Official C12 Clone - Build and Support Thread

[stelin]

Great! I have sent you a "contact message" via store.studio939.com with my address.

Thanks,

Stefan Lindberg

 

[barbaroja]

I *really* need to get the next prototype of my regulated supply done.

I am anxious to demistify the regulated vs unregulated thing...

 

[kidvybes]

...just returned and refreshed from a relaxing stay at the powder-coat spa...

 

[kidvybes]

...say "cheese"!

 

[Category 5]

Looks fantastic!  That was over top of nickel, right?  How much was it?

 

[kidvybes]

Looks fantastic!  That was over top of nickel, right?  How much was it?

...yeah, over the stock finish, no problem...seems very nicely done...this company manufactures aluminum shutters and powder coats large runs frequently...if we put a few mics together for a run of the same color, she quoted me $85 for five bodies (up to 2 pieces per body)...crazy cheap IMHO...single unit jobs like this can be combined with larger in-house jobs of the same color, based on scheduling, for about $20-25 each (which is what I paid this first time)...I got some color-chips I can show you of some of their stock colors (vintage-looking greys, seafoam beige, creme, etc)...

...I may also check out a local motorcycle shop that also does custom powder coating to see their work...

 

[Category 5]

Looks fantastic!  That was over top of nickel, right?  How much was it?

...yeah, over the stock finish, no problem...seems very nicely done...this company manufactures aluminum shutters and powder coats large runs frequently...if we put a few mics together for a run of the same color, she quoted me $85 for five bodies (up to 2 pieces per body)...crazy cheap IMHO...single unit jobs like this can be combined with larger in-house jobs of the same color, based on scheduling, for about $20-25 each (which is what I paid this first time)...I got some color-chips I can show you of some of their stock colors (vintage-looking greys, seafoam beige, creme, etc)...

...I may also check out a local motorcycle shop that also does custom powder coating to see their work...

Wow.  that's cheap.  Any issues with fit after the fact?  Is there a web site with colors and textures to choose from?  Great work man!  Now get us that laser engraving place and our clones will look as legit as they sound.

 

[kidvybes]

Looks fantastic!  That was over top of nickel, right?  How much was it?

...yeah, over the stock finish, no problem...seems very nicely done...this company manufactures aluminum shutters and powder coats large runs frequently...if we put a few mics together for a run of the same color, she quoted me $85 for five bodies (up to 2 pieces per body)...crazy cheap IMHO...single unit jobs like this can be combined with larger in-house jobs of the same color, based on scheduling, for about $20-25 each (which is what I paid this first time)...I got some color-chips I can show you of some of their stock colors (vintage-looking greys, seafoam beige, creme, etc)...

...I may also check out a local motorcycle shop that also does custom powder coating to see their work...

Wow.  that's cheap.  Any issues with fit after the fact?  Is there a web site with colors and textures to choose from?  Great work man!  Now get us that laser engraving place and our clones will look as legit as they sound.

...I asked if it was necessary to tape off areas where fitment was critical, and they explained that the powder-coating is very thin but resilient, so they did not feel it would be an issue...the mic fits together as easily as prior, and there's no sign of binding or such...really very satisfied...

...funny, when she called to tell me the body was ready to be picked up, she left a voicemail, "Hi, your mic thing-a-ma-jiggy is all finished"...say what?

 

[mgronroos]

Wow, that looks really cool!

It is a bit hard to see, is it ELA M-mint or white?
I ask because I just handed my own ELA M build over to a powder coater today, and tried to match the pistageness of the color as good as possible.
I am in terrible angst right now that I will get back something looking like an ice cream.. 

Great looking mic, kidvybes!

Best,
Michael

 

[kidvybes]

Wow, that looks really cool!

It is a bit hard to see, is it ELA M-mint or white?
I ask because I just handed my own ELA M build over to a powder coater today, and tried to match the pistageness of the color as good as possible.
I am in terrible angst right now that I will get back something looking like an ice cream.. 

Great looking mic, kidvybes!

Best,
Michael

...thanks Michael!...it's Nortek powder-coat pigment "Cream":

http://www.nortekpowder.com/pdfs/NortekColorCatalog-2011.pdf

...I have a "seamist-green" 251 clone, so I wanted this to be different...someone told me Neumann used a color very similar to this in the 30's...so, not historically correct for a C12, but very vintage (pretty) looking...I'm very pleased with it...

 

[stelin]

Does anyone know how the original C-12 was finished? Was the body painted or where they nickel plated? If painted, what color? Light gray?

 

[barbaroja]

Hey Mat.  I'm sure it's been covered but I can't for the life of me find it.  What was the determining factor to go for -1V for the bias voltage rather than the original spec. -1.1V and how would that affect the final sound.  The reason I ask is because two of my matched pair (capsules/tubes/etc.) sounded different and one sounded slightly more vintage.  When I re-biased them for the matched tubes I noticed the bias had drifted below -1V on the one that was more "milky" while the second mic was still right at -1V and sounded much more open.

Is there a way to measure the exact bias each tube wants?  Can a .1V variance really produce an audible difference?  It's so difficult to test in application because it become impossible to maintain exact distance, performance dynamic, etc. between adjustments.

Each tube will have a particular "optimum" bias point based on it's exact parameters (rp, gm, mu, etc) and the exact impedances that are connecting it in circuit.

The -1V point was my best swag based on the test tubes I installed in the dozen or so prototypes that were built and tested.  There is no "just one" bias point to cover every tube, just like there is no single source resistor to handle every JFET (since every JFET has a different gm, VGSoff, etc).

"Optimum" is also overloaded, as you can optimize for different things.  The C12 circuit is "middish" gain (like 8-10dB), and I think was optimized to larger signal swings at the grid to give more headroom for very dynamic capsules.  I think sensitive large diaphragm condenser capsules are capable of near 1V signal peaks on close-miked loud sources, so that was the optimization point I picked.

A -1V bias gives 2V peak-to-peak input signal swing (actually a bit more, since the tube will still function properly when the grid goes positive WRT the cathode), so that seemed like a reasonable range.

If you want to get super pedantic, you need to bias the tube just like a JFET:  with a maximal signal swing test.  You dial up the bias resistor to maximum, and inject a 100mV 1kHz sine wave and increase the amplitude until one side of the signal (measured at the plate) starts to squash out and distort (this will generally be the bottom half of the signal, where the grid is driven positive and then rapidly goes non-linear).  Then you lower the cathode resistor until symmetry is perfect again.  Then start increasing amplitude again.  You'll reach a point when as you increase the signal more, both the positive and negative peaks start to squash about equally.  This is the optimal bias point, where "optimum" means "maximum input signal headroom", and NOT maximum gain!

This circuit is very flexible:  you can lower the plate resistor which increases the current and increases the slop of the load line.  Gain drops, but you can move the bias point lower and the circuit can take even higher signal swings with no distortion.  I've used a 68K load resistance and a -1.5V bias point, and the circuit can swing 3V PTP.  It really didn't sound any different to me though (I may have not been really abusing the capsule, like close miking a kick drum from 6" away).

However -1V should work in Most CasesTM.

Can I do this with the capsule connected? I am thinking I should because of the unregulated PSU thing.

 

[kidvybes]

Does anyone know how the original C-12 was finished? Was the body painted or where they nickel plated? If painted, what color? Light gray? Sorry for my bad english. 

...I believe the earlier versions were a kind of matte metal finish as below:

 

[kidvybes]

...the 251's were painted:

 

[mgronroos]

Thx for the color chart, kidvybes, and nice choice!
I will check this prior to choosing color from now on.
Silver Vein and Hammertone look like nice choices for the PSU..

I think I might have chosen a color more like the "Light Ivory". It remains to be seen if it was a wise choice.
If it turns out well, I'll post a pic too.  8)

I friend of mine has an original 251, so it will be interesting to compare them, both sonically and the color matching.

I see that you coated the lower locking nut too. I also asked for that so I am happy to see that it looks great on your mic, I hadn't seen that been done before so it felt like a small risk.
I like it when the mics look as good as they sound! 

Best,
Michael

 

[barbaroja]

Follow up: Tried to do with a very small voltage and the capsule connected, with a very small signal going after the 1000pf cap. Used my daw as a tone generator, 1khz sine, pin1 of xlr going to the grid with alligator clip with mic assembled to avoid nasty RF. Pin 2 was not needed it seems, since I could hear fine the sinewave through the speakers. Used oscilloscope again and, depending on the input signal level, it would start to squash mostly over the top. Careful checking for not saturating the converters, tho. Adjusting bias did not affect at all the symmetry of the wave. Not at all!

Is this maybe a coupling impedance problem I guess? I mean, I do not think the mic is supposed to see a 10Kohm impedance, that would draw too low current in the mic transformer primary... from what derives my questions:

1. Is this a good method?
2. Is this a impedance matching problem? If it is:
3. Would strapping a resistor of "x" ohms in the transformer output give me correct results?

Thanks!

Edit: tested going through a vintech X81 preamp for loading, very clean drive.
Waveform still smashed in the lower region, no bias adjustment could solve.

 

[Category 5]

Follow up: Tried to do with a very small voltage and the capsule connected, with a very small signal going after the 1000pf cap. Used my daw as a tone generator, 1khz sine, pin1 of xlr going to the grid with alligator clip with mic assembled to avoid nasty RF. Pin 2 was not needed it seems, since I could hear fine the sinewave through the speakers. Used oscilloscope again and, depending on the input signal level, it would start to squash mostly over the top. Careful checking for not saturating the converters, tho. Adjusting bias did not affect at all the symmetry of the wave. Not at all!

Is this maybe a coupling impedance problem I guess? I mean, I do not think the mic is supposed to see a 10Kohm impedance, that would draw too low current in the mic transformer primary... from what derives my questions:

1. Is this a good method?
2. Is this a impedance matching problem? If it is:
3. Would strapping a resistor of "x" ohms in the transformer output give me correct results?

Thanks!

Edit: tested going through a vintech X81 preamp for loading, very clean drive.
Waveform still smashed in the lower region, no bias adjustment could solve.

This may not be the right way to do things, but perhaps you could try playing a sine wave through the mic instead of injecting it.  Would that work?

 

[Matador]

Follow up: Tried to do with a very small voltage and the capsule connected, with a very small signal going after the 1000pf cap.

Where were you injecting the signal?  What does "after the cap" mean?  Directly to the grid?  Did you make sure there was no DC offset in the signal?

 

[barbaroja]

Follow up: Tried to do with a very small voltage and the capsule connected, with a very small signal going after the 1000pf cap.

Where were you injecting the signal?  What does "after the cap" mean?  Directly to the grid?  Did you make sure there was no DC offset in the signal?

Yes, directly to the grid. About the DC offset, really do not know, just took the #2 pin for the soundcard out. Sorry If I am doing noob stuff but I am learning as we go on. Thanks for your help!

 

[barbaroja]

Any advice? I have scheduled a recording session, last time I used the mic was distorting easily. I want to use it in this session.