JessJackson

Official C12 - Ela M / Build And Support Thread
« on: June 10, 2013, 12:02:32 PM »
I didn't want to polute the C12 Thread with this conversation.

I'd like this thread to talk about component material and value types pertaining to the original Ela M Circuit. 6072a and Ac701 versions. My next Ela M will be built around AC701.

This thread can be about modding the Chunger C12 to Elam and also next phase which is Ela M only.

Here is photo documentation of my current c12 to 251 conversion.









For now I used what I had on hand, 47uf cap and 2.7k resistor, Tefeunken USA use this in their mics and klaus quotes these values on his forum also. (Althought i'm not too sure why when every schematic I come across online quotes 1.8k and 20uf)

Matador is looking into the possibility of doing another board for the 251.

In terms of pattern switching utilising this body... I came across some internal images of the blackspade um25 and it seems they are incorporating three relays at the top for this purpose. This could definitely be the way to go without needing to hack switches into the body / head basket.

http://www.jessjackson.com/images/C12_251mod/bs1.jpg

http://www.jessjackson.com/images/C12_251mod/bs2.jpg



Winetree

Re: Official C12 - Ela M / Build And Support Thread
« Reply #1 on: June 10, 2013, 04:37:53 PM »
Want to do this to one C-12 Build. Have 2 layer headbasket. Beesneez K12 capsule
What are the correct values for the 3 parts?
47uf electrolitic Cap, (Black)
2.0uf or 20uf PPMF Cap? (Large yellow).
1.8K resistor? (Silver unit in picture looks like 100p)

By doing this 251 mod, it eliminates the Power supply pattern select?

strangeandbouncy

Re: Official C12 - Ela M / Build And Support Thread
« Reply #2 on: June 10, 2013, 05:40:32 PM »
I hate to be a pedant, but the 251 has no capacitor between the capsule and the tube, unlike the C12. I have read many times that this is a significant difference . . . .


     ANdyP


ps you could look up the Oliver Archut C12VR conversion for such an implementation. It works for me in my C12/251-ish mic.


. . . . RUH ROH . . . . .

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #3 on: June 11, 2013, 03:43:59 PM »
I hate to be a pedant, but the 251 has no capacitor between the capsule and the tube, unlike the C12. I have read many times that this is a significant difference . . . .


     ANdyP


ps you could look up the Oliver Archut C12VR conversion for such an implementation. It works for me in my C12/251-ish mic.

For those looking to do this conversion and only want omni and cardioid mode, the gate cap can be pulled out to be more 25x-like.  Otherwise one is stuck with the PSU-selected polarization scheme which needs a cap from capsule to tube grid.

Re: Official C12 - Ela M / Build And Support Thread
« Reply #4 on: June 11, 2013, 06:16:45 PM »
Will that work in this implementation? How would Omni and Cardioid switching work?
« Last Edit: June 11, 2013, 06:44:45 PM by jrmintz »
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #5 on: June 12, 2013, 12:25:50 AM »
1) Get rid of grid coupling cap
2) Connect FC to grid resistor (which grounds FC through the grid resistor).
3) +60V goes to backplate(s) (FB and RB remain tied together)

This is cardioid.  If you ground RB then you get omni - just like a U47.



Re: Official C12 - Ela M / Build And Support Thread
« Reply #6 on: June 12, 2013, 01:18:24 AM »
Thanks, Matador. Is R15 the grid resistor?
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #7 on: June 12, 2013, 11:41:33 AM »
Thanks, Matador. Is R15 the grid resistor?

Yes.  And since R15 gets connected to ground (via P4), C10 can be removed as well.

Re: Official C12 - Ela M / Build And Support Thread
« Reply #8 on: June 12, 2013, 11:13:35 PM »
C13 has to be jumpered, right? C10 can be removed?

Oh, I see. C13 is removed and FC is connected to the grid resistor, not FB.
« Last Edit: June 12, 2013, 11:24:01 PM by jrmintz »
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #9 on: June 13, 2013, 12:06:23 AM »
C13 has to be jumpered, right? C10 can be removed?

Oh, I see. C13 is removed and FC is connected to the grid resistor, not FB.

Yup.

The idea here: the capsule need to see 60V across the membranes.  One side (FC) gets biased to ground through the grid resistor.  Since there's no DC current through a cap, and no DC current into the grid (*hand waving here*, but it gets the point across), then FC looks like 0V to the polarization voltage.  Then the backplate(s) (which are tied together) see's half of the 120V B+ voltage which is 60V.

The rear capsule can be tied to either a) hard ground, or b) the front capsule, which is at DC ground, to get omni mode.  It can also be a) tied to +60V, or b) left floating, to get cardioid mode.

After some more thinking, I also see no reason the rear capsule couldn't be tied to the the existing P3 polarization voltage and retain the PSU-based polar pattern switching.  It wouldn't be EXACTLY the same as a 251, but it would still be a lot closer (overall) due to elimination of the gate cap.

If you'd like me to draw up a new schematic to make it clear, I can do that.


Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #10 on: June 13, 2013, 12:30:38 AM »
Here is the 251 capsule biasing mechanism:  FC = front capsule, FB = front backplate, RB = rear backplate, RC = rear capsule

ModeFCFBRBRC
Cardioid+60VGND (via grid)floatfloat
Figure 8+60VGND (via grid)GND+60V
Omni+60VGND (via grid)short to FB+60V

Signal is tapped from the FB connection.  Figure-8 and omni look suspiciously close together, however the secret between them is:  for figure 8, the +60V connection is isolated from the +60V supply to the FC via a low-pass filter (the resistor and cap in the capsule assembly).  Thus the capsule looks like two, "2-terminal" cardioid capsules stuck "back to back".  However in omni, the FB and RB connections are shorted together, and the FC and RC nodes are shorted together.  So now it looks like a 3-terminal capsule in a regular omni-biasing scheme.

Very different from how U87 does it!

KMB-AUDIO

Re: Official C12 - Ela M / Build And Support Thread
« Reply #11 on: June 13, 2013, 03:26:02 AM »


If you'd like me to draw up a new schematic to make it clear, I can do that.




I would like to see this if you have the time.

Thanks!



Call me Matthew.

Re: Official C12 - Ela M / Build And Support Thread
« Reply #12 on: June 13, 2013, 07:50:40 AM »
So for cardioid FC is not disconnected from R16, it's connected to both R16 and R15? I'd be happy with fixed cardioid, BTW.
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #13 on: June 13, 2013, 01:40:02 PM »
Let me generate a new schematic:  it's much easier than text. :)

Re: Official C12 - Ela M / Build And Support Thread
« Reply #14 on: June 13, 2013, 02:51:46 PM »
Thanks Matador.

I've been trying to redraw the schemo so it matches the old one, but it's a little over my head.
« Last Edit: June 13, 2013, 10:38:48 PM by jrmintz »
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #15 on: June 14, 2013, 12:42:21 AM »
Ok, does this make sense?

C12 to ELAM251 capsule

Re: Official C12 - Ela M / Build And Support Thread
« Reply #16 on: June 14, 2013, 07:40:31 AM »
This shows the capsule connections, the cathode bypass resistor and cap and the 100pF cap would still be as in JessJacksons redraw above?

It's making sense to me. Thanks for taking the time to do this Matador.
« Last Edit: June 14, 2013, 07:49:16 AM by jrmintz »
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Matador

Re: Official C12 - Ela M / Build And Support Thread
« Reply #17 on: June 14, 2013, 10:07:39 AM »
This shows the capsule connections, the cathode bypass resistor and cap and the 100pF cap would still be as in JessJacksons redraw above?

It's making sense to me. Thanks for taking the time to do this Matador.

Yes I only changed the capsule connections in my diagram.

Re: Official C12 - Ela M / Build And Support Thread
« Reply #18 on: June 14, 2013, 11:55:33 AM »
Great.
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

Re: Official C12 - Ela M / Build And Support Thread
« Reply #19 on: June 14, 2013, 11:37:36 PM »
It works great! I can't tell from my own voice if it sounds like any of the good 251s I've used, but it certainly has a nice tone. That's saying a lot with my voice. I have some good singers in next week to try it out on.

I realize now why I was having the spitting and hissing problem - I hadn't removed the direct wire from cathode to ground before adding the 20uF cap and 1.82k resistor. So I was running them in parallel. I finally grasped that after staring at the schemos long enough.

Anyway, thanks Matador, thanks 'Chung, thanks Jess for your schemo.
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but


 

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