Official C12 Clone - Build and Support Thread

[ln76d]

I 'm not having particular EMI issues, it's just that I don't know which perturbation can occur in the future. The more the power line is cleaned, the more the PSU should be efficient, I am right? Or does it have bad sides too?

It's a ******** which you can read on audiophile articles and forums. Original C12 was build few decades ago with power supply which use selenium rectifiers, germanium transistor and diode and old as hell big can electrolytics.
Stored in good condition, original PSU works as it should.  Even with 60 years old capacitors.

 

[cariocaman85]

Thanks for your reply.
I don't understand why it should be bad to give the PSU the most perfect input it is supposed to work with.

So in your experience, filtering the line before reaching the PSU transformer:
Is good for the sound? (in term of noise floor, dynamic)
Is useless for the sound?
Is bad for the sound? (in term of dynamic, does it slow the signal)

As for the bias, I still wondering if an active design could benefit... I don't know if it is a deliberate choice from the engineers back in the 50s or if a passive design was the only option back then.

Anyway as I wrote before the Matador/Chunger design is really really great and my two C12 I built few years ago are my best microphones in the studio! I just want to try to get the best of the one on the way

 

[Matador]

As for the bias, I still wondering if an active design could benefit... I don't know if it is a deliberate choice from the engineers back in the 50s or if a passive design was the only option back then.

Try a AA battery.  You can wire one in to provide -1.5V bias (plus side of the battery goes to ground, and the negative terminal goes to the mike).  If you want to get fancy, wire in a 10K pot and you can vary the bias between 0V and -1.5V.

It would only cost about $1 (battery only) to $5 (including the pot) for the experiment.

 

[pundit]

Okay my C12 clone is up and running fitted with a Tim Campbell C12 capsule and a NOS 1953 GE GL-6072 tube from Bowie.

However when switching to either Fig 8 or Omni mode the rear of the capsule sounds far superior to the front.

It is louder, has more low end and more presence.

B+ Voltage:  120 VDC
Heater Voltage:  6.3 VDC
Bias Voltage:  minus 1 VDC

Any suggestions? - TIA

EDIT: When I set the pattern switch two clicks back from full Omni the front and rear of the capsule sounds more balanced than when set to full Omni. I'll check the pattern switch resistor network in case I've screwed something up.

 

[jrmintz]

I changed some parts in my 251 version today, same values, different types. I thought I'd check the voltages before I put everything back together and I'm finding I can't get my B+ above around 93VDC. I have a vague recollection of this happening before, but I think it might have been trimpots in the wrong spots. I checked them and they're correct. What else would make that happen?

Thanks.

Edit: I just got to page 14 or so of this thread where Matador describes what do do about low B+. I will get on it and report back.

 

[Phrazemaster]

Okay my C12 clone is up and running fitted with a Tim Campbell C12 capsule and a NOS 1953 GE GL-6072 tube from Bowie.

However when switching to either Fig 8 or Omni mode the rear of the capsule sounds far superior to the front.

It is louder, has more low end and more presence.

B+ Voltage:  120 VDC
Heater Voltage:  6.3 VDC
Bias Voltage:  minus 1 VDC

Any suggestions? - TIA

EDIT: When I set the pattern switch two clicks back from full Omni the front and rear of the capsule sounds more balanced than when set to full Omni. I'll check the pattern switch resistor network in case I've screwed something up.

That would't surprise me. In the olden days when they used just trimpots, it was known that sometimes the best cardioid position was a little to the left or to the right of the "true" cardioid position as shown on the trimpot. You're probably fine, but I'd be interested in others' opinions too.

Best,

Mike

 

[Murdock]

Okay my C12 clone is up and running fitted with a Tim Campbell C12 capsule and a NOS 1953 GE GL-6072 tube from Bowie.

However when switching to either Fig 8 or Omni mode the rear of the capsule sounds far superior to the front.

It is louder, has more low end and more presence.

B+ Voltage:  120 VDC
Heater Voltage:  6.3 VDC
Bias Voltage:  minus 1 VDC

Any suggestions? - TIA

EDIT: When I set the pattern switch two clicks back from full Omni the front and rear of the capsule sounds more balanced than when set to full Omni. I'll check the pattern switch resistor network in case I've screwed something up.

How did you test the mic? with headphones on? try to record Your voice, or a guitar with each side, and hear again, or flip the phase on Your preamp when listening with headphones. Does it still sound bad?

 

[chunger]

The c12 capsule is complex and one of the more difficult ones to manufacture.  Even with the tightest Q/C standards, there will be variations from capsule to capsule and front to back.  Sometimes, for specific frequencies your recording system/environment and personal tastes are sensitive to, these can be somewhat dramatic.

All of my builds have featured a side of the capsule that I prefer to the other, and I try both and configure the "better" side to front.

 

[Tim Campbell]

Hey Pundit,
I go to great lengths to match halves to tighter tolerances than AKG did. It might be stray resistance somewhere. If it's the capsule you can always send it back to me. I'll measure it and adjust it if necessary.

 

[pundit]

Hey Pundit,
I go to great lengths to match halves to tighter tolerances than AKG did. It might be stray resistance somewhere. If it's the capsule you can always send it back to me. I'll measure it and adjust it if necessary.

Hi Tim,

It maybe something related to the pattern switch.
I haven't been in the studio since last week so I haven't been able to properly troubleshoot what the issue is.

As I said, two clicks back from full Omni seems to provide the most even balance between the front and rear of the capsule so it's quite possible I may have screwed up the resistor positions on the pattern switch.

I was careful to clean off all the flux and any contaminants.

I'll be back in the studio tommorrow and I'll report back.

Overall the mic sounds good especially with the NOS GE GL-6072 tube from Bowie but I just need to sort out the mismatch problem.

 

[jrmintz]

Just curious - has anyone tried Hexfred diodes in the PSU? Do they have a useful effect on the sound of the mic?

 

[jasonallenh]

Just finished my build. I used one of the Avantone CV-12-style clone (this one was badged as a "PPA" mic, but I can't find it online anywhere)- the one with the pad and HPF switches on the body. I wanted to repaint the mic tube (I did the same with my U87 build), but as I started to sand the tube, I discovered that this one had a BRASS body... it looks too cool like this for me to paint it...

Also worked first try- quiet as heck! Although the rear doesn't seem to reject very well in cardioid when compared to the sides in Bi-directional mode. Is that normal? I don't care that much... it sounds great!  ;D

 

[Tim Campbell]

Jasonallenh, yes you'll get quite a bit more signal from the rear half in cardioid than the side in figure 8. This is normal.

 

[jasonallenh]

Thanks Tim! Makes me feel even better about how well this build went! The mic sounds fantastic, so I really wasn't worried. The mic is VERY quiet. I got my tube from Peluso- so I definitely recommend that.

I am curious about one thing- do you always set B+ before the bias? I found that after setting the B+ to 120 and the bias to -1V, the B+ would be off again. Do you repeat the process until it's right, or just set B+ and then bias and leave it alone? I don't know as much about tube tech, so I'd like to know a little more about this process.

BTW, If anyone needs the .56uF Erse and polystyrene caps for this build, pm me... I have plenty of extras because I had to buy enough to meet the minimum order quantity. 

 

[Matador]

I am curious about one thing- do you always set B+ before the bias?

Yup - bias sets the DC idle current, which in turn effects the B+, which in turn changes the idle current, which changes the bias....  8)

You just need to go back and forth until both are settled where you would like them.  Anytime I set these, I use two DVM's, one on each voltage , and just trim back and forth until it's set the way I would like.  Note that you really need to let the tube sit for a few minutes between adjustments, especially if the tube isn't fully 'warmed up'.

 

[jasonallenh]

Yup - bias sets the DC idle current, which in turn effects the B+, which in turn changes the idle current, which changes the bias....  8)

You just need to go back and forth until both are settled where you would like them.  Anytime I set these, I use two DVM's, one on each voltage , and just trim back and forth until it's set the way I would like.  Note that you really need to let the tube sit for a few minutes between adjustments, especially if the tube isn't fully 'warmed up'.

Nice! It's an analog whack-a-mole mini-game ;D 

 

[Melodeath00]

Hey everyone, it's been quite awhile since I posted.

To be honest, I haven't been using my C12 too much lately (just haven't had the need), but I turned it on last night and noticed a strange intermittent rustling/noise sound.

I tried changing the tube, but the issue was still present. I also tried a different power outlet, and a different mic cable.

What are some other things I could look for? It's fairly quiet noise problem, but at the same time, obvious compared to other mics.

Here is a sound clip:
https://www.dropbox.com/s/qtpvuzjchyxml83/C12%20Mic%20Rustling.mp3?dl=0

 

[OPR]

Most likely moisture in the capsule .
Try sealing it in a bag with desiccant bags for a few days to absorb and excess moisture.

 

[Tim Campbell]

It could also be dirt in a switch or contamination from fingers or flux somewhere. Leave the mic turned on for a few hours and see if it disappears.

 

[micaddict]

Hey everyone, it's been quite awhile since I posted.

Yeah, but we haven't forgotten you.
Good for us you ran into some trouble. 
Nah, just kidding. This sucks, of course.
But good to see you.