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

[lampas]

... another angle ...

 

[lampas]

... someone ask how long does it take to recover, if you blow into it, and it does almost immediately ...

 

[ln76d]

threads from 2006 and 2008

reply 25 is a guess at the circuit in the thread I would use >=4:1, dotted line is microphone and power supply separation.

https://groupdiy.com/index.php?topic=29901.msg362898#msg362898

Another thread https://groupdiy.com/index.php?topic=19965.msg233789#msg233789

Maybe, if the topic circuit would be cathode follower.
Your post can be confusing for some users, so:
6.5:1 - 10:1 transformer is valid for this circuit.
There's no polarisation voltage via grid 1 so your schematic isn't valid here.

 

[ln76d]

Great work Vili!

I would use 0.5uF at the output

For polar patterns, there's a question - switchable via PSU or switch inside housing (better option - less patterns)?

 

[lampas]

Hi,

I made it that way, that I could easy swap output cap... But I owe sound samples to author of the transformer, so I decided to try first with 1uF. But the mike is a lot noiser than Royer mod for example... I will try to switch PSU first (T.BONE PSU with changed output zeners ...)

 

[Gus]

I have built microphones without a grid resistor to ground or some other point.
What I built ended up with a grid to somewhere resistor after testing.
One circuit was  like the MXL2001 Royer I ended up changing it from no grid to ground resistor to 1gig grid to ground with a different cathode resistor value.

Another link for the SD Royer note the grid charges the capsule
http://www.idpmultimedia.com.au/diyfactory/projects/royerproject/royermod_2.pdf

 

[lampas]

It appear that circuit is very sensitive to the PSU. T.BONE PSU with changed zeners for 60V simply doesn't work... Too much noise .. I tested then with my lab PSU's for B+ and H+ and the circuit is very quiet. Now I need to build a proper PSU for it... Maybe I'll try with voltage doubler as I read before ...

 

[ln76d]

Your noise issue is definately from PSU, it's really hard to lower (over) 200V to 60V.
Two guys had problems with lowering t.bone PSU to 120V for simple SCT800 mod. Microphones were noisy as hell.
Problem was only with voltage dropping.

I did my twelve with voltage doubler and it's quiet, so definately it isn't circuit problem.
You could try to lower voltage around 180V, then make one filter stage and divider to get 60V and 120V.
This way you can feed plate and backlapte polarisation as  also get switchable multipattern from PSU.
High wattage resistors recommended.
For that type of multipattern switching, you could try to feed  diaphragm backside directly without filtering, same as with polarisation voltage. I didn't tested that, but it may work pretty good. Then 470pF-680pF between both diaphragms.
if you will get nise trouble then two extra resistors (1G and 10M for example) and one 10nF-22nF.

 

[lampas]

Find time to finally finished this mike ... At the end I did use capacitance multiplier and one another RC stage after zeners and PSU is now quiet. Well I could confirm, really great microphone...

 

[kingkorg]

Would you guys ming making a simple sketch of schematic for psu?

 

[vitopower]

Find time to finally finished this mike ... At the end I did use capacitance multiplier and one another RC stage after zeners and PSU is now quiet. Well I could confirm, really great microphone...

Did you ever make any samples? 

I royer-ized an oktava mc012 and its great...  How does your Sela type mic sound different from the royer?

 

[lampas]

Hi,

please give me a day or two, I owe a schematic for the PSU and samples for Spencerlehorton as I promise him, to make some, because he sold me transformer I did use in it...
I have also Royers and I will include them too...

 

[vitopower]

Nice!  ;D thanks, Iampas!

 

[lampas]

Would you guys ming making a simple sketch of schematic for psu?

So here it is. First I did remove zeners on Chinese PSU and exchange them for 2x30V to get 60V. Then I add
attached capacitance multiplier and RC filter.

 

[kingkorg]

Awesome, thanks!

 

[rmaier]

Could some kind soul please post the PSU schematic one more time (maybe as a PDF)? It doesn't appear in my browser and I'd really like to check it out.
Thanks!
Ralph