Sela T-12 inspired Mic... PSU/Transformer help!

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speakercoil

Well-known member
Joined
Apr 16, 2009
Messages
138
Hey guys...

So yes me again with more mic build questions/info, etc... I know I already have a thread about the U47 clone I am planning on building (which will happen and I have already started buying parts), but since that is a time/wallet consuming labor of love, I am also working on other projects in the meantime. I am going to do a G7 soon, but that is being put on pause until Skylar's bodies are finished.

In the meantime I am going to attempt a Sela T-12 inspired build! It is about as simple as a mic can get in the schematics...

http://www.filmsoundsweden.se/backspegel/sela.html

However I cannot for the life of me find a PSU schematic anywhere. I know Mike has been very interested in building one of these as well and we have been searching everywhere. So I am finally after about a month and a half of asking questions else where deciding to call on the help of the geniuses that reside on this forum!

Are there any other mic PSU's which may work for this mic, or will I have to get a custom design or have someone custom build one for it?

The capsule I plan on using will be an M7, the tube is a 6AK5, and Oliver told me a BV8 would work for this design. However Oliver offered some advice on changing the values around a little.

"just with seeing your schematic the BV8 should work with the 6AK5. Only modification I would do it to change the 0.5 cap to 1uF.
The cathode resistor is a bit low for my taste but a suggestion based on a schematic is always difficult. A 3k potentiometer should help you to figure
the best bias point."

As I know myself and I would sit for days and weeks and still not decide on a value if I had a 3k pot and tested the resistor value... Could anyone give a recommendation on a good value there?

I plan on using a Bees Neez body for this project. Already have some old NOS Siemens 1uf caps lying around as well.

Anyway as I want this to be my next project I was hoping to get info soon. The build will become my second instructional video after I post the Apex 460 mod one.

I think this would be an awesome build and extremely easy to do point to point... Trying to figure out how I will mount the tube, but I will figure that out once I get down and dirty with this project.

Alright enough rambling... Let me know! Oh, and I will pass the word along to Mike, unless he sees this thread... Which I am sure he probably will. lol.

Thanks,
-Grant

PS: I know a few people on here actually own Sela's... If any of them own a T-12 it would be awesome to see the inside of the PSU!!!!
 
PPS... I forgot to mention my Transformer question...

I was thinking of using the BV8, but also wanted to know if anyone else had any suggestions for a different transformer as well!

-Grant
 
hi again grant how about using a G7 PSU cant remember the B+ it puts out I think it was more than 60V tho but you could drop it down with somes resistors....I was experimenting with the Sela t12 circuit used a power supply I made for a pre amp didnt get working at the time had thins to do so had to leave it for a while I will get back to it tho.....good luck  :)
 
Thanks... Yeah I was hoping maybe the G7 PSU would work. i am kind of fuzzy on the theory side of things... I mean I can find my way around schematics, but I wouldn't know where to put the resistors or what value, etc to change the voltage. If someone here could offer suggestions on that, I would be VERY grateful!

Also it is nice to see someone else working on a T12 project!
 
Just drop the HT from a Chinese PSU using a 60V zener, or use dropping resistors.

Alternatively build a new supply like the G7 as suggested but for 60V. Shouldn't be tricky.

There are several transformers probably suitable for the 6AK5 in triode mode.
 
Even better... So when you say Chinese PSU I am assuming you mean the ones for Apex 460's etc...

-Grant
 
Yeah, it's the easy-peasy option.

http://cgi.ebay.co.uk/Tube-Microphone-Regulated-Power-Supply-7-Pin-Design_W0QQitemZ260556257488QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3caa5cf8d0

Just swap the zeners, and jumper the diode hanging off the 6V heater regulator.
 
Wait jumper the diode? What does that mean? I get switching out the zener, that's easy enough. But do you mean reroute the diode somewhere else using a wire?
 
Nah, was just referring to lowering the heater voltage. Don't worry about that just now. Basically, you can pull up a voltage regulator by using a diode to get an extra 0.7V.

Not related to the HT.
 
Awesome... Then I shall start ordering my parts immediately!!!! YAY! Thank you again for your help. This project is going to be a pretty fun one, and I will do a video for this as well. I think this will be a GREAT beginner mic build project for people wanting to get into building their own mics instead of just modding. I mean come on, it doesn't get much simpler than 2 resistors, a capacitor, capsule, transformer, and tube... People modding 460's do more work than that!

-Grant
 
Look closer at the page you linked the t24 looks like a better circuit to me note the added RC filter before the plate resistor, and the voltage divider for the capsule voltage(maybe take the filter and voltage divider section and add it to the t12).  I would read up on power supply design and make a nice supply if I was to use a m7 in a build.  Also note the body and grill designs.  The t12 does not have a grid to ground resistor and is wired more like the Royer MXL2001 tube mod, look close.

The pt6 link has a power supply schematic that might be of help for the other microphones.

It is not a sela this or that number with out the correct transformer and capsule and caps and body etc.  Don't get stuck with this model or that number microphone.  You have your basic triode(or pentode wired as triode) plate out fixed or cathode biased, all kind of microphone models/numbers are based on that basic design.  Then you have CFs.  Plate out to CF.  Hybrid K2, NTK etc.  The details are what can matter.

It is easy to make a tube microphone that will pass sound the trick is to understand what does what to the sound and how to adjust the circuit for the capsule/grill/body/transformer.

If you are going to build microphones google etc for tube microphone schematics and study them.
 
I have studied a lot of schematics... But mostly the classics. I have studied many different U47 ones with the different tube options as I want to build a U47. Those I have studied time and time again... I have also looked at the C12, ElaM 251, the T24... I thought about the T24, but don't want to pony up 500 bucks for the tube. lol... That was the beauty of the T12 as it uses a easily found tube!

Could I modify the T24 circuit to use a different tube? I mean of course it can be done, but I guess I am asking would it be easy and what tube would be a good match!

I am not trying to create a replica mic here, but just sort of a different flavor than is normally offered out there! I know there are others on this board interested in building something like these.

I have heard that the T12 does sound very good, despite being a very simple circuit. However I have heard RAVE reviews about the T24. So yeah... If anyone has info on how to change the tube in the circuit and what tube, and then I guess I can use the power supply listed.

The body and grille will be nice as it is going to be a bees neez body/head.

The other option besides the M7/BV8 I considered would be a K47/Sowter 9995. Mainly just because it would be a different flavor as I plan on using an M7 and BV8 in the U47 clone.

-Grant
 
Gus said:
Look closer at the page you linked the t24 looks like a better circuit to me note the added RC filter before the plate resistor, and the voltage divider for the capsule voltage(maybe take the filter and voltage divider section and add it to the t12).  I would read up on power supply design and make a nice supply if I was to use a m7 in a build.  Also note the body and grill designs.  The t12 does not have a grid to ground resistor and is wired more like the Royer MXL2001 tube mod, look close.

The pt6 link has a power supply schematic that might be of help for the other microphones.

It is not a sela this or that number with out the correct transformer and capsule and caps and body etc.  Don't get stuck with this model or that number microphone.  You have your basic triode(or pentode wired as triode) plate out fixed or cathode biased, all kind of microphone models/numbers are based on that basic design.  Then you have CFs.  Plate out to CF.  Hybrid K2, NTK etc.  The details are what can matter.

It is easy to make a tube microphone that will pass sound the trick is to understand what does what to the sound and how to adjust the circuit for the capsule/grill/body/transformer.

If you are going to build microphones google etc for tube microphone schematics and study them.

Ooooo I like the idea of adding onto the T12 circuit... I am going to compare the two schematics and draw up a hybrid schematic... It may be completely wrong, but it would be fun to end up with something different like that. Once again a unique flavor. I will post it when I finish the hybrid design... That gives me a project tonight.

-Grant
 
OK Gus, or anyone who can take a look at this. I tried to add in the part of the circuit from the T24... Not sure if values need to be changed or if I made mistakes... But let me know how this looks. As I am a completely novice when it comes to theory, I need some feedback. It has hard because the T24 schematic didn't see where the actual points of connection were. So I wasn't sure where connections actually were, but I gave it my best guess.

Let me know what you guys think... The 20M resistor seems a little high to me, but that's what is on the T24 so I left it the same. If that is correct it is going to be a pain in the butt trying to source that part, as 1M's are already hard enough!

Anyway let me know guys!

Thanks again... I must say it was exciting trying to mix circuits... Very fun. It may be 100% wrong, but we shall see! I am going to be very proud if it is correct or if I only made 1 or 2 mistakes.

-Grant

 

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I cant help you with your hybrid design sadly unlike gus I cant study circuits to see how they work so i have to make and see thats why I wanna do this circuit its as simple as they come see would be quick and easy to solder together and try if it dont sound good try another circuit, Iv often read that lowers the parts count in audio the better prob not always the case but fun to try....
 
Yeah that was my understanding of circuit design as well... The fewer the parts, the cleaner the sound. However with mics it may be a little different, because they are at the front end of your signal and need to be protected from different interference, and need to have aspects compensated for.

However I know with Mic Pre's the whole simple signal path idea does provide a clean and clear signal!

-Grant
 
speakercoil said:
OK Gus, or anyone who can take a look at this. I tried to add in the part of the circuit from the T24... Not sure if values need to be changed or if I made mistakes... But let me know how this looks. As I am a completely novice when it comes to theory, I need some feedback. It has hard because the T24 schematic didn't see where the actual points of connection were. So I wasn't sure where connections actually were, but I gave it my best guess.

Let me know what you guys think... The 20M resistor seems a little high to me, but that's what is on the T24 so I left it the same. If that is correct it is going to be a pain in the butt trying to source that part, as 1M's are already hard enough!

Anyway let me know guys!

Thanks again... I must say it was exciting trying to mix circuits... Very fun. It may be 100% wrong, but we shall see! I am going to be very proud if it is correct or if I only made 1 or 2 mistakes.

-Grant

Hi Grant,

The 6ak5 can take more plate voltage so if you use  a power supply that has 125v B+, 10k, rf Filter, and 1megx 1meg (R4,R5) voltage divider to get the polarization voltage near 60v. Then (R3) 100m to 1gig with a cap to ground (C2) will work well with the M7 voltage.
Gus said use a grid to ground resistor, good design.


Jim Jacobsen
JJ Audio
 
Awesome I am going to try and mod the schematic a little and I will repost it with those changes... We shall see if it is better the second go around.

-Grant
 
Oh... Just found this info... Translated from Swedish.

"SELA made no power supply to their microphones in the T-series, without referring to Neumann and Pearl. Neumann had a suitable battery heater, model BB12, with a dc with germanium transistors, which were fed with 3 x 1.5 volt batteries size LR 14th Neumann power supply model NKMa could also be used to SELA's microphones in the T series. Both sets were 6-pin mini-Tuchel connectors on microphone side, and 3-pole normal Tuchel at the end. (Pictured above is the power supply rebuilt and instead equipped with a 3-pin XLR Cannon at the end.)

When we drove T24 on battery (BB12) was lower noise level. Microphone case, an AC701, is direktupphettat so fastidious in pure filament voltage. With BB12 retrieved the tube filament voltage directly from the batteries, while anodspänningen came from a DC / DC converters. "

I thought that was kind of interesting... So theoretically I could use a Neumann KNMa if I could find one!

 
R5 & R6 look to be in parallel. Could just be the way you've drawn it but that's not what you want!
 

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