MK-U47 - build thread

[ioaudio]

Brian,

since you have a original U47 with VF14 you will notice that it heats up in the same way.
No need to change the transformer configuration for the EF14 or dual 6028/408a.
-20dB is a lot. Maybe an wiring issue at the output? Did you try another capsule?

 

[Bri_guy]

I'll try switching out capsules...we've traced the wiring and can't seem to find a discrepancy. Has anyone had any issues with a new Thiersch  cap that you know of?
Thanks for the suggestions btw.

 

[frans]

I read in the old ioaudio U47 thread, that somebody had trouble because he used the Thiersch capsule in a non-Thiersch capsule holder, which gave him a -10 dB level drop? It doesn't hurt to have a cheap capsule in the drawer just to test things/swap/check.

 

[ioaudio]

Brian,

no in fact Thiersch's capsules are amongst the best from a QC / Failure standpoint, it's just one thing to rule out.

Frans,

that was the MK47 project with the capsule holder from the chinese housing - in my MK-U47 housing the whole capsule assembly is perfectly isolated.

 

[barbaroja]

If I have time tomorrow, I can record a comparison between the MK and my old 47.

I can never hear too much clips. Please do.

 

[micaddict]

I can never hear too much clips. Please do.

+100

 

[Bri_guy]

So, We've ruled out the capsule and the tube as possible issues. We still have low gain. Is it possible to get the pinout of the BV08?

 

[ioaudio]

The pinout can be found in my WM thread, scroll down to bv.08 http://groupdiy.com/index.php?topic=35667.0
Looking at the picture above the orientation is correct.
Re-check all connections, verify resistors are in the correct place etc.

-Max

 

[Bri_guy]

We solved the problem. It seems that the Flea capsule holder wasn't completely isolated. I didn't originally fit the plastic washers...once we put a  couple washers at the base of the mount...we were in business. The mic sounds Fantastic! Louder and fuller than my Wunder CM7 (both have EF-14 tubes).  Less mid-forward than my vintage 47 (VF-14). I'm looking forward to the next batch so that I can complete a pair...thanks Max!

 

[ioaudio]

That's good to hear - yes the acrylic washers are crucial for the isolation!
Don't forget to give the dual 6028/408a circuit a try - you might like it!

 

[chefducuisine]

Less mid-forward than my vintage 47 (VF-14).

Did you adjust the 27 Ohms resistor already?
Tune to taste to get closer to your vintage one.

 

[Bri_guy]

Less mid-forward than my vintage 47 (VF-14).

Did you adjust the 27 Ohms resistor already?
Tune to taste to get closer to your vintage one.

We copied the Wunder and used a 2.2k resistor and bypassed it with 47mf cap...I may go back to a 29 Ohms resistor and remove the cap and compare.

 

[frans]

Ah, so it had to do with the capsule holder, as I suspected in my earlier post. What i meant - and sorry if I worded it open to misunderstanding - was that the capsule and holder didn't work together 100%. Of course Thiersch capsules are great, no doubt about that. I won't diss the Flea capsule holder either, sometimes things just behave in tandem as we hope they do...or we overlook something. That's why groupdiy is so great - people who know that stuff contribute to help those who don't know that much. (like me) 

 

[Pip]

So it is done and it sounds wonderful, more on that later. Thank You so much Max for all you do this is truly a beautiful project and it has been a joy to work on. It is truly one of the highest points in my DIY career and I am now what some consider middle aged!

Only a few notes that are build specific. I had to play with the resistor values in the PSU to get to the variable range to the 105VDC range (no big deal). I used Zayance's wonderful PCB for the build (also a thing of beauty) but pay strict attention to the ground plane on this board as it is not complete and you need to ground it properly. Lastly I broke the reed switch the first time trying to align it and had to buy another, again not of issue they cost under a dollar US so I bought 10.

I used the Thierch Blue Line M7. Wow! :-*

Sound is wonderful!!!!!! But I must share a subjective point I started with the kit supplied Phillips 408a tubes and they are very clean sounding and quiet. But they are also harsh on the high frequency with an emphasis that leads to annoying sibilance issues. So I switched out the Phillips to the venerable Western Electric 408a. Now these tubes are problematic as they are, to my ear noisier and often not NOS, but pulls from HiFi gear depending on where you get them so the are also prone to annoying artifacts like pops. so it took a few days of burn in and swap out before I got two that sounded really good but again to my ear slightly noisier than the Phillips. But oh what a sweet sound they have and the high end to my ear is bright but not harsh without sibilant annoyance. So that is my take on that IMHO as it is said.

 

[Pip]

One more pic if you like!

 

[Melodeath00]

Less mid-forward than my vintage 47 (VF-14).

Did you adjust the 27 Ohms resistor already?
Tune to taste to get closer to your vintage one.

Care to explain? I understand the 29ohm resistor in the U47 affects the bias point of the tube. But would this change the frequency response/"forwardness"?

 

[ioaudio]

Thanks Pip! If you give the Jan 6028 tubes more time to burn in, you might prefer the sound.

I think there's some confusion regarding the 29 Ohm resistor, since "Bri_guy" uses the ef14 with another circuit.

Happy new year everyone!

-Max

 

[martthie_08]

just a quick question, I have started to build my mk-u47 and want to use it with the Flea capsule holder. This capsule holder has a orientation pin that gets in the way of the supplied capsule standoff, what is the preferred method of dealing with this? The way I see it one way would be to clip off the orientation pin, or drill a tiny hole into the standoffs... btw. how many layers of standoff discs are people using for the Flea capsule holder? I assume just one?

thank you so much for this great kit, very nice craftsmanship on all the parts!

 

[ioaudio]

You can just pullthe pin out - i use 1, but you might try 2 spacers for the flea holder.

 

[martthie_08]

ok, pulled it out, thanks for the info!

the supplied m3 plastic screw for the capsule holder is pretty long, I shortened it about 5mm...

EDIT: the Flea capsule holder is rather high, so I left out the spacers altogether.
Now the top of the capsule is roughly aligned with the upper metal bracing on the head basket.