Gefell MV101 Tube/Transformer Mod.

[SparkleBear]

Greetings all,

I'm in the midst of an attempt to give a tube head amp to the MV101 mic. Like my other mod, I'm using the body and capsule and mounting my own tube circuit into the mix. This is a different kind of mic than I've ever tried to build before as the capsule is designed for 200v. I'm trying to wrap my head around designing this and this is what i've come up with so far.

Set my universal regulated PSU to 200v for the capsule and divide it down to 120v for B+ voltage...



However, I am only getting 46v coming off the voltage divider before the 100k plate load resistor. My estimation was to see the 120v there... When I pull the 100k out of the circuit I do see the 120v... Am I missing something here?

This is gonna be a cool mic if I can get it going. Cute little tube mic.

Thanks in advance for any advice.

 

[kingkorg]

Greetings all,

I'm in the midst of an attempt to give a tube head amp to the MV101 mic. Like my other mod, I'm using the body and capsule and mounting my own tube circuit into the mix. This is a different kind of mic than I've ever tried to build before as the capsule is designed for 200v. I'm trying to wrap my head around designing this and this is what i've come up with so far.

Set my universal regulated PSU to 200v for the capsule and divide it down to 120v for B+ voltage...

View attachment 135298

However, I am only getting 46v coming off the voltage divider before the 100k plate load resistor. My estimation was to see the 120v there... When I pull the 100k out of the circuit I do see the 120v... Am I missing something here?

This is gonna be a cool mic if I can get it going. Cute little tube mic.

Thanks in advance for any advice.

View attachment 135299

Ohm's law?

You've limited the current available to the tube with the divider. Substitute the tube with the 86.5K resistor.



Even if you remove the 750K the voltage drop is still too large.


Try larger plate resistor without the divider?



This will change the output impedance of the tube, and play with the biasing. There's a reason plate voltage is regulated at desired voltage, dividers are wonky.

 

[SparkleBear]

Thanks @kingkorg ,

Maybe better to add another regulated rail into the supply. I’ve never worked on a mic where the polarization voltage was higher than the capsule voltage.

So when we usually take 120v b+ and divide it down for 60v for capsule it works because the capsule has little to no current draw? But doesn’t work as opposite because ohms law and the impedance of the down stream tube and biasing stuff loads it down too much for the same kind of scheme.

Thanks so much for your response KK. Incredibly helpful and will study into these advisements.

Be well

 

[bremusound]

You could still try out polarizing the capsule with just 120V.
Daniel Andreev of Reso Labs once made tube mics based on the MV102/MK102, you were able to switch the Vpol to 80V IIRC for a different flavour of sound.

 

[kingkorg]

Maybe better to add another regulated rail into the supply. I’ve never worked on a mic where the polarization voltage was higher than the capsule voltage.

Sure, you can do that.

So when we usually take 120v b+ and divide it down for 60v for capsule it works because the capsule has little to no current draw? But doesn’t work as opposite because ohms law and the impedance of the down stream tube and biasing stuff loads it down too much for the same kind of scheme.

Yes. It's still Ohm's law with the divider before the capsule, it's just that there's no voltage drop across the capsule because the lack of current though it.

I am very intuitive and visual thinker. There's no way i can understand any law or calculation until i visualize or create some kind of analogy that makes sense to me.

The screenshot i attached is from EveryCircuit. Not sure if there's a free trial, but it's inexpensive and i believe there are free, but less intuitive alternatives. You can see where current goes in real time, ac signals separate from DC, and much more.

Also many large parts of a circuit can be replaced by a simple resistor to simplify things, like i did here with one resistor to emulate tube's DC conditions. I came to the value based on voltages you were getting. So i used Ohm's law without doing a single calculation or using formulas. It gets much more complex with AC signals tho.

 

[Murdock]

You could still try out polarizing the capsule with just 120V.
Daniel Andreev of Reso Labs once made tube mics based on the MV102/MK102, you were able to switch the Vpol to 80V IIRC for a different flavour of sound.

Yes, I would also do that. It’s the easiest way and You probably won’t really hear that much of a difference between 200v and 120v polarization voltage.
I have a measurement capsule which normally runs with 150V installed in a M50 style mic with 60V on the capsule and it still sounds great.

 

[bremusound]

Maybe the lower capsule output at lower voltage fits better to the small transformer…

 

[kingkorg]

Maybe the lower capsule output at lower voltage fits better to the small transformer…

I have to guess here, but i believe the capsule at 200v still outputs the same level as regular one at 60v. Roughly. Because the diaphragm is much further from the backplate in order to achieve other stuff.

 

[bremusound]

Ah ok.
How would you deal with 50mV/Pa? A pad cap?

 

[Biula]

I own several MV102 (the FET version of MV101), which by the way shares the same power supply (200v polarization, 95v circuit supply, 6v antidew heather (the former tube filament supply...), and I assure you the 200v polarization is key in its outstanding capsule performance. Try to buid a dedicated power supply with separate rails, that's the way to go.

 

[Biula]

A bit of clarification: the original MV101 circuit is a cathode follower typology (hence the big output transformer placed in the power supply), trying to make a standard plate loaded mic out of it seems a difficult achievement to me... probably you will saturate the poor little transformer with that staggering 50mV/Pa.

 

[molke]

I have to guess here, but i believe the capsule at 200v still outputs the same level as regular one at 60v. Roughly. Because the diaphragm is much further from the backplate in order to achieve other stuff.

The MK102/103 (‘1 inch’) capsule apparently has an outer diameter of 23.77 mm and a capacitance of 59 pF (polarized 64 pF), so with some guesses on the diaphragm and backplate diameter, the spacing should be at least less than 50 µm (more likely less than 40 µm).

 

[kingkorg]

The MK102/103 (‘1 inch’) capsule apparently has an outer diameter of 23.77 mm and a capacitance of 59 pF (polarized 64 pF), so with some guesses on the diaphragm and backplate diameter, the spacing should be at least less than 50 µm (more likely less than 40 µm).

I could be totally wrong, cant find image of that specific capsule design, however at 40uM distance diaphragm/backplate and 200V polarization the capsule would arc and burn. Again, just guessing.

 

[molke]

I could be totally wrong, cant find image of that specific capsule design, however at 40uM distance diaphragm/backplate and 200V polarization the capsule would arc and burn.

I don’t have an image either and was just going by the datasheet at the Josephson page.

The somehow comparable B&K design (also 1 inch, for 200 V polarization voltage) in the AIP Handbook lists the unpolarized gap as 25 µm. The main difference to the usual studio capsules seems to be metal membrane, so much higher tension.

 

[Biula]

I don’t have an image either and was just going by the datasheet at the Josephson page.

The somehow comparable B&K design (also 1 inch, for 200 V polarization voltage) in the AIP Handbook lists the unpolarized gap as 25 µm. The main difference to the usual studio capsules seems to be metal membrane, so much higher tension.

I confirm, backplate distance is 25micron. What you must consider is the pure nickel diaphragm tensioned insanely high... nevertheless, one of my mic (used in an aerospace testing facility) was indeed purchased as defective due to an arc burning in the membrane (high humidity during an extreme wind tunnel wing test...).

 

[kingkorg]

I confirm, backplate distance is 25micron. What you must consider is the pure nickel diaphragm tensioned insanely high... nevertheless, one of my mic (used in an aerospace testing facility) was indeed purchased as defective due to an arc burning in the membrane (high humidity during an extreme wind tunnel wing test...).

Wohoah! The puzzle is solved, all makes sense now.

 

[SparkleBear]

After confirming i had a good head amp it was finally time to test my capsules.. Not surprised, but both capsules need service.

One didn't even have the skin in there so it had been, at some point, removed from the case? Or is it possible it crumbled and just fell out of the holes? These were purchased a long time ago and since I didn't have a PSU i could not test them ... Oh well...

Not that you could see the measurements from the photos... but i found it interesting to see the hole pattern and spacing etc... so in a way, its cool to have a skinnless capsule. *making the best of it*

Too bad, (not) so sad!





Hopefully, I'll be able to source a good capsule or find someone who is willing and able to re-diaphragm these pups. Thiersch is like a year long wait at this point. On the USA side of things, I have a friend who is/was a chemical engineer and has been experimenting with growing his own nickel skins. Its above my skillset but its fascinating... Not sure if it will be cost effective but its a possibility.

Contacted MTG and they responded that they likely wont re-skin for cost effective reasons too. They said "get a new capsule" but these are not commercially available anymore... so we shall see.

Do y'all know of any other nickel re-skinners out there?

Cheers!

-Eric

 

[bremusound]

Try to contact Daniel Andreev from Reso Labs, he has sucsessfully reskinned Mk102 capsules. I‘m not sure how it works these days, he‘s from Krasnodar, Russia… but one of the kindest guys i know!!