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Author Topic: Neumann U67 FET conversion back to original  (Read 4748 times)

Theo

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Neumann U67 FET conversion back to original
« on: October 09, 2014, 11:13:16 PM »



My name is Theo,I am a Producer/engineer from South Africa.
Years ago I bought two Neumann U67 microphones from an engineer friend of mine. One of the microphones came original with the PSU included. The other microphone was converted to a FET version somewhere in the eighties. I bought it in this state with the hope of having it converted back to being a tube microphone , hopefully theres a way that can bring it back to the original sound of a U67? I am not that knowledgeable on microphone electronics, and would be very grateful if this forum could assist in suggesting what, in any, my options would be, as well as suggesting a suitable technician for repair?
I have included some photos for you of the FET. I do realise you know these microphones inside out, but  I've also included photos of the FET conversion next an original one just for easy reference for the sake of comparing what parts might be missing.
I would be grateful if this forum could assist me in finding a sollution.
Thank you so much for your time and assistance
All the best
THeo
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polypals

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Re: Neumann U67 FET conversion back to original
« Reply #1 on: October 14, 2014, 05:48:33 PM »


As far as I can see from the pictures most typical components like the output transformer are still there.
That would make a reconstruction possible by a qualified engineer.
I wonder why the U67 was converted to FET technology. Even today there is no shortage of EF86 tubes produced by reputable companies like Telefunken, Mullard and Philips to name a few.

The EF86 in your tube U67 is probably a Mullard or Philips one.
Neumann fitted Telefunken EF86 tubes to their U67.  Klaus says the original Telefunken tube sounds different.
Neumann selected tubes for minimal noise.
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Uwe

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Re: Neumann U67 FET conversion back to original
« Reply #2 on: October 14, 2014, 09:54:05 PM »

Suitable EF 86 or equivalent are still obtainable from various sources. Unfortunately, the printed circuit board with the tube socket has been removed in the (somewhat sloppy) conversion to FET impedance converter. Unless you can scavenge this board from a scrap U67, a faithful conversion to original may be between difficult and impossible. A skilled technician may be capable to recreate this critical part, and then build up the original circuit with contemporary passive components. BTW, in the unprofessional FET conversion the main pcb appears to be turned around as well.

Assuming the tube amplifier module is in good or perfect working order, before further contemplating restoration I recommend to swap the head assemblies to make sure the capsules, pattern-, attenuator- and bass roll-off switches are OK.
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Jim Williams

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Re: Neumann U67 FET conversion back to original
« Reply #3 on: October 15, 2014, 11:26:48 AM »

One can regenerate that missing pcb with some low cost freeware pcb layout tools like Eagle. Then a fab can make the pcb for about $100. If you have access to one of those prototype pcb cutter tables, you can mill out that pcb for about fifty cents.
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polypals

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Re: Neumann U67 FET conversion back to original
« Reply #4 on: October 15, 2014, 06:04:09 PM »

Given you find the right person to reconstruct the U67 it can be done.
Considering the current prices for good U67 mikes it is a good idea to bring the mike back to its original condition.
Your only problem now is to find the right guy for the job.
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klaus

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Re: Neumann U67 FET conversion back to original
« Reply #5 on: October 16, 2014, 03:23:29 AM »

Such 'reconstructed' U67, containing aftermarket circuit boards and other assorted DIYs, will be of considerably less collectible value than an all-original U67.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

polypals

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Re: Neumann U67 FET conversion back to original
« Reply #6 on: October 17, 2014, 03:47:56 AM »

Of course, but it will be a better performing U67 than the current one with questionable FET conversion.
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Theo

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Re: Neumann U67 FET conversion back to original
« Reply #7 on: October 17, 2014, 07:55:16 PM »

Thanks everybody for your advice and your input. I have tested both head assemblies, and they seem to be working correctly.
I cannot seem to locate any PCB spares for a U67, it looks like my only alternative will be to get a PCB made up like Jim is suggesting.
As long as the mic sounds like a tube mic again, and the technician did a good job, I don't mind what it looks like on the inside.  It just has to sound right.
Thanks once again everybody for your advice
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polypals

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Re: Neumann U67 FET conversion back to original
« Reply #8 on: October 17, 2014, 09:33:21 PM »

Like me you do not only see the collectable value but also the use of the U67 as important asset.
Wish you luck finding a motivated craftsman for the job.
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