Official C12 Clone - Build and Support Thread

[barbaroja]

I m wondering about the original capsule holder that telefunken also seems to use (or a similar one) like  a capsule shockmount. The capsule is placed really low in the headbasket and that back post should really do something at least to some extent. I am getting this picture from saturn sound so can be explained clearly:

Reflections in headbasket and things like this should affect sound waaaay more than lets say a cap type change.

 

[barbaroja]

This one taken from telefunken:

 

[Spencerleehorton]

hi,

my B+ voltage @ psu is only at 114v and i'm getting quite a lot of hiss with this mic which is a shame.
I've used a peluso TXF and an Electro Harmonix tube, will investigate and hopefully cure all ills.

regards

Spence.

 

[Spencerleehorton]

hi,

near years resolution, dont take so much work on, slow down, and check wiring on each project at least 3 times!!!
C12 works wonderfully now, quiet as a mouse, wired up the 7 pin wrong!!! doh!!
now for some testing!!

regards

Spence.

 

[TLRT]

Some rock tunes recorded with my ChungermoddedC12(acoustic guitar and all vocals):

https://www.youtube.com/watch?v=h_-sNIW06ao

 

[micaddict]

Thanks!

Details, please?
(Capsule, tube etc.)



Henk

 

[TLRT]

Thanks!

Details, please?
(Capsule, tube etc.)

Tim Campbell CT12, NOS GE6072a(80's green logo, it is really hard to find a good one,most of them are quite noisy out of the box) the rest is "Chunger's stock"



Henk

 

[Matador]

I am getting ready to build a pair for myself (I have many copies, but most were hacks after hacks as I tried different configurations or otherwise tried to break them).  The last three I've built were all sold after sessions were concluded, so I knew I was on to something from the very beginning.

From top-to-bottom:

1) Chunger's 34mm edge-terminated capsule
2) NOS RCA 12AT7 (screened from my stash for lowest noise)
3) Silver Mica caps for on-board filters (C10, C11)
4) High impedance 30M and 250M resistances changed to 100M metal film
5) Very nice NP0/C0G ceramic cap for 1000pF C13 gate coupling cap (my favorite all-time audio coupling cap in 1nF or less sizes)
6) Cinemag CM-13114 12:1 output transformer

I will also go through and do my normal "re-tap every single hole for a stainless steel 2-56 1/4" machine screw" mod, which I have found makes a profound difference to the physical feel of the microphone (it feels tighter, and more "solid", if that makes sense).  Or it might just be my imagination, but I tend to strip the stock 2mm screws out fairly frequently.

I will try and take a few pictures as I go in case anyone is interested.

 

[barbaroja]

3) Silver Mica caps for on-board filters (C10, C11)
4) High impedance 30M and 250M resistances changed to 100M metal film
5) Very nice NP0/C0G ceramic cap for 1000pF C13 gate coupling cap (my favorite all-time audio coupling cap in 1nF or less sizes)

Great! Care to comment on these choices? Soundwise?

 

[Matador]

3) Silver Mica caps for on-board filters (C10, C11)
4) High impedance 30M and 250M resistances changed to 100M metal film
5) Very nice NP0/C0G ceramic cap for 1000pF C13 gate coupling cap (my favorite all-time audio coupling cap in 1nF or less sizes)

Great! Care to comment on these choices? Soundwise?

Mostly just to experiment:
1) C10 and C11 are filters, so I think just about anything will work well.  Silver mica are sturdy and take soldering well.
2) I think anything above 10M works, so 100M for everything makes the BOM easy with less things to mess up
3) C0G ceramic are my all time favorite neutral caps for small value audio coupling:  almost no temp coefficient, no leakage, and can take soldering forever and aren't microphonic.

 

[chunger]

does this mean you can dunk the whole mic board into a vat of isopropyl alcohol after you're done building it and just let it soak over night?  ;D

 

[barbaroja]

Haha. I also got some mica caps so am wondering if it would be worth the hassle.
Guys, any opinions on the capsule holder/shockmount thing? Few posts before.

 

[Melodeath00]

Haha. I also got some mica caps so am wondering if it would be worth the hassle.
Guys, any opinions on the capsule holder/shockmount thing? Few posts before.

There's nothing to really say, unfortunately. I wanted one of these original-style holders for myself, as well, but I see no way to get one, and wasn't sure if it would be easily "compatible" with the Alctron body anyway.

 

[barbaroja]

Haha. I also got some mica caps so am wondering if it would be worth the hassle.
Guys, any opinions on the capsule holder/shockmount thing? Few posts before.

There's nothing to really say, unfortunately. I wanted one of these original-style holders for myself, as well, but I see no way to get one, and wasn't sure if it would be easily "compatible" with the Alctron body anyway.

What do you think about lowering the capsule and this back post changing the sound? Dont get me wrong, the things sounds awesome, we all know this is nitpicking to the point of shame. Anyways, thats the way the originals were made and I vaguely recall a reading on the subject that said it played some role in the C12  signature sound. Was years ago, tho.

 

[Melodeath00]

Haha. I also got some mica caps so am wondering if it would be worth the hassle.
Guys, any opinions on the capsule holder/shockmount thing? Few posts before.

There's nothing to really say, unfortunately. I wanted one of these original-style holders for myself, as well, but I see no way to get one, and wasn't sure if it would be easily "compatible" with the Alctron body anyway.

What do you think about lowering the capsule and this back post changing the sound? Dont get me wrong, the things sounds awesome, we all know this is nitpicking to the point of shame. Anyways, thats the way the originals were made and I vaguely recall a reading on the subject that said it played some role in the C12  signature sound. Was years ago, tho.

The capsule in mine (CT12) looks appropriately/proportionally low despite using a saddle mount. In theory, that mounting bar along the back would play a (small) role  only when the rear diaphragm is in use (so NOT during cardioid, which is what I use 100% of the time anyway). I can't imagine it having a huge effect anyhow. If you hold a pencil or some other small object on the backside of your C12 headbasket when you're in omni/bi-directional, how much of an affect do you notice? I doubt it's much, if anything at all, but test for yourself.

I highly doubt anyone has directly compared a C12 with old-school mounting, and a modified C12 with a saddle. Also, I think people often overstate differences anyway.

 

[colin_maybe]

Hi all!

Is there anything unusual that needs done to configure a 12AT7 in the ELAM 251 circuit? I've got the c12_to_ela251_v2 image up, and the values there appear to be for the 6072A.

Following Chunger's documentation, it looks like the only change should be in R1/R2 (91K -> 33K), and R17 (100K -> 47K). Do these same values apply to the 251 circuit?

Colin

 

[Matador]

Hi all!

Is there anything unusual that needs done to configure a 12AT7 in the ELAM 251 circuit? I've got the c12_to_ela251_v2 image up, and the values there appear to be for the 6072A.

Following Chunger's documentation, it looks like the only change should be in R1/R2 (91K -> 33K), and R17 (100K -> 47K). Do these same values apply to the 251 circuit?

Colin

That's it.  The cathode resistance may need to be tweaked a bit (lowered down slightly from 1.8K), but I would try it first.

 

[micaddict]

After I had asked him, this is what Matador himself wrote in the White Market thread:

Using a 12AT7/6201 would be pretty simple with a few BOM changes.  The plate resistor would need to be dropped to about 47K, and idle current would need to increase up to about 1-1.5mA with a -1V bias.  Gain would be similar as well.  The series PSU filters (90K) might need to be lowered to keep B+ in the right range.  Input headroom would be about the same as well (about 2VPP).

I must add that was on March 22, 2013.
The "woulds" in that quote tell us that he hadn't tried it, yet.

Now he actually uses 12AT7s for his own builds. 



Henk

 

[Matador]

Yup! I think half a dozen or so people have contacted me regarding the conversion, so several have it up and working with success by changing the plate load resistor to 47K and dropping the two filter resistances in the PSU down to 33K.  Bias can be tweaked per normal using the pot in the PSU.

This is a good source for getting a rough idea on cathode resistance: http://www.trioda.com/tools/triode.html

With 120V B+, and the load line tweaked for 47k plate resistance, the curves predict a Q point of 0.97mA.  For -1V bias, cathode resistance is 1.12k (not 1.8k like the ELA M251 uses).  The load lines also suggest that running a Q point at 1.2mA and lowering the bias down to -0.8V (drop cathode resistance down to 680R) yields less distortion for the same input swing.  It's something builders can play around with however: 1K seems like a good starting point.

In terms of sound quality:  I've found the 12AT7's (at least from my stash) have a slightly higher noise floor than Chunger's NOS 6072A tubes that he gets from Christian Whitmore.  I purchase my stash for use in phase splitters, so out of the 30 or so that I had (mostly from RCA with grey plates), 6 or 7 of them fared best on the "pencil test" (ground the grid with a jumper, fire up the mike, and rap on the side of the tube with a pencil, and listen for the volume of the "plunking sound"...which is a microphonics test), and had minimum "whistle/blow" noise after 4 or 5 days sitting in a Fender amp.

If you are recording quiet instruments in quiet rooms, it's probably better to get the tubes directly from someone who will screen them for you for this exact application.

 

[kcatthedog]

I am getting ready to build a pair for myself (I have many copies, but most were hacks after hacks as I tried different configurations or otherwise tried to break them).  The last three I've built were all sold after sessions were concluded, so I knew I was on to something from the very beginning.

From top-to-bottom:

1) Chunger's 34mm edge-terminated capsule
2) NOS RCA 12AT7 (screened from my stash for lowest noise)
3) Silver Mica caps for on-board filters (C10, C11)
4) High impedance 30M and 250M resistances changed to 100M metal film
5) Very nice NP0/C0G ceramic cap for 1000pF C13 gate coupling cap (my favorite all-time audio coupling cap in 1nF or less sizes)
6) Cinemag CM-13114 12:1 output transformer

I will also go through and do my normal "re-tap every single hole for a stainless steel 2-56 1/4" machine screw" mod, which I have found makes a profound difference to the physical feel of the microphone (it feels tighter, and more "solid", if that makes sense).  Or it might just be my imagination, but I tend to strip the stock 2mm screws out fairly frequently.

I will try and take a few pictures as I go in case anyone is interested.

Let me get this straight; you are modding a DIY mod ?  Chung:  is nothing sacred ?