Official C12 Clone - Build and Support Thread

[HellfireStudios]

My 251 schematic shows 1000pF for C5. Though, it would be interesting to hear what a smaller value cap would do to sweeten the top end in the C12 circuit...

-James-

 

[micaddict]

Thanks, Matador.

Here's a comparison by David Bock (who now does his 251 with a 6201 tube BTW). If it's too long or uncalled for, I'll remove it.

"They were both manufactured by AKG, despite the Telefunken badge on the 251.
They do share common a tube type, capsule type, output transformer, psu xfmr, & basic amp topology, all with minor variations and production changes.
For those who don't know the C12 is the skinny one and the 251 the fatter one.
Starting at the capsule, the 251 CK12 was a variant of the CK12, with slightly different backplates, and a little more low end than the C12 CK12. The capsule mount on the C12 hags the capsule, shock mounted with felt, while the 251 capsule sits  on a pedestal which is also shockmounted with felt. The 251 pedestal has a very slight angled face below the capsule, while the C12 has a strictly flat surface below it. I mention this because theoretically, there is high frequency reflection in the headgrille area which is broken up by angles, and not broken up by flat surfaces. This attempt to break up HF reflections may be found in other condenser mic designs as well, and is the basis for the Neumann M49 sloped headgrille.
The 251 mechanical assembly is mostly plastic (!) while the C12's is a metal frame. That 251 plastic often deteriorates & cracks.  The 251 has a pattern select switch just below the capsule, while the C12's is on the power supply. The 251 has two layers of mesh Course and Fine) in the headgrille, whilst the C12 is a very open one layer of mesh.
The amplifier of both mics is a single triode with resistive load, capacitor coupled to a small output transformer. They differ in how the patterns are achieved, and how the capsule is coupled to the amplifier. The 251 is self biased, meaning the cathode is connected to a resistor and capacitor (in parallel)  to ground, and the C12 is fixed bias with a grounded cathode and negative voltage from the psu. The 251 pattern control is achieved by manipulating backplates in the capsule, with no voltages exceeding 60v, and the C12 varies the voltage from 0-120v on the rear membrane. The 251 capsule is directly coupled to the grid of the tube, while the C12 has a coupling capacitor between the capsule and grid. The output capacitor values vary on  the 251 from 1uF-3.2uF depending upon production.
Original C12's used nice polystyrene & PIO caps, while the 251 used wet tantalums, ceramics, & polystyrenes. Both systems operate off 120v dc generated by their psu's.
The first 200 C12's had a large IE core transformer, as such, sound different that all subsequent C12's. After that both mics used the T14/1 tiny output transformer, with many production variations of that part alone.
They both came with large blade type conductor stand mount connectors, which are heavy duty and have stood the test of time.
Both power supplies are well made heavy duty simple designs that have also stood the test of time.
Many vintage 251 & C12 have had modifications and aftermarket psu's paired with them, be careful as the psu is part of the sound in mics like this.
What does all this mean? Well the 251 has a little more gain and is quieter, as well as a generally more pleasing vocal sound (my opinion). The C12 can be a little too bright on vocalists, and seems to find greater use as drum overheads. I think of the 251 as more "forgiving", and falls more into the "flatter the source" category than the C12's "capture the source" category."

Not from the REP forum this time, but from PRW. The whole thread is here:
http://prorecordingworkshop.lefora.com/2011/02/16/c12-v-ela-m251-differences/

 

[Melodeath00]

Thanks, micaddict - I've read that post many, many times myself through various google searches haha.

What is interesting/odd is the talk on PRW about how the decaying plastic int he 251 affects the tone. I wonder if anyone can chime in as to how that would be possible.

Matador, are you outlining the mod just so people know, or does this mean there will be no formal/official 251 PCB and build thread? I almost "assumed" it was coming, since it seems like as soon as I registered on this site, slowly but surely a PCB for the U67, M49, M269, and C12 was released.

 

[Matador]

Thanks, micaddict - I've read that post many, many times myself through various google searches haha.

What is interesting/odd is the talk on PRW about how the decaying plastic int he 251 affects the tone. I wonder if anyone can chime in as to how that would be possible.

Matador, are you outlining the mod just so people know, or does this mean there will be no formal/official 251 PCB and build thread? I almost "assumed" it was coming, since it seems like as soon as I registered on this site, slowly but surely a PCB for the U67, M49, M269, and C12 was released.

Those modifications I posted will get you 95% of the way there without having to spin another PCB and kit.  To truly match the exact circuit configuration means we'd need a mike body that has the proper polar pattern switches, etc.  Seems like a lot of effort for that last 5%.

 

[chunger]

The problem is not spinning another pcb. . . that is easy. . . well, for Matador anyways.  The bigger problem is indeed spinning a new body in order to "delete" what I feel to be a nice feature of the C12 - the 9 position variable patterning.

The turret-board style construction of this mic kit lends itself easily to this kind of component swapping.  We are out of test microphones right now, but another shipment should be inbound soon.  What I'm hoping to do soon after new mics arrive is to document this hybrid ELA configuration clearly and see where that takes us.  A switch can be hacked into the HT-11A body and I don't think it would be terribly difficult, but I'm not sure how many people on forum are keen to taking a dremel and file to their shiny new donor microphones.

 

[Melodeath00]

Those are good points. I don't feel the switch is particularly important, since we are using non-replica mic bodies anyway. I guess it would only matter if the switch caused a difference in sound compared to the 9 pattern PSU. "The 251 pattern control is achieved by manipulating backplates in the capsule, with no voltages exceeding 60v, and the C12 varies the voltage from 0-120v on the rear membrane." I'm not experienced enough to even make a guess if there COULD be a difference in how the capsule reacts in those two different situations.

 

[JessJackson]

just to confirm, I received Tim's TC12 4 terminal capsule. Do I tie together the backplates or keep them separated for this build?

Original c12's are tied together and in chungers build notes they are tied together but when i soldered a bridge across RB and FB on the circuit board, i can hear tube sound but no capsule sound at all.

 

[Tim Campbell]

Dear Jess,
Yes, you tie the backplates together. If you get no signal something is very wrong. Did you try the mic with the other capsule?
You must be very gentle when soldering the solder tags. All these screws thread into plastic. If they are over heated at all the screws can become loose. Check that the screws are snug (but don't overtighten them).
Scroll through the pattern selector and see if you get any signal in omni or figure 8.

If you can hear the amplifier then you have a missing connection rather than a short. A picture might help.

 

[chunger]

Did you verify prior to connecting Tim's capsule that the B+ voltage absolutely did not exceed 120V?  When I installed my non-expendable capsule, I adjusted B+ well to the low end of the trimmer pot's range and slowly inched it up after the capsule was installed.

 

[JessJackson]

Dear Jess,
Yes, you tie the backplates together. If you get no signal something is very wrong. Did you try the mic with the other capsule?
You must be very gentle when soldering the solder tags. All these screws thread into plastic. If they are over heated at all the screws can become loose. Check that the screws are snug (but don't overtighten them).
Scroll through the pattern selector and see if you get any signal in omni or figure 8.

If you can hear the amplifier then you have a missing connection rather than a short. A picture might help.

Thanks Tim,

I just checked all continuity, Everything is fine. When soldering the capsule I was very quick and made great connections.

Although one thing i just noticed. On the schematic ( http://musicalsparks.com/images/c12/c12_schematics_v3.pdf )

It shows Front Capsule going to R16 (30M) and Rear Capsule going to GRND

On my circuit board RC PIN goes to R16 and FC seems to be on grnd. although this would just reverse the capsule.

I found that i had accidently r17 and r13 swapped values, so i switched them just now. about to try it.

 

[JessJackson]

Did you verify prior to connecting Tim's capsule that the B+ voltage absolutely did not exceed 120V?  When I installed my non-expendable capsule, I adjusted B+ well to the low end of the trimmer pot's range and slowly inched it up after the capsule was installed.

my B+ sits around 123 at minimum adjustment.

 

[JessJackson]

WORKING! perfect, capsule sounds great tim, looking forward for head basket and ELA mod.

was the plate cap in most 251's 1000pf or 100pf? i see the schematics vary. thats a big difference.

 

[micaddict]

Those are good points. I don't feel the switch is particularly important, since we are using non-replica mic bodies anyway. I guess it would only matter if the switch caused a difference in sound compared to the 9 pattern PSU. "The 251 pattern control is achieved by manipulating backplates in the capsule, with no voltages exceeding 60v, and the C12 varies the voltage from 0-120v on the rear membrane." I'm not experienced enough to even make a guess if there COULD be a difference in how the capsule reacts in those two different situations.

I'm not qualified to offer full proof either, but the word goes that remote pattern control adds some noise and can (relatively) thin out tone. This is why some "remote controlled" tube mics have a hard switch, pure cardioid setting as an extra.
This doesn't have to be a clearly visible toggle BTW. From what I remember, the Horch (now Urton) RM2J has a very small, recessed "button" that has to be pressed with a pen. This completely switches off the back half of the capsule and makes for a considerably fuller tone IME. Added (=extra) proximity effect might play a part, too.

There are zillions (well almost) documentations about polarization. This one is more in depth than most and at the same time concise and readable IMO:
http://www.foxaudioresearch.ca/Polarpatterns.htm

This one is interesting, too:
http://www.google.nl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=22&ved=0CDsQFjABOBQ&url=http%3A%2F%2Fwww.neumann.com%2Fdownload.php%3Fdownload%3Dlect0003.PDF&ei=swOWUcjJJ-me0QXlvoC4BA&usg=AFQjCNFc6SJ8kStFG5hKFmXMmsARj-7vVA&sig2=C7oCgyyMsmPMG0pmy7ArnQ&bvm=bv.46471029,d.d2k

 

[Matador]

Dear Jess,
Yes, you tie the backplates together. If you get no signal something is very wrong. Did you try the mic with the other capsule?
You must be very gentle when soldering the solder tags. All these screws thread into plastic. If they are over heated at all the screws can become loose. Check that the screws are snug (but don't overtighten them).
Scroll through the pattern selector and see if you get any signal in omni or figure 8.

If you can hear the amplifier then you have a missing connection rather than a short. A picture might help.

Thanks Tim,

I just checked all continuity, Everything is fine. When soldering the capsule I was very quick and made great connections.

Although one thing i just noticed. On the schematic ( http://musicalsparks.com/images/c12/c12_schematics_v3.pdf )

It shows Front Capsule going to R16 (30M) and Rear Capsule going to GRND

On my circuit board RC PIN goes to R16 and FC seems to be on grnd. although this would just reverse the capsule.

I found that i had accidently r17 and r13 swapped values, so i switched them just now. about to try it.

I thought I had fixed that schematic.   The PCB is correct:  the "front capsule" should be grounded.  The capsule doesn't care either way it's just the patterns will be switched front to back.

I'm not convinced that the polarization changes make any difference, but I haven't A/B'ed them in a scientific manner either.  In one case, the capsule "sees" -60V by having 120V on the back diaphragm and 60V on the center diaphragm.  In the other case, it's zero and +60V, or the exact same bias flipped in polarity.  I don't think the capsule cares either way.

Now using the capsule as the grid coupling element (eliminating the coupling cap) probably makes much more of a sonic difference (comparing the 251 to the C12).  For those who are looking for cardioid/omni only it may be possible to hack up the design to achieve this:  I'll give it some thought.

 

[Tim Campbell]

I always find it preferable to have the front membrane at ground. The capsule is usually quieter ( the membrane acting as a shield to the polarized backplate) and it attracts less dust.

 

[tonycamp]

Hi fellas, i got my tim campbell capsules, and my new styrenes for the matachung c12(i apparently burned em, use hemostats instead of alligator clips for heat dissipation), i'm going to be calibrating with christians test tube, and installing Tims capsule this weekend, then i'll let everyone know my results.

In the meantime, i modded my rode K2 with a NOS mullard tube, uped the capacitors with hi Q's in the mic/psu, and put my 2nd Tim Campbell capsule in, it sounds amazing to me, transformerless, scouped mid/extended hi's, modern c12 like? I attached a clip that was done through a VP28 pre.

The mic already sounded really nice to me with the upgrades, but Tims Capsule just raised the bar on it to unreal good, i hope that translates in the clip?

https://soundcloud.com/tonycamp/modded-k2-samples-1

 

[HellfireStudios]

I know it is probably nuts, but I tend to put heat sensitive components (especially germanium diodes) in the fridge for an hour or two before I heat-sink-clip, and solder them. It's like insurance.

-James-

 

[tonycamp]

I know it is probably nuts, but I tend to put heat sensitive components (especially germanium diodes) in the fridge for an hour or two before I heat-sink-clip, and solder them. It's like insurance.

-James-

Hellfirestudios and Ice! never occurred to me James, good one i think ???

 

[MicDaddy]

Are there any negative ramifications from such a temperature swing?  I don't know just posing the question. 

 

[tonycamp]

Are there any negative ramifications from such a temperature swing?  I don't know just posing the question.

Hence the "i think ???" lol! Don't know either, I've done freeze and heat maneuvers with auto fab/mechanics, but i think i'll stick to hi heat, quick touch, with a high mass hemostat heat sink for now, again this may be totally wrong as i'm no expert :