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

[Whoops]

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?

How much is the voltage with the POT on Max position?

 

[C12VR]

well to be honest I'm using a 250k pot instead of the 25k so I don't think that measurement would be useful; it can go well below 100v, I know that. Yes, I know it's dumb, I just left the correct pot off the order by mistake and this is my interim solution. Nonetheless, I don't see how a pot set to almost no resistance should differ between values enough to matter in this application. This is also has insufficient wattage for the job, so it will be replaced for dissipation concerns.

I have just noticed that there is a small negative voltage (-25mv) and negative resistance (-15ohm) between the chassis ground/safety earth and the mic body, but only when the mic is on. I can touch the mic without getting shocked, but I'm worried that this is unsafe, and I'm not quite sure what is wrong. Might this have something to do with the negative heater voltage derivation? I checked earth connections with a multimeter when the power was off, and all were equal to the inherent resistance of the multimeter itself (less than 0.5ohm). I have read that this occurs due to DC leaking to ground; is this then normal?

 

[C12VR]

My research says it is likely the presentation of interference on the shield/screen. As long as there isn't any voltage potential on the mic body I should be fine right? I would like to be able to post on GDIY lol.

 

[dmp]

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?

The pot should give you adjustment to get the correct voltage, 210v, if things are correct.
You should double check R1,R3,R7 and confirm that the B+ in the power supply is also 175 (no voltage drop along cable). If the psu is built correctly, the mic is drawing too much current.
You can check the B+ current being drawn by the mic by measuring the voltage drop across the 150k plate resistor. The 'correct' values from the schematic are 210-75=135v, which is 0.9 mA
Yours is going to be different.
Something is off that your B+ is dropping more than expected.

I have just noticed that there is a small negative voltage (-25mv) and negative resistance (-15ohm) between the chassis ground/safety earth and the mic body, but only when the mic is on. I can touch the mic without getting shocked, but I'm worried that this is unsafe, and I'm not quite sure what is wrong.

The mic ground should be connected to the chassis ground. You need to doublecheck the ground scheme

 

[C12VR]

The pot should give you adjustment to get the correct voltage, 210v, if things are correct.
You should double check R1,R3,R7 and confirm that the B+ in the power supply is also 175 (no voltage drop along cable). If the psu is built correctly, the mic is drawing too much current.
You can check the B+ current being drawn by the mic by measuring the voltage drop across the 150k plate resistor. The 'correct' values from the schematic are 210-75=135v, which is 0.9 mA
Yours is going to be different.
Something is off that your B+ is dropping more than expected.



The mic ground should be connected to the chassis ground. You need to doublecheck the ground scheme

I have checked voltage drop across the cable. It is negligible. I will check the resistor values and current draw, thanks!
If by "mic ground" you mean the metal shell, then yes, this is definitely connected at the chassis. 0V from the mic is also connected to the chassis via mains earth bolt star ground.

 

[C12VR]

Just confirmed, and you were right to suspect increased current draw. (255VDC-80VDC)/150KOhm=about 1.16 mA. PSU resistors are within spec. Might this be a normal deviation between tubes? or perhaps the used Mullard I put in is beyond its service life and I got screwed. I also tested the R14 plate load resistor and it is a healthy 149k, so that's not the issue.
But given the transformer secondary is rated for over 100mA, wouldn't this tiny variation in current draw be negligible?

 

[dmp]

Could be your tube bias. Higher biased tube will pull more current. Check the heater voltage and the voltage at r9,10,11,12

 

[dmp]

Is the b+ 255v? I thought it was 175v?

 

[C12VR]

Is the b+ 255v? I thought it was 175v?

I swapped resistor coupled to the potentiometer. 22k instead of 56k gave me that result. Sorry for not clarifying.

 

[dmp]

Which resistor is that on the schematic? I don't see a 56k, unless it is the trim in parallel with the pot. But if the pot is at 0ohms it won't do anything. And why bring the b+ way up to 255v? I'm very confused

 

[Daniel Faustino]

Hello everybody,
I'm in Europe with 230v on mains. I don't understand why the guys are using the mains transformer to do 230v to 230v.
To clean possible DC current from the outside?
Thanks

 

[dmp]

galvanic isolation

 

[rock soderstrom]

Hello everybody,
I'm in Europe with 230v on mains. I don't understand why the guys are using the mains transformer to do 230v to 230v.
To clean possible DC current from the outside?
Thanks

Yes, galvanic isolation. It is a safety feature for your house, humans and animals. It is mandatory by law in Europe. If something happens, no insurance will pay and you will go to jail for any personal injuries. Don´t do it!

 

[Daniel Faustino]

Got ya!
Thanks

 

[EarthTone]

I have an incredibly stupid question I’m sorry but how do you know which tubes to use in these Poctop builds? For example, do you use the u67’s classic ef86 tube? Or something else? I see a few of the designs on Poctop’s site have been modified to use different tubes than the original specs called for but I can’t see the tube requirements in any of the BOM’s or build guides. Sorry I know this is a basic question. I’ve built a lot of solid state gear but tubes are new to me…

Thanks!
Matt

 

[Whoops]

I have an incredibly stupid question I’m sorry but how do you know which tubes to use in these Poctop builds? For example, do you use the u67’s classic ef86 tube? Or something else?

EF86 and/or EF806

 

[EarthTone]

EF86 and/or EF806

Thanks. So from that I gather that unless specified on the product page at vintagePCB, each mic model would use the same tube that its vintage counterpart would’ve used. I’ll roll with that for these builds (looking forward to trying one!) and go from there.

 

[C12VR]

Which resistor is that on the schematic? I don't see a 56k, unless it is the trim in parallel with the pot. But if the pot is at 0ohms it won't do anything. And why bring the b+ way up to 255v? I'm very confused

Yes, the resistor is not on the schematic. It is in the build guide for the PSU and the BOM. It is in series with the potentiometer, creating max resistance of 80kish ohms. I had a 22k resistor on hand and stuck it in there to see if that was enough, the 255v is the undesirable effect. I changed tubes and put a new 25k pot in, and was able to get B+ to 210v. Thanks

 

[EarthTone]

Hi, I have 2 of these capsules in a pair of old Studio Projects C1's, 797 Audio K67 style capsules. I never use those mics. I don't know enough to know if these capsules are isolated backplates or what. They were also used in the Studio Projects C3 multi-pattern mics.

Question is this: Can I use these 797 Audio K67 capsules in Poctop's u67 build? I was thinking of doing a pair of u67 builds, getting them working, and then eventually upgrading to vintage Nuemann capsules if all goes smoothly with the builds and they work well. Thanks!

 

[Whoops]

Can I use these 797 Audio K67 capsules in Poctop's u67 build?

Yes you can.
People actually praise the 797 K67 capsules, there's a lot of fans of those capsules around here.
I actually bought 4 of them directly from 797, still haven't use them, but I will also use them in some U67 builds.

If you use them and you like the sound don't stress about having a vintage Neumann capsule, a vintage capsule might haven't aged that well, and if you like the sound of the 797 it's because it's good and a good reason to leave it be.