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Author Topic: Improving Gefell UM92  (Read 22955 times)

Klokkern

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Improving Gefell UM92
« on: February 27, 2008, 11:05:06 am »

Hi all

I have decided to try to improve my UM92. As far as I know of, the first thing I should do is change the tube to a good quality NOS tube. I have just bought one from ebay, which I hope will work well.

  http://cm.ebay.com/cm/ck/1065-29392-2357-0?uid=101529067& ;amp ;amp   ;site=0&ver=EOIBSA080805&lk=URL&Item=26020944557 9

I am in no way capable of determining if this is a good tube from looking at the pictures, so if anybody out there can tell from the pictures if this is a genuine telefunken tube or just a rebranded lower quality tube (meaning I have spent my money in a silly way...), I would be happy to know.

Apart from upgrading the tube, the output transformers were improved from the UM92 to the UM92.1, so I guess an upgrade in my mic would also be a good thing. The first thing that comes to mind is to get a Lundal transformer ( http://www.lundahl.se/ ). Does anybody have any other suggestions on this??

As most of you have already guessed, I am NOT going to do this myself, so this thread is meant to be forwarded to the SLIGHTLY more skilled guy who will help me with this.

Thank you!

LarsK
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Klokkern

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Re: Improving Gefell UM92
« Reply #1 on: February 27, 2008, 11:10:39 am »

....I forgot the reason for posting this....

Does anybody have other improvment suggestions while I am at it??

lk
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Barry Hufker

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Re: Improving Gefell UM92
« Reply #2 on: February 27, 2008, 11:30:57 am »

If you search Klaus' forum you'll find a lengthy and extremely helpful discussion on modifying the UM92.1S.
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Klokkern

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Re: Improving Gefell UM92
« Reply #3 on: February 27, 2008, 11:34:40 am »

I did (and found the thread where you did a mod on yours, one of the reasons I decided to upgrade tube & transformer on mine), but since this is an UM92 and not UM92.1, I thought there might be more / other things I could improove too:o)
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compasspnt

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Re: Improving Gefell UM92
« Reply #4 on: February 27, 2008, 01:14:41 pm »

The big change can come from transformer and electronics, including the valve.

Oliver has a lot of good changes to do.

He is highly recommended for this.

And of course Klaus, as always.
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J.J. Blair

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Re: Improving Gefell UM92
« Reply #5 on: February 28, 2008, 12:16:57 am »

Replacing the tube with a high grade Telefunken EC92 should improve the noise floor, but I'm not sure that it will improve the sound quality.  

As far as I understand it, and somebody please correct me if I'm wrong, the difference between the mics is the tube used.  If that is the only difference, the NFB circuit should be the same, and Oli's modification, in the aforementioned thread, should probably be the same.  

I can say for sure that Oli's replacement transformers make a huge improvement.  
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Klokkern

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Re: Improving Gefell UM92
« Reply #6 on: February 28, 2008, 04:25:22 am »

Hi, and thank you for your help.

I have found the following about the UM92 vs 92.1:

"The difference between the Gefell 92S and the 92.1S is.... the 92S had a heater voltage adjustment inside the microphone. It was made for running cable lengths of longer than 50 meters. Gefell wound up not being satisfied with having it in the body of the mic. With the 92.1S the heater voltage adjustment was moved and is housed in the power supply.

In addition to the heater voltage adjustment location change, they also changed from an EC92 triode tube to an EF86 pentode tube. There was some additional RF noise rejection circuitry added (a bad idea which most people who use them to record have removed). According to Oliver Archut at TAB Funkenwerks, the output transformers on the newer versions were also improved.

I highly recommend that you replace the original East German tube with a Telefunken tube. You will notice an immediate improvement in the sound quality. As far as I can tell, the mic was built according to the same schemeatic as a Gefell UM57 but with improved components."


As I understand it, the NFB circuit was added to the 92.1, and is NOT in the 92. The heater voltage adjustment is something I wasn´t planning to move at this point.

I am still not certain if this is all the differences between the two mics, so further input is welcome:o)

Best regards
larsK
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ricknroll

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Re: Improving Gefell UM92
« Reply #7 on: February 28, 2008, 12:54:45 pm »

J.J. Blair wrote on Wed, 27 February 2008 21:16

Replacing the tube with a high grade Telefunken EC92 should improve the noise floor, but I'm not sure that it will improve the sound quality.  


J.J.,

I'm not sure if the same is true for the UM92S (different tube, I know), but regarding the UM92.1S, Klaus wrote, "You can improve the sonics by switching to any good NOS European EF86. But the difference in noise floor between a selected Sovtec and Euro tube is 1 dB at best.

The overall marginal noise floor of this mic can only be remedied to standards we are used to from Neumann et al if we up the B+ power supply voltage considerably, redesign the entire circuit, replace the transformer, the power supply.... i.e. junk the amp and supply altogether, to get at the sound the M7 is capable of."

Maybe Klaus will clarify whether or not his findings above can be extrapolated to the UM92S.  I believe there is another thread in the forum in which either he or Oliver explains that earlier UM92S does not contain the same feedback that Oliver recommends removing from the UM92.1S, which is one of the big differences between the two mics.  

I have a UM92S and I will say that replacing the stock tube with a NOS Telefunken from Tube World did not make a noticeable difference in the noise floor, so I suspect what Klaus wrote about the UM92.1S is also true of the UM92S (and its different tube).

Rick Hedges
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J.J. Blair

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Re: Improving Gefell UM92
« Reply #8 on: February 28, 2008, 06:04:49 pm »

OK, I guess I was wrong on both accounts!  Thanks for straightening me out on that, guys.

I do know this, from my UM57s: Using a Telefunken EC92 and replacing the Geffel transformer with Tab Funkenwerk transformer resulted in a better sounding mic.  If the UM92 is based on the UM57, and doe snot have the NFB that the UM92.1 does, which I find to make that mic too dark, then maybe just the things I did to my UM57s should make your mic as improved as it did mine.  

BTW, I bought tubes from that same seller listed in the first post several times.  He's a good seller.  You made a good choice.  
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studio info

They say the heart of Rock & Roll is still beating, which is amazing if you consider all the blow it's done over the years.

"The Internet enables pompous blowhards to interact with other pompous blowhards in a big circle jerk of pomposity." - Bill Maher

"The negative aspects of this business, not only will continue to prevail, but will continue to accelerate in madness. Conditions aren't going to get better, because the economics of rock and roll are getting closer and closer to the economics of Big Business America." - Bill Graham

Barry Hufker

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Re: Improving Gefell UM92
« Reply #9 on: February 28, 2008, 07:16:55 pm »

When I modded my 92.1S (per Oliver_) replacing the original tube with a Telefunken made a +*world+* of difference.  Not in noise floor necessarily but in sound quality.  Worth every penny.  And I bought from Tube World.
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J.J. Blair

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Re: Improving Gefell UM92
« Reply #10 on: February 28, 2008, 08:16:07 pm »

Wow.  I only heard the noise floor difference when I swapped mine.  But perhaps it's different with the EF86s.  I mean, the RFT tubes in my Gefell weren't too bad sounding.
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They say the heart of Rock & Roll is still beating, which is amazing if you consider all the blow it's done over the years.

"The Internet enables pompous blowhards to interact with other pompous blowhards in a big circle jerk of pomposity." - Bill Maher

"The negative aspects of this business, not only will continue to prevail, but will continue to accelerate in madness. Conditions aren't going to get better, because the economics of rock and roll are getting closer and closer to the economics of Big Business America." - Bill Graham

Barry Hufker

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Re: Improving Gefell UM92
« Reply #11 on: February 28, 2008, 08:30:41 pm »

My tube wasn't bad sounding, but there was nothing much good about it either.  Did you get the tube with the Hilumin shield (per Oscar)? If not possibly that's the difference?  If you did then I don't know what the difference is (tube mod only)...

Also I had a Sovtek tube.

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davebl

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Re: Improving Gefell UM92
« Reply #12 on: February 29, 2008, 11:45:09 am »

I sent my UM92.1S to Andreas Grosser who installed a new RWN tube selected for low noise. Some work was done on a repair to earth the cable correctly also. It came back sounding very sweet. It was not expensive either. I would strongly recommend using a good mic tech though with complete respect to every one, I think therein lies the secret !

Dave Blackham
UK
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Lukas Heyer

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Re: Improving Gefell UM92
« Reply #13 on: February 29, 2008, 05:54:45 pm »

J.J. Blair wrote on Thu, 28 February 2008 05:16

Replacing the tube with a high grade Telefunken EC92 should improve the noise floor, but I'm not sure that it will improve the sound quality.


The original factory-selected RFT EC92 tubes are usually quiet, in my experience, if they are not too underheated (they develop discharge noises etc.) or weak. Telefunken and Siemens tubes I have tried sounded usually slightly better, but they too needed to be selected from bigger batches (tens of tubes) to get the ones with least noise and microphonics, i.e. to be usable in microphones at all.

I do not have much experience with Russian EF86s, but I have tried many EF95-type tubes, incl. military-selected Soviet 6Z1P-EV etc., and while microphonics (and sometimes noise floor as well) seemed to be usually OK, I have never liked their sound character - Siemens, Telefunken et al. sounded way better in this case.

Lukas

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Oliver Archut

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Re: Improving Gefell UM92
« Reply #14 on: March 02, 2008, 10:21:18 pm »

The UM92 was build upon request of the former Gefell US distributor G-Prime. However it was nothing else than a slightly modified UM57, only difference was the output x-former that was taken from the MV amplifiers. Aside the low grade metal composition, the core was/is not able to get any resolution an M7 requires.

The UM92 was original shipped with an RFT Neuhaus EC92, even selected that tube is low grade compared to a Telefunken or Philips/Mullard EC92, I prefer the Tefefunken but the Mullard is equal in noise specs.
A simple tube swap might help, but without proper changes/modification the full potential of an Telefunken tube nor the M7 won't be usable.  

After Gefell could not get any longer the RFT EC92 the EF86 was chosen as a replacement, also the regulator inside the mic was replaced with an updated PSU as well as some crowbar EMI/RFI filters. There are many problem that come with the EF86 in a relative low voltage set up, but the biggest of them is that Gefell choose a Svetlana EF86 tube.

Both mics have the x-former connected to the cathode resistor acting as a feedback to stabilize the load.

In order to move the UM57/92/92.1 closer to the full potential of an M7, here are some steps that will help.

Replacing the stock 50K plate resistor to 100k with a 1uF (UM92.1 1.5uF) high grade capacitor.
Replacing the stock x-former with a high grade one and grounding the x-former instead of laying it on cathode potential.
Also adding a 50uF cathode cap, and a 1G high quality grid leak as well as polarization resistor.
A good NOS tube of western origin!
Changing the C12 style remote to an M49 style and most of all replacing the crummy PSU of the UM92.

Hope that answers most of the questions,

Best regards,

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Oliver Archut
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We are so advanced, that we can develop technology that can determine how much damage the earth has taken from the development of that technology.
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