Murdock

Re: MK47 PCB tube mic kit - build thread
« Reply #1420 on: April 21, 2019, 02:51:59 PM »
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.
« Last Edit: April 21, 2019, 03:27:32 PM by Murdock »


Re: MK47 PCB tube mic kit - build thread
« Reply #1421 on: April 21, 2019, 09:14:39 PM »
Ok, cool - and thanks for the suggestion on trying different valves - I have some spares I can also try.

Re: MK47 PCB tube mic kit - build thread
« Reply #1422 on: April 21, 2019, 09:17:24 PM »
Overview of the two MK47’s

Pip

Re: MK47 PCB tube mic kit - build thread
« Reply #1423 on: April 21, 2019, 11:38:25 PM »
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
« Last Edit: April 26, 2019, 12:38:08 PM by Pip »
Pip
New York City
http://geosonixlab.com

Re: MK47 PCB tube mic kit - build thread
« Reply #1424 on: June 03, 2019, 06:42:44 PM »
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

Re: MK47 PCB tube mic kit - build thread
« Reply #1425 on: August 05, 2019, 12:03:42 PM »
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.
Pip
New York City
http://geosonixlab.com

mrclunk

Re: MK47 PCB tube mic kit - build thread
« Reply #1426 on: December 03, 2019, 08:54:41 AM »
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

Re: MK47 PCB tube mic kit - build thread
« Reply #1427 on: January 14, 2020, 05:18:45 PM »
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

Re: MK47 PCB tube mic kit - build thread
« Reply #1428 on: May 23, 2020, 04:40:55 AM »
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

Re: MK47 PCB tube mic kit - build thread
« Reply #1429 on: May 25, 2020, 12:49:26 PM »
Am I the last man on earth building an MK47 ?

This thread is  dead quiet !


 

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