The Ultimate DIY U47 - Build Thread (And some PSU questions)

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

StormsofVogue

Active member
Joined
Jul 2, 2017
Messages
25
Looks nice. Isn't PCB a bit overkill with so few components. Why not point to point like the original resistor tree?

It is indeed overkill. I’m actually planning to use some small brass turrets. The PCB will complete a couple of connections but will mostly just serve as a mounting plate for the turrets. It's gonna look super clean with a node to the vintage aesthetic.

1AADDDA6-FFBB-42F2-9CDD-B23C901C7EBA.jpeg
 

JerryPbury

Well-known member
Joined
Jun 6, 2004
Messages
118
Location
Charlotte, NC
The closest I have gotten to the “u47” sound was using the 13cw4 nuvistor. I have build two ptp 47s, one with all proper components but different bias for the nuvistor suggested by Andreas Grosser and Moby and the other one with the Neumann bias but bv11 transformer ( David Bock suggestion)
What was the bias setting on the nuvistor (Moby and Andreas) which do you prefer? Thanks!
 

Purplenoise

Well-known member
Joined
Sep 19, 2019
Messages
465
What was the bias setting on the nuvistor (Moby and Andreas) which do you prefer? Thanks!
12 ohm cathode resistor. They both sound very similar. I find the one with the bv11 a little more relaxed in the mids but that could be a difference in capsules. In the bv08 I use a Neumann k49fet ( cardioid only-one side gold sputtered) and in the bv11 a Neumann k47fet ( both sides gold sputtered but one side wired)
 

JerryPbury

Well-known member
Joined
Jun 6, 2004
Messages
118
Location
Charlotte, NC
12 ohm cathode resistor. They both sound very similar. I find the one with the bv11 a little more relaxed in the mids but that could be a difference in capsules. In the bv08 I use a Neumann k49fet ( cardioid only-one side gold sputtered) and in the bv11 a Neumann k47fet ( both sides gold sputtered but one side wired)
Is this 12ohm with the 68 ohm in parallel as shown on the schematic or is it with ~15ohm with the 68ohm in parallel to get ~12ohm? What is the bias voltage afterwards?
 

Purplenoise

Well-known member
Joined
Sep 19, 2019
Messages
465
Is this 12ohm with the 68 ohm in parallel as shown on the schematic or is it with ~15ohm with the 68ohm in parallel to get ~12ohm? What is the bias voltage afterwards?
There is already the original 29ohm resistor and in the Neumann nuvistor schematic there is a 68ohm in parallel with that. You can add a 27ohm parallel to the 68ohm to get total of 12ohm. Or you omit 29 and 68 and just use a 12ohm. Haven’t checked voltages since…I trust the info source haha
 

JerryPbury

Well-known member
Joined
Jun 6, 2004
Messages
118
Location
Charlotte, NC
There is already the original 29ohm resistor and in the Neumann nuvistor schematic there is a 68ohm in parallel with that. You can add a 27ohm parallel to the 68ohm to get total of 12ohm. Or you omit 29 and 68 and just use a 12ohm. Haven’t checked voltages since…I trust the info source haha
Hmmm, I calculate 19ohms. I was going to use 68 and 15 ohms for 12 ohms. Did you do the power supply mod that Neumann described?
 

Attachments

  • IMG_0108.jpg
    IMG_0108.jpg
    452 KB · Views: 2
  • IMG_0109.jpg
    IMG_0109.jpg
    314.2 KB · Views: 2
  • Neumann-U47-schematic-ed.png
    Neumann-U47-schematic-ed.png
    311.4 KB · Views: 3

Purplenoise

Well-known member
Joined
Sep 19, 2019
Messages
465
Hmmm, I calculate 19ohms. I was going to use 68 and 15 ohms for 12 ohms. Did you do the power supply mod that Neumann described?
29,68 and 27ohm in parallel equals 12ohms. If you build an exact replica using the 1780ohm drop resistor in the mic then you have to do the Neumann mod in the psu.
You might also separate h+ and b+ in the psu feeding the heater directly from psu like conventional psus. This way you avoid excessive heat in the mic from the 1780ohm drop resistor
 

JerryPbury

Well-known member
Joined
Jun 6, 2004
Messages
118
Location
Charlotte, NC
29,68 and 27ohm in parallel equals 12ohms. If you build an exact replica using the 1780ohm drop resistor in the mic then you have to do the Neumann mod in the psu.
You might also separate h+ and b+ in the psu feeding the heater directly from psu like conventional psus. This way you avoid excessive heat in the mic from the 1780ohm drop resistor
I was only calculating the 68 & 29, brain fart. Good info, I‘m trying to do as little as possible as I am building the MK47 first then swapping in the Nuvistor circuit and if it works I’ll build it out. I’ve built 14 mics in the last couple of years and modded 3. They all sound great. Can’t wait to hear these..
 
Last edited:

Moby

www.mobytransformers.com BV.8, Bv.11, Bv.12, T14/1
Joined
Jun 3, 2004
Messages
2,021
Location
Serbia
I was only calculating the 68 & 29, brain fart. Good info, I‘m trying to do as little as possible as I am building the MK47 first then swapping in the Nuvistor circuit and if it works I’ll build it out. I’ve built 14 mics in the last couple of years and modded 3. They all sound great. Can’t wait to hear these..
The nuvistor "mod" is to force the nuvistor operating point from 0.3ma to 0.55ma. Anode curent change the output impedance of the tube, to match the Bv.08. Especially the early models with slightly lower inductance at the primary side. If it's propery matched the sound will be much closer to the Vf14, the low end also the right K² distortion point.
If you want to have the same output level, than change the 6,8pF capacitor
between the grid and the anode to 3,3pF.
I hope this helps.
 

Purplenoise

Well-known member
Joined
Sep 19, 2019
Messages
465
The nuvistor "mod" is to force the nuvistor operating point from 0.3ma to 0.55ma. Anode curent change the output impedance of the tube, to match the Bv.08. Especially the early models with slightly lower inductance at the primary side. If it's propery matched the sound will be much closer to the Vf14, the low end also the right K² distortion point.
If you want to have the same output level, than change the 6,8pF capacitor
between the grid and the anode to 3,3pF.
I hope this helps.
And as per your generous sharing of knowledge change the grid resistor from 60M to 100M and output cap from 0.5 to 1uf.
Thanks again for all your help man.
 

Moby

www.mobytransformers.com BV.8, Bv.11, Bv.12, T14/1
Joined
Jun 3, 2004
Messages
2,021
Location
Serbia
And as per your generous sharing of knowledge change the grid resistor from 60M to 100M and output cap from 0.5 to 1uf.
Thanks again for all your help man.
Of course. 100M grid resisitor, but not more than 250M because nuvistor (and some other tubes) can oscillate with higher values
 

JerryPbury

Well-known member
Joined
Jun 6, 2004
Messages
118
Location
Charlotte, NC
The nuvistor "mod" is to force the nuvistor operating point from 0.3ma to 0.55ma. Anode curent change the output impedance of the tube, to match the Bv.08. Especially the early models with slightly lower inductance at the primary side. If it's propery matched the sound will be much closer to the Vf14, the low end also the right K² distortion point.
If you want to have the same output level, than change the 6,8pF capacitor
between the grid and the anode to 3,3pF.
I hope this helps.
Thanks Moby that helps. I’m using your Bv08 in this build😁
 

Latest posts

Top