MK-U47 - build thread

[stelin]

Hi Max. The two resistors for the highZ board seems to have the same value (the same color coding). Is that correct? In the U-47 schematic I'm looking at, R1 is 100Mohm and R2 is 60Mohm.

Regards,
Stefan

 

[o3misha]

Best resistors  for U47 are both 100M .  60M resistor was used  with 0.5uF coupling capacitor in U47. This 60M-0.5uF version was built for radio broadcasts , it has less low end than 100M-1uF version. Sometimes, 60M-0.5uF version even better: for vocal or other instruments ( natural low cut).  Grid resistor ( as a part of the highpass filter of the first order) and coupling capacitor (as part of a second-order highpass filter) affect on cutting of low frequencies. But, already a grid leak resistor 100 M you have 3 Hz filter cutoff. It is sufficient to apply the microphone in a studio.1G - too high value(!!!!) for most of old tubes. It is believed that higher grid resistor value may increase low end. But in reality,  you will not hear marked increase  low end  in comparison with 100M. Even smallest fluctuations temperature or humidity will cause fluctuations of 1G  resistors value, - it is not good. Of course, you can use laquer for avoiding of humidity influence. But that, huge  grid resistor may also  affect on stability of  work of the tube. Use normal value- 100M, believe me. I tried 220M  grid resistor with a few types of old German steell tubes (including VF14M), for example, and I did not hear any pleasant improvement in sound.

 

[stelin]

Thank you, o3misha. Very helpful information!
/Stefan

 

[stelin]

One more "stupid" question: What about the curled up green wire from the tube section to the highZ section? What does that do? Is there a specific length that is "enough"? Is it for increasing the resistans of the plastic cover of the wire?

/Stefan

 

[ioaudio]

I saw the curl or the coil in some of the vintage U47's. When i asked A.Grosser about it, he told me that Neumann service incorporated this very low-value coil of the grid wire for mics used nearby radio transmitters. I'm not sure if it has any effect, but i dig the looks ;-)

-Max

 

[kcatthedog]

". I'm not sure if it has any effect, but i dig the looks ;-)"

Finally, the truth behind your mad (but Devine) reverse engineering endeavours !

 

[Melodeath00]

Best resistors  for U47 are both 100M .  60M resistor was used  with 0.5uF coupling capacitor in U47. This 60M-0.5uF version was built for radio broadcasts , it has less low end than 100M-1uF version. Sometimes, 60M-0.5uF version even better: for vocal or other instruments ( natural low cut).  Grid resistor ( as a part of the highpass filter of the first order) and coupling capacitor (as part of a second-order highpass filter) affect on cutting of low frequencies. But, already a grid leak resistor 100 M you have 3 Hz filter cutoff. It is sufficient to apply the microphone in a studio.1G - too high value(!!!!) for most of old tubes. It is believed that higher grid resistor value may increase low end. But in reality,  you will not hear marked increase  low end  in comparison with 100M. Even smallest fluctuations temperature or humidity will cause fluctuations of 1G  resistors value, - it is not good. Of course, you can use laquer for avoiding of humidity influence. But that, huge  grid resistor may also  affect on stability of  work of the tube. Use normal value- 100M, believe me. I tried 220M  grid resistor with a few types of old German steell tubes (including VF14M), for example, and I did not hear any pleasant improvement in sound.

I thought the change from 1uF to .5uF was due to transformer changes. I currently have 1G/1uF in my U47 clone, and I have been planning to swap to 60M/.5uF, but haven't gotten around to it yet.

 

[o3misha]

There were no big differences between historical BV08 transformers concerning its parameters , inductance, impedance etc. So, there was no other  reason for decreasing of coupling cap from 1uF to 0.5uF,- only to reduce low end, which was undesirable for  usage of U47 in radio broadcasts. With  simple program- simulator  you can find that with 1uF you will get ~30Hz resonance  . When you use  grid resistor 100M  ( first-order high pass filter),  frequency response is linear with 1uF cap. If you reduce grid resistor value to 60M, than with 1uF cap you will find decrease of useful low frequencies from 100 Hz, already. it is not good.  That's why, if you use 60M you should also decrease coupling cap value to 0.5 uF.  0.5uF cap produces  resonance~40Hz. Linearity with 60M-0.5uF will be recovered.
So, with 100M-1uF and 60M-0.5uF you will have linear freq. response, but in first case you will have low end a bit deeper in very low frequencies.

 

[chefducuisine]

Success!!!

 

[ioaudio]

Very nice Chef!
Important update: there was a error in the tube deck wiring schema regarding the yellow wire (thanks Chef!)

 

[Category 5]

Very nice Chef!
Important update: there was a error in the tube deck wiring schema regarding the yellow wire (thanks Chef!)

So schems are wrong but build blog is correct?  Where are the schems and support docs btw?

 

[ioaudio]

Yes, the pics are correct, i was refering to this "schema" which has been corrected now:

 

[bancho]

Success!!!

Great! 
Where did you get the shockmount from?

 

[chefducuisine]

The shock mount is from Flea.

If you want to go the modern route I recommend this one:
http://www.rycote.com/products/shock-mounts-suspension/invision-studio/invision-usm-vb/

Regards,

Christian

 

[ioaudio]

I was just informed that the second batch of headbaskets was shipped from FLEA
so i'll be accepting orders soon - as usual in my WM thread: http://groupdiy.com/index.php?topic=35667.0

 

[fishdrop]

Hey just finished my U47. I also started burning in the tubes.

So now i have a question.
i read that the voltage should be 105VDC. i built my PSU accordingly a calibrated it properly using a dummyload.
with the mic connected the voltage dropped to 95VDC. I readjusted my trimmer, but i cannot get more than 103,7VDC.

can i go go on burning in tubes??
or does it have to be 105VDC???
does the voltage change during burn in procedure??


all the best

bernhard

 

[chefducuisine]

Bernhard,


Did you wire the heaters in series as ioaudio's drawing above?
If so please check that both tubes are slightly glowing.

Other than than that:
- Please specify your power supply. I assume you did something like the MK-47 Power supply. As this is a passive supply it is heavily dependent on the voltage present on the primary winding of the power tranny - for Europe it can be 230V +/-10%.
- Can you please specify your dummyload. How many Ohms???

You can simulate all this using this sofware:
http://www.duncanamps.com/psud2/

 

[fishdrop]

Did you wire the heaters in series as ioaudio's drawing above?

yes i did it exactly like that

- Please specify your power supply.

It's the MK47 PSU by Zayance.
during building the psu i measured the tranny. it gives me 210 vac.

- Can you please specify your dummyload. How many Ohms???

2,2k

at the moment i have 237VAC of the wall. the PSU voltage dropped to 102,2 VDC

 

[Matador]

The output voltage will depend heavily on the actual tube quiescent current:  sounds like you just need to re-center the pot travel to give you some better adjustment room (right now you have it dialed to 0 and still aren't high enough on B+).

Did you use 330R or 680R for the third filter resistor?  I would start by soldering another resistor on it in parallel to drop it down some and give you an output voltage boost.

 

[chefducuisine]

OK  -  this sounds good. (BTW: Having only 103,7V is a minor issue - the mic will work. )
Zayance's PCB is based on the schematic Max posted for the Mk-47.
What did you install on position R4 of the PS PCB?
If I interpret the schematic correctly you can put a 680 Ohms resistor here and - if needed - 330 Ohms resistor in parallel.

If the parallel resistor is installed this should result in an equivalent resistance of approx. 222 Ohms.
Together with the pot in maximum position this should result in approx. 700 Ohms.
Without the parallel resistor you'll get ca. 1300 Ohms.
This should give you enough room to get the output voltage right.
If it doesn't you can change the value of R4.
Please use the suggested simulation software - this will greatly help you understand what is going on and to what value you need to change R4 to move the trimming to the needed range.

Edit: Just saw Matador's post - sorry for parallel posting