AKG C12 circuit

[baadc0de]

So in the docs section of this site, there is this nice AKG C12 schematic, including a power supply
http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315

Since I've recently acquired most of the stuff like body, output transformer, tube, PSU case, .. and am about to get the capsule.

I have a couple of questions though, first about the head amp..

There are 7 wires indicated in the PSU-mic cable (and that's what you usually get for a tube mic) but there is no wire identified as '7' though there is one '8'. Typo, 7 = shield or is there a deeper meaning that I missed?

Can C12 be .47uF? I'd assume so, what are the benefits / repercussions in changing this value? Bass bump / resonances?

When wiring the capsule, are the wires from the backplate different or it doesn't matter? You can see on the schem one goes to C13, the other goes to R14, so is it the same or do these carry different 'signal'.

I guess I can keep resistors in the head amp at 1/2W?

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The power supply is bothering me a lot though. I don't know what voltages the power transformer has on the secondaries, I don't know which diodes are used (some random googling proved futile for me).

It looks like regulated heaters to 6.3V and no idea what ends up on the B+.

I have no idea on how the pattern selection thing works. And why it works.

Would be very nice if someone could shed some light on these

 

[gemini86]

Read the circuit description for the g7 on gyrafs site. That will explain how polarization of the capsule works. As for diodes, 1n4007 will work fine. I think the b+ is in the 250 range? Really don't know without being at my PC.

 

[MagnetoSound]

B+ is more like 120v.

 

[baadc0de]

B+ is more like 120v.

I saw that apex 460 has a recommended b+ of 120 - 125V and people stick in the same tube, so I assume that would be a safe starting point.

 

[dissonantstring]

B+ is more like 120v.

MagnetoSound is correct.  i've seen a brochure page which states 6.3V DC filament voltage and 120V DC plate voltage.  i'll try to load it up when i find it in the tech docs (AKG thread).
kind regards,
grant

 

[Kamel]

Check C24 schematic, information are very interesting, and except 12,6 instead of 6.3 shemo are very similar

http://www.groupdiy.com/index.php?topic=44737.msg563663#msg563663

 

[baadc0de]

Yes, very interesting..

So, if I get this correctly, the pattern selection is a 1-pole 9-position rotary switch with 50k resistors forming an arc on the position contacts, the pole is connected to the 1M and eventually to pin 8 of S24 device, while the two ends of the arc connect to pins 4 and 5. Looks like a tapped voltage divider to my nooby eyes. So the omni position is esentially ground potential and the other extreme is +120V?

I see this schematic has the output capacitance upped to 1.5uF, could anyone explain how this affects the signal?

And the plate power supply has changed from half wave to full wave, the heater one seems adjustable. I thought to add both plate and heater to be internally adjustable.

I've also taken a look at the more modern C12VR schematic.. do you guys think it's better / worse as a head amp? I see it uses both halves of the tube, so that makes me wonder what that changes.

 

[Kamel]

yes patern selector is a voltage divider with low pass filter to polarize the rear of the back to back capsules

1,5 uF value give more low end

i use the C12 Vr, it's a good mic, but nothing to see with the historic C12, 2 différent mics, same name surely for marketing aspect

 

[Gus]

I do not know the level of skill the first poster has from the questions asked.

Fixed bias tube microphone power supplies need good designs.  I would build a different supply than shown in the schematic however it might not be what people would expect.

One should be able to use  Ohms law and tube curves to get close to the voltages in the circuit from just looking at the schematic.

As for the other questions in the first post.  The selection of values and how you wire thing comes from experience and math and learning.  The part used depend on a number of things.  The answers you might get from the web should be questioned.

IMO microphones like the real C12(what C12 do you want to build) are often more about the capsule, grill and  transformer.  The other parts and voltages matter but sometimes can be adjusted for the build.

What output transformer will be used and who makes it?
What capsule will be used?
What tube? (good luck with current built tubes)
What body?
What PS transformer VA rating etc?
Are things that would help others to help you if they want to.

 

[baadc0de]

Hi Gus,

thank you for chiming in, I appreciate it. Let me try to answer your questions..

My skill with microphone building is not really existent. I did DIY a few boxes of equipment like 1176, la2a, pultec etc.. and did some mods to them, but nothing more serious like replacing a gain stage, just tweaking filters, adding switches to defeat certain parts of a circuit etc. I had some supervisory/troubleshooting help in these matters either by forum members or some el. engineers I know (though none of them are analogue or mic building experts).

I have toyed with the idea to have someone design me a power supply for these microphones. I know I wouldn't be able to make a good one, I've seen some rectification and filter circuits and soldered nigh dozens of them, but I just found out how a diode works recently, so while I'm learning, I'm hardly in a position to design my mic supply I did ask one of the engineers I know to design me a DC-DC schematic to regulate between 110-130V for plate supply (from 120V) that looks very much like heater regulation in C24. This was more for fun, because I wanted to know how it would look like (what kind of devices are in there) and perform.

I'm looking to build a C12 microphone that I can tweak. I would like to test different mods to it, driven by my imagination, feedback from recording and scrounging the net for ideas and hints. It doesn't need to be historically accurate, but that would be somewhat of a bonus, as I heard a few raw tracks recorded with C12 and I liked them a lot and thought I could use a tool like that.

The output transformer is an AMI T14/1 http://www.tab-funkenwerk.com/id76.html
Capsule is a C12 chinese capsule and either a beesneez C12 or tim campbell's version. To be swapped in and out..
Tube is a GE 6072 with 5 red stars I got from a poster here and was supposedly tested in a mic, though I could use that in an LA2A and get something else if that one won't do.
Body is roughly an ELAM shape done by a poster here. It's not terribly sophisticated, but it is a tube and has a grill Supposedly some G7 projects done with such bodies were well accepted.
PS transformer I haven't bought yet, but I was looking at a hammond 100CT and 6.3CT with I think enough juice to run at least 10 of these tubes (I looked at tube datasheet plate and heater current).

Basically I'm looking for a tube, 9-pattern modding platform with good parts. The idea is to build two mics and adjust one, compare them side by side.. repeat process.

 

[mad.ax]

There are 7 wires indicated in the PSU-mic cable (and that's what you usually get for a tube mic) but there is no wire identified as '7' though there is one '8'. Typo, 7 = shield or is there a deeper meaning that I missed?

Can C12 be .47uF? I'd assume so, what are the benefits / repercussions in changing this value? Bass bump / resonances?

The schematic shows only a 7 wires + shield cable. Numbers refers to the Tuchel connector pin numbers. As you can see in the pic (found on the Flea site) the C12 connectors have 8 pins. Pin nr7 was not used.

Given the tolerance of capacitors, there's no practical difference between 0.47, and 05µF...

Axel