R/E/P > Klaus Heyne's Mic Lab Stickies

Telefunken AC 701 Tube Primer

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Klaus Heyne:
The first chapter, below, is a slighly edited condensation of Oliver Archut's historical knowledge of the AC701.

"In the beginning Telefunken had two different AC701 versions, both low noise but one with the K designation was a frame grid construction vs. a round grid for the none K. That means that the K stands for 'klingarm' (low microphonics) for the frame grid version. The AC701 without the k was made for the audio section in AEG-Telefunken handheld two way radios for the west German army as well as some other military equipment.

"Neumann only used AC701k tubes that were pre selected in Ulm at Telefunken's factory and then selected them again for random noise and microphonics, if they passed, it got the wrapper (with an internal ser.#) around it. The "k" versions that didn't make it at Telefunken became standard AC701.

"Since 1965 Telefunken only made frame grid AC701 so there was no need to specially mark them. But careful:Telefunken also sold RFT Neuhaus-made ones under the "Telefunken" name that can be quite noisy and more microphonic than the Tele-made ones. Those ones had a white dot vs. a red indicator dot (for identification of the anode/plate-filament)

"The AC701 works nearly with any gain and mostly for impedance conversion so distortion test is not an issue here. I never had any problems with high distortion AC701 tubes, mostly with noise in the first case, rather than microphonics.

"A noisy AC701 you can sometimes save via ionization discharge over a 50 Meg resistor, but you have to come up first with a 1KV power supply and then the chance is about 30% that it will work properly over a longer time. Also it is quite likely you may get shocked!

"You can see on the tube date code when the tube was made:

If you have U for Ulm and then two letters, sometimes with two numbers, it is made before 1965 if you have U and seven numbers then it was made after 1965.

"If you buy an AC701 make sure you get an original, still in its sealed box, because 95% of all AC701 tubes out there for sale are Neumann rejects. If the tubes are not in the sealed box, get a money back guarantee for at least a week before buying.

    ------------------------------------------------------------ ---------------------

To add to Oliver's primer, 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 to two out of ten Neumann-select AC701 (in Neumann-box, with Neumann serial number on paper band around tube, or Neumann number hand written in white ink) 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. Measuring noise, whether steady-state or sporadic, in tubes to be placed in super-high input impedance circuits is not possible with conventional testers. Those that are designed to work in that application cost several thousand dollars, even used. And even then, certain types of noise are not captured, accounting for quite a high percentage of returns from Neumann back to Telefunken during the period AC 701s were manufactured.

That's why I never bothered to invest in one of those testers, but still use the good old ear method of testing: wire it up, set the correct voltages, plug in, let run for seven days, and be satisfied that 90% of all tubes that are still quiet after withstanding that type of testing for a week are guaranteed to stay quiet for at least a year.

6. 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.

7. 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.

7. Please note that the AC 701 is very sensitive to oversupply of heater voltage and can easily self-destroy under those circumstances. Therefore, many dealers are reluctant to sell a tube worth several hundred dollars with a money-back warranty unless they have control over the installation and are satisfied that the customer's power supply was correctly adjusted before installation.

8. I adjust my AC701 buying prices to the rule of thumb explained above: I discount each tube price so that the percentage of failure per tube is built into the price. That way I don't need to bother with money-back warranties which few sellers will give me anyway.

John Bordon:
Thanks Klaus and Oliver for the AC701 Primer!

Where did AKG C60 fall into the distribution of the AC701 tubes?
I have an AC 701 k in my C60  with a post-1965 date code U, then 7 digits.  But a separate 3 digit serial number is also on top of the tube. Is the 3 digit  added by AKG?


Oliver Archut:
The three digit number is the "Sperrlager Number" (locked stock) that means nothing else than a warranty number.

Telefunken made the AC701 in sets of 1000. For every batch 50 to 100 tubes were randomly selected for simulated lifetime tests. If the test revealed the probability of the required min. 5000 hours, the "locked stock" was released for final testing (the test circuit was developed by 'Schalltechnik', better known today as Schoeps.)

There are several none-Telefunken-made AC701 (labeled AC761, made by RFT) that say "Telefunken" Those tubes do not have this three-digit number.

Best regards,


Sometimes used AC701's pop up on ebay, which have twisted / cut legs and thus obviously have been unsoldered from mics.

Since installation of an AC701 requires much more effort than swapping out standard, socketed tubes, I figure that these tubes are not coming from studios which used to change tubes in their mics every year or so by default.

Obviously, the only reason to replace this tube in a mic would be that it actually went bad, no? So these used tubes for sale either must be noisy, have not enough gain, or even may be completely dead?

Agreed. Used AC701s offered on e-bay or elsewhere should only be bought with a money-back warranty.

Hallo Klaus,

Is this real AC701K? I have never seen AC701 stamped like this....

Thank you.


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