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[kdueck]

Hello everyone! I am writing here for the first time today and would like to take this opportunity to thank you for your valuable contributions, which I have been following for a long time!

I have an Um92 - still with the EC92 and would like to carry out Oliver's mod. I have 3 questions that I'm sure you can help me with:

1. can I adopt Oliver's circuit but leave the EC92 in? Or will the voltages and currents then not fit?

2. can I continue to use the original UN920 power supply for the time being if I leave the Heating Vortage adjustment in the microphone?

3. if both are possible, what would be the sonic advantages of changing the tube to an EF86 and changing the power supply to the AMI 49RP? Can this reduce the noise floor, for example?

Translated with DeepL.com (free version)

[klaus]

Hello Kornelius,
If you could post Oliver's schematic (Schaltzeichnung) that details his changes to the UM92's circuit, qualified advice can be given.

[kdueck]

https://www.tab-funkenwerk.org/articles-tech-documents-2/ami-microphone-schematic-archive/um92-and-um92-1-schematics/

[klaus]

1. can I adopt Oliver's circuit but leave the EC92 in? Or will the voltages and currents then not fit?

First, the schematic shown you linked of the "original" is not that of the UM92(S) but of the UM92.1S. They differ quite a bit. On a UM92, you will have to work from the actual UM92 schematic and rewire most of the DC supply and audio routing circuit; you need to rewire the tube socket, as the pinouts differ quite a bit: the original tube is a single triode, while the EF86 is a pentode; then you need to eliminate L1 to L3 and replace with better means for RF protection.

2. can I continue to use the original UN920 power supply for the time being if I leave the Heating Vortage adjustment in the microphone?

You can continue the original supply after carefully calibrating the voltages: currently, B+ is 127VDC which needs to be brought down to 120VDC by increasing R6 (3.3kΩ), and the heater voltage needs to be calibrated for the EF86 to 6.2VDC by reducing R11 (100kΩ).

3. if both are possible, what would be the sonic advantages of changing the tube to an EF86 and changing the power supply to the AMI 49RP? Can this reduce the noise floor, for example?

It will reduce the noise floor quite a bit, and make the mic perform much more like the 92.1S

[kdueck]

Hello Klaus,
Thank you very much for your help!
Yes, the UM92S is different. R1 and R2 have 20M, C3 has 0,68uF, no inductors with their related capacitors.

I have now found a wiring diagram that uses Oliver's mod without changing the tube. I have ordered the AMI UM351 transformer.
My plan would be to leave the original power supply and EC92 for now, but modify everything else. I have a pretty good Telefunken and a Siemens tube available.
To change the tube, I need the original tube socket from the UM92.1, as this is a special plastic plate with an integrated socket. And the AMI 49RP power supply is quite expensive, as I still have to pay the expensive shipping, customs and taxes in Germany. But if it's worth it, I'll at least replace the power supply unit later.

Thanks for the description of how to adjust the voltage R6 and R11! These are in the power supply unit, right? Do you have a circuit diagram?

[kdueck]

...so, I have found the 3.3k in the power supply. If I replace it with a 6k, I initially have 119.5V with the microphone connected and after approx. 20 min. exactly 120V. Is that correct?

[klaus]

119.5VDC is more than close enough. 120 is perfect!

[kdueck]

So, the modification is finished and definitely sounds better. A note on the previously attached drawing: This is from the UM75 mod. Apart from the pin assignment of the connector and the tube, it is identical to that of the UM92.1 mod. However, the original power supply is less suitable in terms of voltage. Without adaptation, B+ has 125V and the voltage for the patterns is 77V for cardioid and 155V for figure eight. In order to achieve the desired B+120V and the pattern V of 60 or 120V, the R6 (3.3k) must be replaced by a 6K. However, the voltage only stabilizes after approx. 30 minutes and is then between 120-121V. If the 20k resistor in front of it is replaced by a 39k and the 3.3k remains, the voltage seems to be more stable.
To get the pattern voltage to 60V, I had to place a 120k resistor in front of the voltage divider on the pattern switch board and cut the circuit board track.
Now I have the desired values, but whether this is technically ideal is up to you professionals to decide.

[kdueck]

...and the other side with the EC92

[kdueck]

In the meantime, however, I have received the socket for the EF86 from Gefell, which is already pre-wired. I noticed that, unlike in Oliver's circuit, Grid3 does not go to the anode together with G2, but G3 goes to the cathode together with the shield. G2 is connected to anode. Both variants are probably possible, which would be the more recommendable for the UM92?

[klaus]

I would make a comparative listening test, as nothing else matters in the end.