Telefunken AC701 vs. AC701K

[Max Crace]

Could someone please enlighten me as to the difference between the AC701 and
the AC701K tubes? I used to have a Neumann SM2 and I am pretty sure that it
had AC701K tubes in it but I don't have that mic anymore. I just bought a
Neumann SM23c and supposedly the mics are the same with the exception that
the SM23 uses a different power supply. But I notice that the SM23 has AC701
tubes without the K. Any comments from a friendly tube guru?

Thanks.

Max Crace

[Kurt Drillen]

Max Crace schrieb:

Hello,

The " k " in several old Telefunken tubes means " klirrarm ".

Means " less distortion ".
In most cases the standard AC 701 works proper !
The AC 701 is VERY VERY rar and expensive nowadays.
Here in germany about 130.oo USD up each NOS, if exists ...

Regards,

Kurt

[Lowell Cross]

I am certainly not contradicting what Kurt Drillen has written below, but
it is my understanding that the "k" in "AC 701k" stands for "klingarm"
(low ringing, or low "microphonics").

My information comes from a German physicist living in the Koeln/Bonn
area who, as a hobby, collects rare Telefunken tubes. He has informed
me that many, but perhaps not all, of the AC 701k tubes had the grid
welded at each contact point on the support rods, to reduce the
possibility of mechanical vibration.

It is possible that the AC 701 tubes (no "k") did not undergo this
special step in manufacturing, but I have no direct information on this
issue. Perhaps I'll ask my friend the next time I exchange email with
him.

In any event, one does not want a "microphonic" tube in a microphone!

Lowell Cross

[Kurt Drillen]

Lowell Cross schrieb:

Hello,

RIGHT: "k " means low ringing, low microphonics and less distortion !

Some "k" AC 701 types has more mirophonic effects than standard
AC 701. Best is to choose them before buy ...

Regards,

Kurt