 hours, two used, and these are terrific driver tubes. I do not know about Telefunken or another brand C3Gs, but I suspect there is little difference between them. All were made in the sixties, or early seventies, under a strict West German, mil spec requirement typical of cold war for undeground telephone signal amplifiers that not only will survive a nuclear EMF pulse, but will amplify and forward even the tiniest communication signal, with utmost fidelity, and last a lifetime...cost no object, this class of tubes were designed to perform, with the very best and very expensive exotic metals, a Thorium alloy (I think) to absorb oxygen atoms inside the tube to preserve absolute vacuum...an all out assault to design a super reliable hybrid pentode/triode audio amplification tube. The specs of the C3G are unmatched, transconductance is enormous, something to expect in a tube designed to amplify weak signals reliably...this in itself does not relate necesarily to sound quality, but it does in the C3Gs. And power, dynamics, bandwidth, they will reveal ruthlessly the limitations of your power tubes. For example, 6SN7G double triodes were a clear step ahead over the stock tubes in my LD MKIII, and the C3Gs make them sing clear and loud, the differences just jump out, only a few were up to the task. I went all out to mod my little preamp to accept the mighty 6AS7G big bottles, and after that even the best 6SN7Gs sounded small, limited, restricted. The C3Gs clearly had the power and the amperes to control the big ones solidly, superior dynamics, fast, tight bass and a soundstage to die for... and the little thing is working as a preamp driving 2 mono SS NAD Amps. Extremely powerful as a Pre, I learned to set the NADs at about 1/4 output... Cheers to all !