Official C12 Clone - Build and Support Thread

[HellfireStudios]

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

With the heat sink in place, the temperature swing only occurs on the leads from the component. Also, I haven't destroyed anything yet. I got this tip from one of the guitar effects sites (I can't remember which one). BTW, I use the fridge and NOT THE FREEZER. I can't guarantee that your components would like that large of a swing.

-James-

 

[JessJackson]

getting back to plate high freq filter cap.

ela m 251e shows 100pf
ela m 261 shows 1000pf
Neumann M249 shows 500pf
Neumann M269 shows 80 to 160pf
Neumann M250b shows 600pf
Neumann M49 shows 150pf

 

[jrmintz]

3) In parallel with this resistor goes the cathode bypass cap (C4 on the original schematic).  It's value should be 20uF/16V (although anything from 10uF to 100uF should work).  If you get a radial cap, you can just wrap the leads around the resistor that you added in step 2 since these are in parallel.

Thank you for posting this Matador. For the cathode bypass cap does the negative end go to ground and the positive to the TK hole?

 

[tonycamp]

Calling Matador!

Long time no talkie ;D Hey i got the new styrenes in at all 3 places, and put a christian test tube in, fires up glows nicely, readjusted, b+ to 120v, set 6.3v heater at the tube, set the biasing to -1v, then i rechecked B+v and it went down to 113.4v with the trimmer all the way open? (insert best homer simpson voice) Doouuughp!

Any suggestions? looks like my saga saga continues :-[

Thanx
T

Edit: I read the whole thread, and conclude i need to change the values on resistors R1 and R2, to get the B+ up, does someone know the exact values on those? I read a couple different ones? I'm testing with a 12at7, and going to run a 5star 6072A from christian

thanx boys
T

 

[Matador]

Calling Matador!

Long time no talkie ;D Hey i got the new styrenes in at all 3 places, and put a christian test tube in, fires up glows nicely, readjusted, b+ to 120v, set 6.3v heater at the tube, set the biasing to -1v, then i rechecked B+v and it went down to 113.4v with the trimmer all the way open? (insert best homer simpson voice) Doouuughp!

Any suggestions? looks like my saga saga continues :-[

Thanx
T

How does it sound?

 

[tonycamp]

Hi matador,

thanx for responding!, I just threw an edit on my previous post, I read through the whole thing to get current(no pun ;D) I haven't plugged it in yet, i wasn't even going to bother until i had the 6072A and Tims capsule in it(which i have). Check my edit on my previous, what do you say?

thanx
T

 

[sr1200]

8) 8) 8) 3Hrs 51min Start to finish!!! Great guide and great project thank you so much!!!!!!

 

[micaddict]

Matador wrote:

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.

Cool, especially if there are sonic advantages.

Anyway, thanks for taking all feedback into account. Much appreciated.

 

[Melodeath00]

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.

Ah yes, I forgot about the grid coupling difference. I have read that this extends bass response. Is it not possible to eliminate the coupling cap while keeping the remote pattern switching? It seems then we would have the exact 251 circuit (with your other mods) other than the pattern switch.

By the way, could anyone send me a 251 schematic? I searched google images, but am only finding very small ones I can't read.

 

[sr1200]

Hey, the mic sounds AMAZING!!!!  I have one small question/gripe and perhaps its something that i did.  The new head basket has a helluva resonant frequency around 430 cps.  Anyone have a good way to dampen that? (besides EQ)

 

[JessJackson]

Went to use mine in session last night... really low output level by about 20db... hiss way too high. damn i was almost there.

 

[tonycamp]

Hey, the mic sounds AMAZING!!!!  I have one small question/gripe and perhaps its something that i did.  The new head basket has a helluva resonant frequency around 430 cps.  Anyone have a good way to dampen that? (besides EQ)

Yes, i have a great way to dampen that, go to home depot, go to the electrical section, find box/duct seal, It comes in the shape of a giant candy bar w/white wrapper, it costs under $2, its for sealing the back of electrical boxes from weather. roll up a small amount into mini hot dog shape the width of the vertical posts, then line it up and push it into the offending rails from the inside. Works awesome, and poses no threat of falling off or damaging anything, and soaks up those resonances. I've done this to a dozen mics with great results. It's not too sticky and messy at all, just make sure you wash your hands before handling the re assembly of the mic.

good luck

anyone know the exact value of R1 R2 resistor change? my B+ is maxed at 113.4V

thanx
T

 

[sr1200]

Cool ill look into that.  What if its the screen that resonating?

 

[chunger]

Can you check to see if there are any defects physically in the headbasket construction?  Broken/missing solder joints, etc.

In our testing, a sharp resonance should have been detected, but our primary tracking room inherently has a 500 Hz resonance that we know about and work around, so that could have partially masked the issue in our test environment.

Do you have clips?  Does the stock Alctron headbasket eliminate this resonance?

 

[HellfireStudios]

Cool ill look into that.  What if its the screen that resonating?

You could try dampening the rails with your fingers, and then tap/flick the screen to check. Same idea for testing the rails.
If the screen is the issue, the two screen version may be the only option, but some sort of clear spray-on plastic on the inside of the headbasket could work.
You've got me a little worried. I ordered the from first round of C12 headbaskets...

-James-

 

[tonycamp]

Chungs point aside, if you do the lower circumference and the entire U shaped rail, it should tamp down the mesh very well. At least that has been my experience, it works EXTREMELY WELL.  I'll post a pic of a head basket that has it later on tonight if i can.

 

[tonycamp]

Using 400K is fine.

There is obviously much higher tube tolerance than the original datasheets suggest.  Getting more current through the bleeder is one way to get more current flowing through the bias network.  In reality, the original C12 schematic shows a 300K resistor (R9) as the B+ bleeder, but that design was also only half-wave rectified, so it's not apple to apples in any case.  This had to be done in order to make use of the stock transformer in this circuit.

[size=14pt]As a general principle, if the B+ voltage is too low (can't be trimmed high enough), then one has several options:  a) reduce R1 and R2 in value, b) increase the value of the bleeder R6, or c) make the bias voltage more negative.  B and C reduce the current through the network which increases B+, and A reduces the voltage drop while keeping the current roughly the same.
[/size]
If the B+ voltage is too high (can't be trimmed low enough), then reverse the direction of everything above.

I was on the fence about making the B+ part of the supply regulated (with a MOSFET pass element and a Zener diode bias with feedback), however this is a drastic departure from the original supply and likely has more sonic implications.

Hey fellas, which option is best? my B+ is low at 113.4V, is there an exact value on R1 and R2 for a 6072A?, is a,b,or c a better option? I'm a first year newb, i'm getting better, but i'm just not comfortable making these decisions on my own

help a brother out?

 

[sr1200]

No defects that i can tell in the headbasket.  Only thing i can think of is that the chrome bath that it got made that screen mesh just REALLY hard.  (kinda like when you tune a drum head)

(sorry im trying things as i type this so it may seem all over the place)

I "massaged" the mesh a little (hearing the chrome crackling a bit as i did it) and it actually seems to have helped.  Maybe it was just to solidified from the chrome plating.  To answer your question Chung, no didn't get it with the original head basket and its 15 layers of mesh lol.  Im only hearing the ring now if i tap it with my finger, and if anyone taps this with their finger during a session im gonna be pretty annoyed anyway lol.  So, cheers n beers to you and matador for putting this awesome sauce together!!!! 

 

[chunger]

Just a clarification. . . this is a nickel plating. . . same as the microphone body except the surface is polished instead of bead blasted prior to plating.  And yes, all of the mics I put together for testing required a little bit of bending to straighten out the side rails because I'm anal like that, but when I made those bends, I did hear the crackling.  I'm assuming the plating while thin,  "glued" some of the mesh wires together and when I mechanically moved the mesh, the wires separated back to their original state.  This is all before audio was put through the mics.

I suppose it would not take much to dampen or move the resonant frequencies around (kindof like putting tape or those gel thingy-majigs on a drum head).

 

[Pip]

Using 400K is fine.

There is obviously much higher tube tolerance than the original datasheets suggest.  Getting more current through the bleeder is one way to get more current flowing through the bias network.  In reality, the original C12 schematic shows a 300K resistor (R9) as the B+ bleeder, but that design was also only half-wave rectified, so it's not apple to apples in any case.  This had to be done in order to make use of the stock transformer in this circuit.

[size=14pt]As a general principle, if the B+ voltage is too low (can't be trimmed high enough), then one has several options:  a) reduce R1 and R2 in value, b) increase the value of the bleeder R6, or c) make the bias voltage more negative.  B and C reduce the current through the network which increases B+, and A reduces the voltage drop while keeping the current roughly the same.
[/size]
If the B+ voltage is too high (can't be trimmed low enough), then reverse the direction of everything above.

I was on the fence about making the B+ part of the supply regulated (with a MOSFET pass element and a Zener diode bias with feedback), however this is a drastic departure from the original supply and likely has more sonic implications.

Hey fellas, which option is best? my B+ is low at 113.4V, is there an exact value on R1 and R2 for a 6072A?, is a,b,or c a better option? I'm a first year newb, i'm getting better, but i'm just not comfortable making these decisions on my own

help a brother out?

http://www.groupdiy.com/index.php?topic=51377.msg666445#msg666445

I think this is best option. Again 75K was my choice with the same problem.