MK47 PCB tube mic kit - build thread

[Murdock]

and does it matter if the second mic only sees 102v instead of 105v?

I would not worry about 3v difference. Each tube draws different amount of current, so that's probably where the 3v come from. That's also why you should keep power supplies with there respective microphones. So you set up the correct voltage for that particular tube/mic once and leave it (except for checking it every few months) until you change the tube again.
Concerning the gain difference; that could also be the tube. Maybe swap the tubes and see if the gain changes with it.

 

[EvLoutonian]

Ok, cool - and thanks for the suggestion on trying different valves - I have some spares I can also try.

 

[EvLoutonian]

Overview of the two MK47’s

 

[Pip]

I would not bring those two wires through the transformer bobbins like that. Seems like a good way to induce voltage where you might not want it.

I would not worry about the gain difference unless its drastically different. All the things others have said is true in my experience with this mic design. The 408a tubes vary greatly in their noise floor and sonic quality, I tested dozens before I settled on two tubes that I liked the sound of.

As to the power supply that is suggested for this mic remember it is an unregulated design so therefore the voltage will change with varying states such as line voltage fluctuation and component variance.

Lastly those are drastically different capsules and it does not surprise me at all that the mics vary in voicing and timbre.
Check out this awesome website for some real testing and data on the mic that inspired this design.

http://www.moxtone.com/mU47_U47.html

 

[Mraihala]

Hello fellows,

I built an MK47 5 years ago. It worked perfectly and sounded amazing until the beginning of this year, as it just died suddenly.  We examined it with a local tube electronics guru and didn't find any faults in the PSU or the mic. Suddenly it worked fine again, until going silent again after few days. My friend started to test it and while holding the mic in his hand got a massive electric shock in his arm turning the power on... and a few seconds later another one in the arm holding the mic body. He had dizziness and aching arms for few days but was lucky to survive alive. The only damage I can find are those burnt legs of the USSR capacitor.

 

[Pip]

Hello fellows,

I built an MK47 5 years ago. It worked perfectly and sounded amazing until the beginning of this year, as it just died suddenly.  We examined it with a local tube electronics guru and didn't find any faults in the PSU or the mic. Suddenly it worked fine again, until going silent again after few days. My friend started to test it and while holding the mic in his hand got a massive electric shock in his arm turning the power on... and a few seconds later another one in the arm holding the mic body. He had dizziness and aching arms for few days but was lucky to survive alive. The only damage I can find are those burnt legs of the USSR capacitor.

The fuse didn't blow? Please check that the ground is intact through-out the whole circuit. The body of the mic should never have been able to get energized without the fuse in the PSU blowing. Your friend is very fortunate.

That cap is the output cap yes so if it failed or the solder connection failed that would explain the no output and possibly how the body was energized but again the fuse should have blown.

 

[mrclunk]

I've been building one of these for years...

Anyway i hired a real U47 for a session the other day and in comparison my MK47 has zero low end.

I've probably cocked something up somewhere?
Where should i be looking first?

It has a thiersch PVC M7 capsule.

I'm hiring  the Neumann again on friday so should have a fiddle with mine before then so i can compare.

(Mine's also really noisy but i think that's a separate issue as mentioned on here many times)

 

[vitopower]

Yes, it's typical for 6028 - i don't know how those tubes can be considered as related to VF14.
Specs are different. Good that many folks like it, but is it anyway close to U47?
You readings are fine for 6028. There 's no option to get real U47 specs here.
For plate is correct, for polarisation - yes your multimeter input impedance is too low, for heaters - it should be a little underheating. 40V was a big underheating for VF14. To get similar result (in theory of course) for 6028 you should get around 13-14V I doubt that these tubes can work with that low heater voltage.

Does it just kill the tube, and are there others that are similar that might do better underheated?  Is there a way to tell by construction which ones could stand underheating.. 6AK5 possibly?  I have a nice stash of NOS and would like to compare if I decide to build this.

 

[innercityman]

Hi,

I started to build this mic and it's been my first DIY microphone project build so any advices would be welcome. I have few questions though.

- I read the topic from the beginning an even if I still have a lot of pages left to read, I'd like to understand what to do with the 1.5K ciment resistor that came with the kit. It seems that it's a heater resistor and needs to be close to the mic body and not soldered directly on the PCB right ? But does it do the trick or do I need to replace it by a chassis resistor ?

- For the PSU I'm using the Zayance MKPSU pcb. I saw on a lot of pictures that the choke resistor was missing. Does it means that if using the choke (like I will do), I have to leave the location empty ?

- And finally, I bought the components referring to the bom from Chunger but I read that if using a 200V transformer, one of the 1K 10W resistor needs to be changed by one 680R and one 330R, also the pot needs to be changed to a 500R 5W. I bought the power transformer which is recommended on the bom which is a 200V and 6.3V secondary.

Here's the link :  https://www.tube-town.net/ttstore/toroidal-30va-8808.html

So could someone tell me if I understood everything...

 

[innercityman]

Am I the last man on earth building an MK47 ?

This thread is  dead quiet !

 

[sedit1]

Hi,

I’m having a trouble with my mk47. It makes lots of cracking and popping sounds. First it started with few pops every now and then and it was quite useable. But now it pops all the time.

I have used it with  thiertch M7 and beezneez capsule and same thing.

Do you have any pointers where to look. Could it be the 6028 tubes or the tube holders?

I will post a video where you can hear what it sounds like.

https://youtu.be/XM8BllqbL_I

Thank you!

Edit.. Read somewhere that the 1.5k cement resistor could be the case???

 

[georgerarnold]

I have a similar thing. Mine is more crackly though and less poppy.
I'm going through the tedious process of trying to change components.
First thing, I was told to change the 1.5 k resistor, didn't make any difference with me.
I've changed the other two larger resistors and there is an improvement but not solved.
Next I want to change the carbon comp resistors 100k and 30k but I don't know what the wattage is of these two and if that is important. Anyone have any advice?
Cheers.

 

[georgerarnold]

Hello fellows,

I built an MK47 5 years ago. It worked perfectly and sounded amazing until the beginning of this year, as it just died suddenly.  We examined it with a local tube electronics guru and didn't find any faults in the PSU or the mic. Suddenly it worked fine again, until going silent again after few days. My friend started to test it and while holding the mic in his hand got a massive electric shock in his arm turning the power on... and a few seconds later another one in the arm holding the mic body. He had dizziness and aching arms for few days but was lucky to survive alive. The only damage I can find are those burnt legs of the USSR capacitor.

Scary. I'm going to look for a replacement capacitor.

 

[Pip]

Hi,

I started to build this mic and it's been my first DIY microphone project build so any advices would be welcome. I have few questions though.

- I read the topic from the beginning an even if I still have a lot of pages left to read, I'd like to understand what to do with the 1.5K ciment resistor that came with the kit. It seems that it's a heater resistor and needs to be close to the mic body and not soldered directly on the PCB right ? But does it do the trick or do I need to replace it by a chassis resistor ?

- For the PSU I'm using the Zayance MKPSU pcb. I saw on a lot of pictures that the choke resistor was missing. Does it means that if using the choke (like I will do), I have to leave the location empty ?

- And finally, I bought the components referring to the bom from Chunger but I read that if using a 200V transformer, one of the 1K 10W resistor needs to be changed by one 680R and one 330R, also the pot needs to be changed to a 500R 5W. I bought the power transformer which is recommended on the bom which is a 200V and 6.3V secondary.

Here's the link :  https://www.tube-town.net/ttstore/toroidal-30va-8808.html

So could someone tell me if I understood everything...

- The 1.5k Resistor (drop down filament resistor) should be mounted to a heat sink of some kind the body will work in most cases but I would recommend something dedicated to it like a piece of aluminum mounted between the mic body rails. In one of my builds I actually kept the resistor in the PSU due to heat and space concerns in the body. Some recommend against this for a plethora of issues but it worked just fine in my case.

- If you use the choke you do not use the resistor.

- I don't remember this being the recommended transformer but it was a long time back. I think it is a TRIAD VPT230-110 see link below. You don't need the 6.3 volt winding as the 105 VDC supply voltage in this build does it all (thus the drop down resistor). Yes to the component swapping as a result of the output. I forewarn you you may have to do some resistor games to get to the right voltage.

https://www.mouser.com/ProductDetail/Triad-Magnetics/VPT230-110?qs=wkKrz7WmEgNNNgyT8w4YqA%3D%3D

 

[Pip]

Scary. I'm going to look for a replacement capacitor.

It also sounds like your grounding scheme needs review. That much energy in the body should have blown the fuse IMO!

 

[Delta Sigma]

It also sounds like your grounding scheme needs review. That much energy in the body should have blown the fuse IMO!

That was an old post he was quoting.

Scary. I'm going to look for a replacement capacitor.

I didn't read the follow up but I doubt the cap was the cause. You should never have a mic build that doesn't have the mic body grounded. If something shorts inside the mic, the power supply fuse should blow and eveyone should be safe. It is important to understand safe grounding before proceeding with a build.

 

[JMc]

Looks like Dropbox changed it's links to https, just fixed the links.

I know this is coming MANY years later, but the image links are not working any longer and I sure could use them to complete a new, 2021 build of an old MK47 kit I bought from you years ago. Any chance of restoring the links just one more time or send me the build instructions in a .PDF with the photos, if possible?

 

[reginator]

I know this is coming MANY years later, but the image links are not working any longer and I sure could use them to complete a new, 2021 build of an old MK47 kit I bought from you years ago. Any chance of restoring the links just one more time or send me the build instructions in a .PDF with the photos, if possible?

Chunger's thread still has all the photos of his build. It's the thread I used when I built my MK47

https://groupdiy.com/threads/newbie-builds-ioaudio-mk47-microphone.47112/

 

[reginator]

I know this is coming MANY years later, but the image links are not working any longer and I sure could use them to complete a new, 2021 build of an old MK47 kit I bought from you years ago. Any chance of restoring the links just one more time or send me the build instructions in a .PDF with the photos, if possible?

The mic build starts on post #100.

https://groupdiy.com/threads/newbie-builds-ioaudio-mk47-microphone.47112/post-632950

 

[JMc]

I looked at post #100 and didn't see what you were talking about? I was referencing post #1 in this thread and all the missing photos. That's the post I assembled my first Mk47 with and it went off without a hitch.