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[Steve Culp]

#2

[Lukas Heyer]

Oliver Archut wrote on Thu, 16 December 2004 05:13

[...]the "locked stock" was released for final testing (the test circuit was developed by 'Schalltechnik', better known today as Schoeps.)

Oliver, it would be very interesting to see the test circuit's  schematics - can you or anyone else here please share it with the rest of us?

Many thanks,
Lukas

[Frankenheimer]

Hello Klaus,
i found a tube from 1965 from the Ulmer Factory.
Details: U3206503, lfd. Nr.4333. (Bj. 1965).
Is it worth buying fo a C60 ?
The dealer told me that the tube is original, no thrown out neumann or AKG stock.
The cost is about 300

[Klaus Heyne]

Whether it's worth it depends on your love for the AKG C60 mic, and how much you have already invested.

You should read the beginning of this Stickie once more:

Quote:

here is some empirical data, from my 20-year experience with the tube: 1. Two out of three non-Neumann select AC701s (non-k) are not usable in microphones: they will have steady-state white noise or irregular discharges, beyond an acceptable level for critical recording situations. 2. One out of three non-Neumann select k are unusable, for the same reason. 3. One in 10 Neumann-select (in Neumann-box, with Neumann serial number on paper band around tube) are unusable. 4. Never judge an AC701 before at least a 48hr. burn-in (i.e. the tube is installed in the mic for which it is intended, and voltage-adjusted to the proper operating voltages): One in three tubes which were fine within the first hour of testing will develop discharges later on, especially once the tube has reached its thermal stability ( thermal stability = operating temperature of the tube, once the tube's heat dissipation through the heat sinking around it has stabilized, and the tube and its surroundings won't get any hotter). 5. Unlike tubes which you plug in, you cannot easily and thoroughly test an AC701 for noise outside of the mic, for example, with a tube tester. This tube needs to be soldered into the mic for testing. 6. Unfortunately, you rarely will be able to return AC701 tubes once you have installed them because the seller cannot easily sell an AC701 to the next customer as new, because its filament wires have already been cut and twisted for installation. This is especially true for Neumann KM-series.

[John Stafford]

I really want to buy an SM69. However, one major concern is the long term viability of a stereo mic based around the AC701. Like everybody else, I am having trouble finding single specimens for mics that need replacement valves. Is it realistic to hope to find a pair that is suitably matched for a stereo mic (should anything go wrong), or would any two good performers do the trick?

Many thanks
John Stafford

[Klaus Heyne]

To my knowledge, there was no tube selection at the Neumann factory beyond the low-noise preselection which was performed regardless of final mic destination.

I.e., pair selection for tubes was probably not performed, as far as everything I have read and know.

So you should be fine once you have found a low-noise specimen AC701, even for a stereo mic application

[Klaus Heyne]

Not all genuine Telefunken tubes have the embossed diamond on the bottom. Besides, the AC 701 has four closely spaced filament wires sticking out of the bottom of the tube- there would be no space to add a diamond.

[Klaus Heyne]

Not all genuine Telefunken tubes have the embossed diamond on the bottom. Besides, the AC 701 has four closely spaced filament wires sticking out of the bottom of the tube- there would be no space to add a diamond.

[Lemoine]

Hi,
I have a AC701 (no K) inside a Schoeps M221 I just got. I don't know if the mic is working yet as I have no PSU for the moment.

If I read the primer correctly, it seems to be a post 1965 tube. And if I read correctly, it does not necessary mean that this is a non selected tube or a switched out one right? In other words, this might be the original Schoeps factory tube even if there is no "k", right?

[Klaus Heyne]

Please contact Bernhard Vollmer ar Schoeps: vollmer@schoeps.de

Bernard is the preeminent authority on what kind of AC701 tubes might have been used by Schoeps in their M221 models.

In general, non-'k' tubes are not inherently second-choice tubes, but were and still are quite often used in first-rate mics. (see more about the 'k' designation earlier in this thread.)