R/E/P Community

The above replies are why I love this forum!
Thanks to both Klaus and soapfoot for sharing their knowledge!

Klaus—if your last (pre-P.S.) sentence was addressed to me, first thank you for the kind words. I own two mics that utilize AC701s and thankfully both are behaving themselves at this time, so if (when?...) I find myself needing to no longer leave well enough alone, then I'll consider installing the Phaedrus and reporting back.

I look forward to seeing if any other forum readers have any experience with it.

I see two problems with the way Phædrus advertises its 6D-H3:

1. It is not a tube, as soapfoot stated, but a solid-state emulation. I find it troubling that a manufacturer refers to a solid state device as a tube and uses terms germane to tubes ("heater voltage", "triode", etc.) when the common definition of a tube is, per Wiki:

A vacuum tube or electron tube is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
It utilizes thermionic emission of electrons from a hot cathode for fundamental electronic functions such as signal amplification.

2. This solid state device is advertised as a direct replacement, therefore claimed equivalent in performance, to the AC701 triode.
All tube emulations I have heard, including those made by Phaedrus, sounded pedestrian to me. They will pass common function tests with more or less noise associated with it, and may be ok for beginners not familiar of the timbre and dynamic behavior of tubes. I.e. a perfect starting point for DIY's.

But there are better and cheaper alternatives in that category, like the 6S6B, a Russian military type triode tube. (Neumann uses what looks like a relative of that tube in its M49V.)

In the end I question the idea of installing a tube simulation in any of the most revered, iconic, breathtaking mics ever made- those originally designed and optimized for the Telefunken AC701.

Knowing your excellent ears and discrimination for good sounds, I encourage you to buy one of these simulations, install it and report back with what you hear.

P.S.: I also invite anyone who has personally heard and compared solid-state emulations to post his/her experiences.

It's left vague in the manufacturer's literature, but you should be aware that this is not a vacuum tube, but rather an equivalent-circuit substitute utilizing semiconductors.

Earlier today I learned of a company that is apparently currently manufacturing an AC701 replacement tube: http://www.phaedrus-audio.com/6D-H3%20triode.htm. Has anyone had any experience with these tubes?

Klaus,Thank you very much.

Both videos show how the mic comes from the factory: plastic wrapped in the form-fitting tray inside the suitcase.

If your mic does not have the protective wax seal on the head, there are only two possibilities:

1. someone had opened the head, and Neumann likely will not honor its one-year warranty for the mic.
2. someone forgot to put the factory seal on (this would be a first)

Open the head
(https://germanmasterworks.com/publish/articles/Advice/Step-By-Step-Instructions-On-How-To-Open-A-U87-Head-%28Same-as-U67%29.html) 
and make sure that you have the original capsule inside (it should be dated "21" or "22").

Thanks for the reply Klaus, btw you were the last to work on it when I sent you the head back in 2002.
Do you know anyone who might have an original capsule 1/2?  That would be my preference, otherwise i have to decide if I want to put a new capsule in its place.

Klaus, thanks.
Found a couple of videos on youtube:

https://www.youtube.com/watch?v=WB5ZAIOV6Pk&list=LL&index=17

https://www.youtube.com/watch?v=P0qRZamthFM

If you look at my photo, there is no protective seal.

Given the amount of conductive gold remaining, the rear diaphragm may still function (you did not mention whether it does).
But the rear may have suffered too much loss of s/n, due to loss of gold and/or heavy contamination.

Though the front side looks also moderately contaminated, the mic should work fine in cardioid, as long as the membrane tension of the rear is still ok (it should be unaffected by the gold loss).

Your options, then:

1. Leave the capsule as is and use the mic, at least in cardioid.

2. Find a used, period-correct (second generation) K67 and either replace the whole capsule, or, if you are really happy with the mic in cardioid, sandwich the current frontside with one half of the donor capsule.

3. Replace with a newer K67. If you can find a K67 made before 2000, the overall timbre of the mic should be relatively similar to that of your original capsule. (You had mentioned you thought that new K67/870 are scarce. That is not the case: you can buy a new one from Neumann without any wait, but that version has an audibly different timbre.)


What is not an option: to assemble a non-brass K67 half on the rear of your current brass K67. I have not been able to mak ethat combo work in the past, due to various design differences.

Let us know what you decided to do, and how it affected the sound.

I have an old Telefunken U67 w/orig capsule that has a good front diaphragm, and a questionable back diaphragm. Please see the pix. Looking for feedback on its plight.

It works fairly well, but I am concerned about the back diaphragm. Seems new U67 or 870 caps are scarce, I am wondering if it would be best to leave alone, or try to procure a 1/2 capsule to replace the back.

Is the back diaphragm repairable? I contacted Neumann USA, but their only option was to send it in.