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Author Topic: Telefunken AC 701 Tube Primer  (Read 32844 times)

Klaus Heyne

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Telefunken AC 701 Tube Primer
« on: December 07, 2004, 07:01:15 pm »

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 in ten Neumann-select (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.
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Klaus Heyne
German Masterworks
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John Bordon

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Re: Telefunken AC 701 Tube Primer
« Reply #1 on: December 08, 2004, 01:03:22 pm »

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?

JP
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John Bordon
Sound Sculpture USA

Oliver Archut

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Re: Telefunken AC 701 Tube Primer
« Reply #2 on: December 16, 2004, 12:13:57 am »

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,

Oliver


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Oliver Archut
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volki

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Re: Telefunken AC 701 Tube Primer
« Reply #3 on: July 12, 2005, 11:33:21 pm »

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.
K.H.


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Volker Meitz

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Re: Telefunken AC 701 Tube Primer
« Reply #4 on: December 17, 2005, 01:12:49 am »

Hallo Klaus,

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

Thank you.
TM
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Takashi Miyahara

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Re: Telefunken AC 701 Tube Primer
« Reply #5 on: December 17, 2005, 01:13:50 am »

pic2...
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Takashi Miyahara

Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #6 on: December 19, 2005, 03:05:02 am »

Looks totally fake to me.
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Klaus Heyne
German Masterworks
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Oliver Archut

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Re: Telefunken AC 701 Tube Primer
« Reply #7 on: December 19, 2005, 11:02:58 am »

As I stated in my earlier post: there are several copies of Telefunken AC701 tubes on the market, and this is one of them.

The tubes like you show on your photo were relabeled with a Telefunken logo by H&B [Heinze & Bolek] in Coburg, Germany, and found their way to Hong Kong, were a tube dealer has been offering them for a couple of years now.

Mostlof these tubes do not conform to AC701 specs, but the Telefunken logo made it appear real.

As info for all Telefunken AC701(k) buyers, who like to buy the real thing, attached is the stamping drawing from the original Telefunken blue prints.

Best regards,
index.php/fa/2058/0/
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Oliver Archut
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sekim

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Re: Telefunken AC 701 Tube Primer
« Reply #8 on: January 08, 2006, 12:05:15 am »

In case anyone needs a substitute submini for use as a capsule amplifier, you might want to try out a Raytheon 5784WB wired G2+G3+P=triode.  This is one of the very few submini pentodes that can be wired this way - most have G3+K as an internal connection.  

Note I have not used an AC701 and can't compare if it would be a similar sound.  I can only say that after testing many different submini triodes and pentodes (wired as triode), I found the 5784WB to be quite good in all aspects.  IMO similar to FET sound, only the things I don't like about FET sound are fixed - i.e. it's unconstrained and open sounding while being otherwise similar.  

Worth checking out if you need a decent submini and exact restoration is not a big issue.
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halljams

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Re: Telefunken AC 701 Tube Primer
« Reply #9 on: May 11, 2006, 01:39:41 am »

Is this a real Telefunken AC 701?


  http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=741357 6213&rd=1&sspagename=STRK%3AMEWA%3AIT&rd=1
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #10 on: May 11, 2006, 06:01:08 pm »

The tube frome the e-bay auction looks very real to me.
(Note the brazen use of whole sentences from this thread in the seller's description, to bolster his chances and make him sound more authoritative.
Bad manners not to attribute the source!)

I have added another shot of this Neumann-selected AC701k, because the eBay pictures may disappear after the auction:
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Klaus Heyne
German Masterworks
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ratite

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Re: Telefunken AC 701 Tube Primer
« Reply #11 on: June 07, 2006, 11:08:42 am »

For the tube to be installed in a KM or M mic how much length should be left on the leads?

Also do these appear to be real AC701 (left) and AC701K  (right) ?



index.php/fa/2927/0/
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Richard Horner
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #12 on: June 07, 2006, 02:03:12 pm »

Your two AC 701 look authentic.

The final length of the filament wires, at the time of installation, is determined by the location inside the specific mic:

You can cut a filament in a KM by about 10mm, but remember that you need plenty of length to position the wires AFTER they have been twisted, or curled (for heat sinking.)

No general length can be quoted for "M" mics, as they are all diffenent in physical layout.

In case of doubt, leave them uncut, or as long as possible.
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Klaus Heyne
German Masterworks
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Steve Culp

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Re: Telefunken AC 701 Tube Primer
« Reply #13 on: December 13, 2006, 04:00:22 pm »

Here are some photos of a pair of Ulm factory AC701 tubes in a SM2 serial number 128. This dates it around 1957 I think. I see that there are two vertical stripes on the tape... What do these mean? Some serious Telefunken trivia here... Help me out here Oliver! Or anyone else who knows... Winner gets .....  a warm feeling for knowing the answer!

Steve Culp
Triad Studios
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Steve Culp

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Re: Telefunken AC 701 Tube Primer
« Reply #14 on: December 13, 2006, 04:03:04 pm »

Photo #1

index.php/fa/3862/0/
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Steve Culp

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Re: Telefunken AC 701 Tube Primer
« Reply #15 on: December 13, 2006, 04:06:02 pm »

#2

index.php/fa/3863/0/
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Lukas Heyer

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Re: Telefunken AC 701 Tube Primer
« Reply #16 on: March 12, 2007, 06:23:40 pm »

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
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Frankenheimer

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Re: Telefunken AC 701 Tube Primer
« Reply #17 on: March 20, 2007, 09:04:28 am »

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
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #18 on: March 20, 2007, 01:42:12 pm »

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.

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Klaus Heyne
German Masterworks
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John Stafford

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Re: Telefunken AC 701 Tube Primer
« Reply #19 on: June 10, 2007, 04:12:09 am »

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
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #20 on: June 10, 2007, 11:11:08 am »

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
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Klaus Heyne
German Masterworks
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #21 on: November 21, 2007, 04:20:27 pm »

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.
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Klaus Heyne
German Masterworks
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #22 on: November 21, 2007, 04:20:30 pm »

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.
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Klaus Heyne
German Masterworks
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Lemoine

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Re: Telefunken AC 701 Tube Primer
« Reply #23 on: October 10, 2009, 12:08:00 pm »

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?
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Klaus Heyne

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Re: Telefunken AC 701 Tube Primer
« Reply #24 on: October 12, 2009, 03:40:18 pm »

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.)

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Klaus Heyne
German Masterworks
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