MK-U47 - build thread

[fishdrop]

Hey matador  & chefducuisine,


thanks for your help. R4 is aready 330 & 660 in parallel which makes 222 Ohm and my trimmer ist at 0 Ohm.
so i definitely will change the resistor after burning in the tubes. kinda strange. i mean with 237 VAC coming out of the wall????

just wanted to know if there is something to worry about or if this whole thing is a hint for something i should check.

the mic sounds good, and even better the longer it's turned on. no noise at all. it sounded awesome from scratch  ;D
i'm all excited about it.

 

[fishdrop]

so is this the last, recent and correct tube wiring guide???

  : i think i wired it wrong

 

[Matador]

Hey matador  & chefducuisine,


thanks for your help. R4 is aready 330 & 660 in parallel which makes 222 Ohm and my trimmer ist at 0 Ohm.
so i definitely will change the resistor after burning in the tubes. kinda strange. i mean with 237 VAC coming out of the wall????

just wanted to know if there is something to worry about or if this whole thing is a hint for something i should check.

the mic sounds good, and even better the longer it's turned on. no noise at all. it sounded awesome from scratch  ;D
i'm all excited about it.

To be honest, I wouldn't worry about it.  103V is close enough for jazz.

If you really wanted to sweat it, I would first lower both R1 and R2 down to 820 ohms to try and get the pot re-centered.  You want to try and hit 105V with the pot at 250ohms to give you adjustment range up and down.

 

[ioaudio]

so is this the last, recent and correct tube wiring guide???

  : i think i wired it wrong

Sorry for chiming in late - Yes there was an update on the yellow (heater) wiring!

 

[fishdrop]

Sorry for chiming in late - Yes there was an update on the yellow (heater) wiring!

did i destroy anything?? or is everyhting alright, when i change this connection to the right pin?
whta's the effect of the misconnection?

the mic sounded good to me, but i never tried a original U47, so who knows
i swap the wire later and tell you whats going on.

 

[ioaudio]

Nothing got hurt - only one tube was heated at a slightly higher voltage but it will be fine.

-Max

 

[fishdrop]

i corrected the miswired cable.

now i can perfectly adjust the PSU Voltage to 105VDC. both tubes are glowing.

 

[ioaudio]

Another 25 Kits available now --->http://groupdiy.com/index.php?topic=35667.0

 

[fishdrop]

i already told some musicians about my new great mic.
right now i'm organising demo sessions, which i invite musicians and engineers to.

so they can try out the mic in a relxed atmosphere - i hope i will get some decent vocal/instrumental takes which i can put on my website for demo purpose.
and of course i will share this to the forum people as well.

 

[kcatthedog]

Advice at soldering caps stage

I mistakenly cut the legs that protruded out of the pcp after soldering each cap from the inside. ( thought I was smart )

As the ends of the cap and the  bottom resister legs were present on the outside of the pcp , I simply used a small piece of the leg to solder a bridge in place for each cap to its respective resister.

Will this be ok ?

thx!

Kcat

 

[ioaudio]

Yes, no problem with that.

-Max

 

[stelin]

Hi Max, do you have a schema for the MK-U47? I have seen original U47 schemas, and I know that you have followed the original very close (even the color of the wires are like the original), but the schemas I have found are of poor quality. They are also missing the parallell tube configuration, needless to say.
I want to have good documentation for the microphones I build. The MK-U47 will last for a long time and may very well end up i someone elses mic-collection some day.

 

[ioaudio]

I will work on a clean Schematic for this build.

 

[rainton]

Great!

I'm building mine right now, but I'm still confused with the tube wiring.
As far as I can see the photos of your wiring differ from the scheme that shows the wiring or am I mistaken?

In the scheme the heater wire goes from the 3rd pin (from right to left) of the one tube to the third pin of the other and the heater wire from the resistor is going to pin 4 of the first tube.

But in your build pics it seems the wire from the resistor is going to pin 3 of the first tube and pin 4 of the first tube is jumpered with pin 3 of the second tube...

I'm just asking because I just fired it up the first time and I measuring 114V at the power supply when the the pot is fully clockwise - so I can't get lower than that.
Disconnecting the mic and using the dummy load - I can dial in 105V perfectly fine...

 

[ioaudio]

The pics can be a bit unclear, but if you wire following this schema everything's fine:

 

[rainton]

Ok,
that's how I wired it Max

But when checking the voltage at my power supply I can't get below 110V with the mic connected.
I built the MK47 power supply with 230V toroid, a C-3X choke and three 1K ohm resistors in series with another 1K pot.
Using a dummie load of 2.2K I can dial in 105V pretty easily even though the pot is pretty far from being "centered" then.

Anyway should I try to raise the resistance in the power supply to get a voltage drop, or does the above mean something's wrong in my mic?
Not sure.
I haven't really tested it for sound yet since I switched it off after not being able to lower the voltage below 110V - didn't want to do any harm to the mic...



Also another question regarding the two 100 Mohm resistors in the Hi-Z part of the mic.

o3misha posted some great info on that, but he wrote that either you use 100Mohm with a 1uF coupling cap, or a 60Mohm with a 0.5uF coupling cap.
But the kit included two 100Mohm resistors for the Hi-Z and 0.5uF coupling cap?

I used the original schematic I have here (it's from 1959) and it says 60Mohm + 0.5uF coupling cap.
And since I the kit included a 0.5uF one and I had an 68Mohm resistor laying around I tried that.

Any more info on that matter?

Thanks!!

 

[ioaudio]

The test load is used to bring the voltage to a ballbark figure - it ultimately should be dialed in with the microphone connected.
The (passive) U47 Psu is easy to adjust, just like Ohm's law - if you have to much voltage, add resistance.
Add a 220R in series and you'll be fine.

Re: HiZ resistors.
I found the 100meg works best. A.Grosser also told me that Neumann used 100meg in most of the u47's instead of 68meg although it was noted in the schematics.
Of course you can experiment, if you like the 68meg better (you can use two 33meg in series)

-Max

 

[Category 5]

Anyone interested in pre-made PSU's for this mic?  Beesneez mentioned a possible group rate for done PSUs.  the regular price was $300 U.S. so if a few more people are into it maybe we could get the cost down.

I want do to Chunger's BadAss PSU as soon as it's available, but that might not be sooner than later so I'm looking into this option for right now.

 

[ioaudio]

Here's the Schematic:

 

[Category 5]

Max, thabks so much for the schematic.  That will help.  It would also be helpful for you to do a diagram similar to the tube wiring diagram for the connector, resistor/tx board too since the pics are dark and difficult to see.  I have heard from at least two others that had some confusion doing this part and may have mis-wired as a result.

Also, what were the screws you recommend for the tube sockets?  Our hardware store didn't have anything shorter than 1/2"