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 this
https://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