ST12 - 6AK5 - SELA T12 rip-off - SIMPLEST TUBE MICROPHONE IN THE WORLD, FULL PRO

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ln76d

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[PROJECT FINISHED]

This time "inspired" by swedish  SELA T12.
As you can see - probably simplest tube mike in the world - so there's not much too change.
I have always was curious how this circuit sound :)
It utilize  NT1a edge terminated capsule, 5654  tube and 6.3:1 Funkwerk Leipzig transformer.
Heater and B+ voltages regulated - 5V and 62V.

Capsules recommendations:
- NT1a edge terminated
- 32mm K67 (alctron, mxl etc.)
- K47 ("RK47" type) for "transparent" headbasket.
- K12 ("RK12" type)

Transformers worth to check:

- 3U Audio GZT-47, GZT- 87
- Neutrik NT10/3

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All microphones projects:
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https://groupdiy.com/index.php?topic=62262.0
https://groupdiy.com/index.php?topic=62682.0
 

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Khron

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Wouldn't you rather do the 251 thing and turn the tube upside down, to minimize capsule wiring length? :p
 

ln76d

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No, i avoid mounting tubes upside-down.
Not every tube works good upside-down and also am avoiding noodles from the rest of the wires, which would have to hang near the tube.
Two twisted capsule wires if are even 50mm longer doesn't change anything. Yes i know high impedances blah blah.
In the real circuits i build it doesn't change anything. Let to the theory sometimes  stay in the book ;)
 

micaddict

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No, i avoid mounting tubes upside-down.
Not every tube works good upside-down

Tube mics were often used upside down in the old days, though.  (Some still are.)
Perhaps out of convenience, perhaps to keep the heat away from the capsule.

 

ln76d

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Please remember that in the days, most of the clasic microphones was used also for radio and television broadcasting as also recording studios. They were used really often upside down, on the statives and also boompoles.
When microphone was mounted upside down tube was in it proper position.
I believe that this was the real reason of mounting tube upside down.
I think that there wasn't (maybe except the real first designs) microphone where tube was so hot.
Even in U47, where the heat was high - it wasn't tube fault but heater dropping resistor ;)
 

ln76d

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I found the picture of Stevie Wonder with chunger C12 kit microphone ;)

And now the tube is upside down!
 

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shot

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Maybe this mic could be made to fit into MXL990 body?
That way tube wouldn't be neither "up" or "down" - it would have to lay horizontally!  8)

But seriously, would that be possible?

And also, a bit offtopic, please ld76d don't abandon the S216 project!
I'm very keen to do it!
 

ln76d

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If you will find small transformer you can use 990 body without any problem :)

Even using original PCB without components.
Currently i don't have 990,  so can't check but there could be possibility to mount it upside down :)
Also PSU isn't nothing hard to make. I'll be using 25V/9V transformer but it could be done similar to the royer PSU but only with the doubler not tripler.

I'll back to the S216 soon. Still didn't found good (easy)  solution for minus pattern voltage.
 

ln76d

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First test!

IMPRESSIVE- really great design!

Worth to build!!!!

It's really simple microphone - great project for beginners due to simplicity and low voltage but this is really pro - so if you need great microphone, don't hesitate!

PSU need to be well filtered, but with 60V there's no problem, capacitors 100V-160V rated are easy to find.
I used 5654 military tube and chinese K47. I need to test also 10:1 transformer, it may even sound better.
Now, with 6:1,  the output is really high.
In my build only inconvenience is that the headbasket boost hi and hi-mid range.
Probably i will have to remove internal mesh.

 

lampas

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I found that 5702 is submini replacement for 6AK5 and I will try with it as I don't have 6AK5 on hand ... Is it possible to buy somewhere that mini transformers you did use on your microphones ?
 

ln76d

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lampas said:
I found that 5702 is submini replacement for 6AK5 and I will try with it as I don't have 6AK5 on hand ... Is it possible to buy somewhere that mini transformers you did use on your microphones ?

I don't know 5702, later i will look for datasheet. Pleaase remember that ultra submini tubes have more inconviniences due to little bulb. Maybe it will be good?!
That mini transformer is from Funkwerk Leipzig small mixer/console KSG. It was used as microphone input 1:6.
Channels with this transformer are to find on ebay for cheap, but even in tha same mixer were used two types of this transformers 631.1 and 631.2. Second one is a lot bigger and also great! These transformer are ultra linear in both ways!
They sound fantastic. I'll be using it in G9 preamp as input transformers.
Wait a while for choosing transformer - i need to test 10:1 (it will be beyer -  due to size).
I think 10:1 will be much more suitable for 6AK5.

From the other hand - i need to test ultra cheap 10:1 neutrik transformers, from what i know they are (or were) made by Haufe.
In other applications, from what i remember they were fine.
It'sa 11$ transformer :)
http://www.neutrik.com/en/accessories/nte10-3

For the tube - probably i will buy huge stock of military 5654 (these are really impressive) and i will put some in blackmarket.
If you will need it - let me know ;)
 

ln76d

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shot said:
Damn! I'll do this one!

Just have to get some cash to get the parts...

Bravo ln76d !!!

Thanks ;)

It's really worth to build, it's also a lot of modification options, but for now i think there's no need of any mod.
 

ricardo

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ln76d said:
This time "inspired" by swedish  SELA T12.
As you can see - probably simplest tube mike in the world - so there's not much too change.
There's no Grid connection in your schematic.

How is the Grid biased  ...  1G or other high value resistor?
 

ln76d

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Yes there is :)

Diaphragm to grid - and that's the funny story :D
It's really work :D

Design is 1948 SELA rip off:

http://www.filmsoundsweden.se/voxbilder/filmhist/sela-t12-schema.jpg

IKEA microphone :)

Royer mod circuit also don't use grid resistor.
 

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