I builded a Tube U87

[oscarhuang]

Tube u87 should be a u67, right? But u67 need a special transformer and hard to find it. Why we don’t build a tube mic with u87ai schematic? They don’t need a feedback transformer and any tube you wanted. Here is my diy tube u87. A 5654 tube with LL1935 transformer and MP RK87. I used 220pf cap for it. I will try other tubes and find the best for it.

 

[soliloqueen]

very cool!!

 

[kingkorg]

@RuudNL did a take on u67 without the "special" transformer. Thanks for sharing your work, awesome!

https://groupdiy.com/threads/u67-ish-microphone.75162/

 

[LevinGuitar]

Is the R7 really neded there? There is no voltage across it unlike the U87 circuit. And could it be just one cap for the feedback?

Also I have read somewhere that 10n should be optimal for C7 instead of 0.1uf.

 

[Khron]

Also I have read somewhere that 10n should be optimal for C7 instead of 0.1uf.

"Optimal" how? That's just a DC filter cap there. In theory, the larger value, the better, but then there's the charging time (RC time constant) through R9...

 

[LevinGuitar]

You are right.
I have read time ago this thread about u47, but now I see that the point wasn't to be optimal but just no need to be larger than that.

Reply#34 from Kai
https://repforums.prosoundweb.com/index.php?topic=35368.0
Different question:
In the schematic of tube 87 as well in the original U87, C6 could be just grounded if there will be no measuring test made?

 

[Khron]

Doesn't C6 in conjunction with R4+R6 set the high-pass rumble-filter corner frequency? Wouldn't that be nullified if those resistors were nulled?

(I'm just raising the question, not being facetious - LTspice or measuring a real circuit would illustrate that though, i'm sure.)

 

[lampas]

Tube u87 should be a u67, right? But u67 need a special transformer and hard to find it. Why we don’t build a tube mic with u87ai schematic? They don’t need a feedback transformer and any tube you wanted. Here is my diy tube u87. A 5654 tube with LL1935 transformer and MP RK87. I used 220pf cap for it. I will try other tubes and find the best for it.

Great. I really like 5654 tube...

 

[ubxf]

5654_ Sela microphones _ here is a document originally shared by Dennis (API)

 

[oscarhuang]

Great. I really like 5654 tube...

Yes I like 5654 as well. Cheap but work very good for microphones.

 

[oscarhuang]

I will build other u67 or m269 with5654 and LL1935. Should be sound better than this one.

 

[rock soderstrom]

There is a small error in your schematic. The screen grid must not be connected to the anode because it is internally wired to cathode potential. Right?

 

[oscarhuang]

There is a small error in your schematic. The screen grid must not be connected to the anode because it is internally wired to cathode potential.

Thanks for point out my mistake.

 

[oscarhuang]

I’m modify this mic to M269 style mic just for test. With 5654 and LL1935. Looks we don’t really need a special feedback transformer. That’s a great news for me.

 

[rock soderstrom]

I’m modify this mic to M269 style mic just for test. With 5654 and LL1935. Looks we don’t really need a special feedback transformer. That’s a great news for me.

Can you please post a schematic of how you did it?

 

[oscarhuang]

 

[oscarhuang]

Works perfectly without special transformer. Better sound than my Tube u87.

 

[rock soderstrom]

Works perfectly without special transformer. Better sound than my Tube u87.

Thanks a lot!

 

[ubxf]

Thanks for sharing, Sorry for asking something that might be obvious but what makes it a M269 more than a tube U87.

 

[oscarhuang]

Thanks for sharing, Sorry for asking something that might be obvious but what makes it a M269 more than a tube U87.

I’m not a professional engineer. I have not idea why M269 sounds better my tube u87. But I have good ears and passion to do something I like. I think that’s why Mr Neumann designed M269 so complicated to achieve nice sound and this mic became to one of the best microphone.