Groove Tubes MD1b FET possible mods, upgrades, schematic?

Hi,
After watching this video I just had to find this mic somewhere, and I did score one for 100e.
https://www.youtube.com/watch?v=B33N5zuHRn8

I am going to say this mic is veeeery underestimated, and there is not a lot info on it on the internet. Does anyone have a schematic, or idea for any mods, improvements?

In the original tube mic there is Cinemag transformer, but I cant tell which one this one uses. I can't tell either if it has any kind of high frequency attenuation since it uses k67 style capsule, but it is not too bright mic. It is in my Audio Technica 4040 range, but I feel it has a bit higher resonant peak and sweeter esses. 

Board 2

Capsule

Back

Searching the forum I found out this guy has almost identical PCB and components as GT55. There was a thread on moding GT55 which seems like a failure, but I am interested in implementing some HF de emphasis  components more than anything else.

I am not quite sure, but it seems to have some body resonance issues and seems to pick up a lot of room ambience compared to my other mics. It's a bit strange that nobody else noticed this on MD1b Tube anywhere on the internet as they share the same enclosure.

Try this one.
It should work, potentially one cap to tune circuit.
Member which tried to apply that wasn't even sure that microphone was working before.
It's easy mod.

Wow! 2.2 nF, isn't that a bit too much?!

RuudNL said:

Wow! 2.2 nF, isn't that a bit too much?!

Click to expand...

It's a cap which value is for "tune".
In simulation looks like proper value.
Usual all values in different circuits implementations, which i did, were simulated properly.
I added deemphasis network into few different topologies (including two tube microphones) and values were totally different for each circuit.

I would not build what ln76d posted
Something much simpler can be done
I have worked on a few ST microphones
Only thing  I agree on is the 1uf output coupling cap

Gus said:

I would not build what ln76d posted
Something much simpler can be done
I have worked on a few ST microphones
Only thing  I agree on is the 1uf coupling cap

Click to expand...

I am all ears

Gus said:

I would not build what ln76d posted
Something much simpler can be done
I have worked on a few ST microphones
Only thing  I agree on is the 1uf output coupling cap

Click to expand...

Kingkorg asked about HF deemphasis - so here it is.
If you are able to build deemphasis network easier - enlighten us. (note - cap to ground or simple eq isn't deemphasis type of circuit). If not, please don't "troll" topics.

I have a few ST51s with no switches otherwise it is mostly like the st55 circuit.
One thing I do is clean the flux off the board in the ST51s.

I have not seen a MD1b can you determine if the resonance from the body or from a PCB mounted part?

What other microphones are you comparing to for the room ambience?
You can do a simple test walking around the microphone  with a source of sound checking the pattern.

I change the input cap in the ST51 you can direct couple to the gate with a circuit change, however that loses charge voltage because the gate is above ground so you have less voltage difference for the charge voltage.

@in76d i think i might turn this one into your ST12. There is plenty of space  inside and glowing tube can be seen through that carved GT logo, way too cool. Even tranny could match, i haven't measured it yet tho.

It's pretty good circuit, but you will need at least 6:1 transformer. GT tranny is rather 2:1.
3UAudio gzt47 or gzt87 is pretty good choice.
Check PM i sent you link for other donor worth to consider.

One more interesting thing is that I actually believe this same capsule is used in Manley Reference Cardioid. This is totally possible as the capsule for Manley R.C. is produced by Feilo.

And Sterling Audio with green wire. I am 99% positive this is the same thing!

That Sterling Audio one reminds me a lot of my several sE2200A's, with the single circuit-board, and mechanical structure.

Measured a ST51 transformer ratio at about 4.5:1