U47 Build using EF80

[Lowfreq]

Interesting thread.

I built a U47 using Oliver's schematic and an ef800 a few years back, and I'm interested in this whole parallel tube thing.
Like a lot of others, I haven't noticed any weird top end or funny low end.

I am using the Cinemag CM-2461NiCo which from the website, sounds like it might cope with higher impedance better, but 40k still seems kinda high.

From Cinemag website:
"Excellent performance even when driven by high impedance source (up to 20K). Upgrade transformer for Neumann U-47 microphone"

Are there any component changes to Oliver's schematic when you parallel the tubes? I'm keen to experiment with the circuit, but at the same time I'm wondering if it ain't broke, don't fix it.

 

[OPR]

Hey low freq,
No changes are needed as far as I can tell but don't quote me that!! But make sure that your psu can handle the extra currant draw for the two heaters. 

 

[saint gillis]

Few questions about grounds for the gurus ...

According to this :

  - Should the filament ground (pin5 of the cable) and the audio ground (pin4) be tied together on the mic board?
  - If not, should the screen of the the EF80/EF800 (pin6 of the tube) be connected to the heater ground or to the audio ground?

 

[rmburrow]

Connect as shown.  Don't set up a ground loop with the filament ground.  The screen and suppressor grids are tied to the anode for triode operation. 

 

[saint gillis]

Connect as shown.  Don't set up a ground loop with the filament ground.  The screen and suppressor grids are tied to the anode for triode operation.

I was talking about the internal screen, look closely to the pin6 of an EF80 or EF800 http://www.mif.pg.gda.pl/homepages/frank/sheets/128/e/EF800.pdf

 

[saint gillis]

Ok I found this : https://groupdiy.com/index.php?topic=44442.0

 

[saint gillis]

  Hello here's my build :

Board with the tube socket, the relay, C2 (from O.Archut's schematic) and all the resistors except R4 and R8 :

Seen from below :

The board with the Cinemag CM-2461 NiCo transformer :

The board with C1 C3 R4 and R8 :

Those 2 boards mounted on the body :

The whole mic wired, without the capsule :

The PSU :

The capsule and the EF800 :

The finished Mic :

We still didn't made any recording but it seems to sound awesome.

 

[shabtek]

Something to be proud of there.

Whats the oposite of a fetish?— like a turn-off:
That is what I have for zip ties.

Other than that a very cool build.

 

[saint gillis]

Whats the oposite of a fetish?— like a turn-off:
That is what I have for zip ties.

Hey, I'm sorry my english is not sharp enough to get the subtlety of these sentences...

 

[shabtek]

I like everything about  your build—But have a dislike for those wire tie things that are fastening the transformer in the mic.

 

[saint gillis]

I like everything about  your build—But have a dislike for those wire tie things that are fastening the transformer in the mic.

  Heheh, yes so do I...  Have you another idea to fasten those hole less transformers?

 

[shabtek]

I’ve made retaining brackets from brass flat stock that follow contour around lams and bolt to fiberboard. Also have sandwiched xfmr between 2 fiberboards with relief cutouts for coils.
Wire ties are convenient but are meant for ...? Bothers me as much as lock/tooth washers on toggle switches visible from the exterior.

 

[saint gillis]

Also have sandwiched xfmr between 2 fiberboards with relief cutouts for coils.

Yes that would have been the classy way, with spacers of the same height as the laminations.

 

[saint gillis]

Just to share the psu :