Neumann U67 Clone : D-U67 Tube Microphone Build Thread.

[dmp]

Ahh, they have a 50 ohm resistor in parallel to take a portion of the current (and power) - so does the du67 psu schematic. That changes things.

 

[dmp]

You can work out the current through the pot with Ohm's law and it looks like the max power dissipation is 0.5W
Even if the pot is 100R it is fine. Just need the resistor in parallel. Neumann schematic was correct.
The resistor power does go over 0.5W with the pot resistance >50 so put a 2W in there if you use a 100ohm pot

 

[Whoops]

The resistor power does go over 0.5W with the pot resistance >50 so put a 2W in there if you use a 100ohm pot

For the 2W are you referring to the wattage of the Pot or the Parallel resistor?

 

[dmp]

For the 2W are you referring to the wattage of the Pot or the Parallel resistor?

I was referring to the parallel resistor. I should have been more clear. With the pot at 100 ohms, the power dissipation in the resistor gets to about 0.9 W (orange line). So the resistor must be of wattage higher than that. Since 2 W resistors are cheap and easy to get, I'd recommend 2 W to have a ncie safety factor, keep it cooler and avoid any problems down the road.

 

[Whoops]

What source are you guys using for the 7 Pin XLR insert?



Any advises or link?

I found a link in this thread for a Switchcraft insert from Mouser, but it's out of stock and lead time is more than 6 months

Thanks

 

[Wordsushi]

What source are you guys using for the 7 Pin XLR insert?

View attachment 98686

Any advises or link?

I found a link in this thread for a Switchcraft insert from Mouser, but it's out of stock and lead time is more than 6 months

Thanks

Chunger has them on his site. Or, you can find them on Aliexpress.
2.2US $ |7 Pin Xlr Connector For Diy Microphone Golden Plating Studio Microphone Connector Microphone Accessory - Shell&body Parts - AliExpress

Just for fun, order from both and we can start a betting pool over which one arrives first.

 

[Whoops]

Thanks mate,
I actually searched on Aliexpress before posting but the search results showed a lot of stuff but not those inserts.

Thank you so much for the link

I would preffer to buy the switchcraft one because I'm a bit afraid of the quality of this Chinese part, but as the Switchcraft is out of stock I will have to order from Aliexpress

 

[Greenie]

Where do they sell 400m and 80m resistors for the tube pcb u67? One is labelled 989 and seem small enough to fit??

 

[Daniel Faustino]

Hello everyone,

I'm in the process of getting all the guts for this build and i can't find a 400M ohm resistor in mouser (nor a 60M). Is it ok to replace the 400M with one 300M and another of 100M in series?

And does it need to be 720mW or higher? can it be of 1/2W?

Thank you

 

[dmp]

If you had 120v across either of those resistors to ground they wouldn't come close to .5W
Power is V^2/R
As long as the resistor is rated for ~200v it is fine.

Having the exact value is less important than having clean, uncontaminated high impedance area. The only issue with resistors in series might be that.
The mic will work with different values than 60M & 400M, up to 1G, don't need to be exact. Might affect the sound subtly, but not necessarily worse

 

[Whoops]

I'm in the process of getting all the guts for this build and i can't find a 400M ohm resistor in mouser

400M:

https://pt.mouser.com/ProductDetail/Ohmite/MOX200004006JE?qs=LlUlMxKIyB06u5IfN9aihg==
470M:

https://pt.mouser.com/ProductDetail/?qs=ip69W3eHERXPnniX7vee4A==

(nor a 60M).

60M here:
https://www.aliexpress.com/item/100....order_list.order_list_main.51.22e81802pRcbHb
68M:

https://pt.farnell.com/te-connectivity/rgp0207chj68m/res-68m-5-250mw-axial-thick-film/dp/2805259
https://www.mouser.ca/ProductDetail...thy/RGP0207CHJ68M?qs=ip69W3eHERWQmjEselA81g==

 

[Whoops]

Where do they sell 400m and 80m resistors for the tube pcb u67? One is labelled 989 and seem small enough to fit??

you probably wanted to say 60M and not 80M.
Check the links I posted above.

They're not easy to find, but the exact value is not detrimental, the 400M could be anything from 300M to 500M

And the 60M anything from 50M to 70M, or whatever is closer

 

[Daniel Faustino]

Thanks,
My concern (besides the impedance value) is the wattage.. the resistors indicated in the BOM are all rated 720mW.
I'm not doing the math, but if you (@dmp) say it's okay with 1/2W then it's solved.

 

[Whoops]

Thanks,
My concern (besides the impedance value) is the wattage.. the resistors indicated in the BOM are all rated 720mW.
I'm not doing the math, but if you (@dmp) say it's okay with 1/2W then it's solved.

yes, it's fine, I think some pages back there was a talk about the wattage of those resistors

 

[ratrace]

Maybe stupide question, do wirewound resistor could be ok in U67 build ?

 

[Delta Sigma]

Maybe stupide question, do wirewound resistor could be ok in U67 build ?

For which resistor(s)?

 

[ratrace]

the ones Whoops and Daniel was talking about :
-R1 and R8 (400M)
-R2, R9 and R16 (60M)

 

[Whoops]

wirewound resistor

Circuit wise (electronic wise) I don't see any problem if the resistor values are the same but there's a Physical problem, you need the resistors to be small enough to fit in a limited space and wirewound resistors are quite big.

The links I posted are a better option as those resistors will fit the space available.

check here the size of the original resistors on an U67:

 

[Whoops]

Just to let everyone know, that I ordered the 7 pin XLR insert from Aliexpress, and it looks pretty good and for the time being it works pretty well.
So if anyone needs one, here is the source for it:

https://www.aliexpress.com/item/100....order_list.order_list_main.35.485f1802g859ng

 

[C12VR]

Is it normal for the B+ voltage to read at approximately 175 at the little pad in the mic with the B+ pot set to least resistance? The output voltage before the pot is well over 350v, so I don't think it's the transformer. Can I just drop the value of the resistor in series with the pot, or should I be looking for a mistake?