Modding a Gefell UM92 to Oliver Archut Specs

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

corgan4321

Well-known member
Joined
Nov 30, 2008
Messages
205
Location
Brooklyn, NY, USA
Hi all,

A couple years ago I got to use a UM75 that I fell in love with. I bought a UM92 recently that I plan to mod to UM75 (Oliver Archut's design) specs based on his schematic. My plan was to get polypropylene WIMA caps for all C's except for the one polarized cap in the schematic which will be an electrolytic. I haven't built a mic before so want to make sure I'm on the right track and that there aren't any rules of thumbs when selecting components that I'm missing. I was very surprised to find the 1G resistors at 16USD a pop :eek:
 
You can get 1G resistors for less than a dollar:
https://www.digikey.com/en/products/detail/te-connectivity-passive-product/RGP0207CHK1G0/3477617

I don't know why the R5 is specified 1G, it could be smaller.
 
What is wrong with the microphone that you want to change it?

The first things to try is different NOS tubes before you change anything else.

I have not worked on a UM92 however, has anyone one measured before and after some of the mods you find on the web. Will it pick up radio stations with the RF  filters removed?

Found thishttps://www.tab-funkenwerk.org/articles-tech-documents-2/ami-microphone-schematic-archive/um92-and-um92-1-schematics/
Removing the chokes and caps from the B+ and fil supply makes no sense.

I have not found a ps schematic.

You should not hear the RF filtering if it is done right and you might want that. FWIW if you look at a U67 schematic you will find caps at input and output pins.

The other "mods" change the circuit some so what happens to the resale value of the microphone?

Why do you want to change to polypropylene caps?

Why do you want to change the 220megs to 1gigs?


Found a ps picture in a Reverb listing
https://reverb.com/item/2386121-microtech-gefell-um92s-um92-s-large-diaphragm-tube-studio-microphone-ps-needs-repair


 
mhelin said:
You can get 1G resistors for less than a dollar:
https://www.digikey.com/en/products/detail/te-connectivity-passive-product/RGP0207CHK1G0/3477617

I don't know why the R5 is specified 1G, it could be smaller.

That's a 10% tolerance resistor. Might be fine for R5 but the other two should be closer matched I think since they are on either side of the capsule, no? I was quoting the price for a .5W 1%

 
Gus said:
What is wrong with the microphone that you want to change it?

It does sound good as is, but it doesn't do what the UM75 was doing for me. From my understanding, the UM75 is built to Oliver's schematics here so I am just hoping to replicate that. Here's a link  https://repforums.prosoundweb.com/index.php?topic=24668.0 to some well respected folks talking about how the UM92 series is lacking. In my experience, I've found this to be true, and have also been very impressed with what the UM75 can do

The first things to try is different NOS tubes before you change anything else.
Unfortunately the mic I have is the UM92S (not the 92.1S) so it uses a EC92 triode. If I do intend to swap out the transformer at the least, it wouldn't make sense to invest in a NOS EC92

I have not worked on a UM92 however, has anyone one measured before and after some of the mods you find on the web. Will it pick up radio stations with the RF  filters removed?
As I stated before, this schematic is the same as the UM75 which is a highly regarded microphone. In my use, I didn't find any interference...

Found thishttps://www.tab-funkenwerk.org/articles-tech-documents-2/ami-microphone-schematic-archive/um92-and-um92-1-schematics/
Removing the chokes and caps from the B+ and fil supply makes no sense.

I have not found a ps schematic.

You should not hear the RF filtering if it is done right and you might want that. FWIW if you look at a U67 schematic you will find caps at input and output pins.

The other "mods" change the circuit some so what happens to the resale value of the microphone?
Considering the UM75 usually sells for twice that of a UM92, I'm not worried.

Why do you want to change to polypropylene caps?
Not sure what you mean by change? I need to buy new parts and and am wondering if it's appropriate to use PP caps for these values in this circuit. I'm used to preamp builds where C0Gs or even polystyrenes so want to make sure there aren't any rules of thumb I don't know when it comes to tube mic builds

Why do you want to change the 220megs to 1gigs?
Because that's what Oliver's schematic calls for, maybe not important?

Found a ps picture in a Reverb listing
https://reverb.com/item/2386121-microtech-gefell-um92s-um92-s-large-diaphragm-tube-studio-microphone-ps-needs-repair
 
I would suggest C1 polystyrene, C2, C4 WIMA film, C5 electrolytic, C3 polypropylene/polyester (Cornell Dubilier or Vishay) or PIO, similar to Oliver's other designs.
 
I was in a discussion at least 7 years ago with Oliver about various tubes used in condenser mics...He effectively said many of the tubes I mentioned would "work", but certain ones were better.  Has anyone made a recording with a UM75 with the EC92 (6AB4) tube?

Oliver mentioned it was very difficult to obtain quiet EF86's  and the best were from NOS.  NOS AC701's and VF14's are expensive and rare...but Oliver "liked" the 5840 for some reason...since he complained about the high cost of the AC701..

None of the tube manuals published characteristics of certain tubes operating at 100 megohms grid resistance or above...over the years some tubes (6072, 7586, 6AB4, EF86, etc.) found their way into respected mics.  BTW the 7586 was used in the AKG C12 export models and also as a "front end" tube in Tektronix scopes...

As far as I am concerned, NOS is the way to go since environmental regulations, high cost of labor and rare earth materials, etc. ruined a lot of industries, especially "niche" industries.  Look at the cost of 300B tubes for example....  I am happy with PP EL34's....

 
Interesting information! Thanks.

I wouldn’t consider anything but NOS for a capsule like a real M7! In other tube microphones, I’ve heard the difference it can make.

Good to know about the grid resistors, maybe I’ll just give the existing resistors a try and swap if I have issues.

Am I totally wrong here: I’m pretty sure the EF86 and EC92 are totally incompatible. I didn’t think you could just pop a EC92 into a UM75 or a EF86 into a UM57

rmburrow said:
I was in a discussion at least 7 years ago with Oliver about various tubes used in condenser mics...He effectively said many of the tubes I mentioned would "work", but certain ones were better.  Has anyone made a recording with a UM75 with the EC92 (6AB4) tube?

Oliver mentioned it was very difficult to obtain quiet EF86's  and the best were from NOS.  NOS AC701's and VF14's are expensive and rare...but Oliver "liked" the 5840 for some reason...since he complained about the high cost of the AC701..

None of the tube manuals published characteristics of certain tubes operating at 100 megohms grid resistance or above...over the years some tubes (6072, 7586, 6AB4, EF86, etc.) found their way into respected mics.  BTW the 7586 was used in the AKG C12 export models and also as a "front end" tube in Tektronix scopes...

As far as I am concerned, NOS is the way to go since environmental regulations, high cost of labor and rare earth materials, etc. ruined a lot of industries, especially "niche" industries.  Look at the cost of 300B tubes for example....  I am happy with PP EL34's....
 
When I posted try another NOS tube I meant try the same number. Tube microphones plate out, capacitor to output transformer type are a circuit you can hear differences in tubes, noise etc.
You often have to try a number (10 or more)of the same type before you find one that you might want to keep in the circuit

What is the sound difference between the UM75 and UM92S? Less bass, more bass less, treble etc.?
Sometime it can be a tube(same number) and/or output cap change (value and type)

An 6ab4 is 1/2 of a 12AT7. I did search the prices are not that bad if you look around.

Do you know the transformer ratio? That can be a hint as to what you can build with it.

A lot of the tube microphone builds you find on the web are triode plate out cap coupled to transformer. The pattern switching can be different.

You need to look at the tube curves and the transformer specifications for how to set up the tube for the circuit.

IMO there is a bunch of stuff on the web that you should question.

The EF86 is close to the 5840 operating points when you place it in a microphone circuit however, 5840s can have noise and sound differences between different ones.




 

Latest posts

Back
Top