Official C12 Clone - Build and Support Thread

[junkface]

thx chunger for the clarification. was pretty sure about all that but your post set my mind at ease moving forward for sure, especially the part about the skin being glued to the tension ring. one last question I have is whether I should be biasing the power supply to the specs for the CT12 as in the OP, or if there's another +/- dc I should be shooting for to have it running it's best. thanks again!

 

[gyraf]

Been trying to contact Tim
to get another capsule pair for a few weeks now but haven't heard back.

He's very overworked these days - some micromechanic polyethylene-based devil plays tricks with him, and perfectionism is a hard thing when that happens...

Jakob E.

 

[Category 5]

Been trying to contact Tim
to get another capsule pair for a few weeks now but haven't heard back.

He's very overworked these days - some micromechanic polyethylene-based devil plays tricks with him, and perfectionism is a hard thing when that happens...

Jakob E.

Being the best is tough.  Lol.  I will keep at it.  Hopefully his business slows by just two capsules very soon and I can slip in there.

 

[Enchilada]

Hey guys,

I've finally got around to building my C12 kit. It has replaced my C12 circuit I built from a schematic ages ago. This one seems to have a lower output that the one I built. One major difference between this one any my old one is I had paralleled both sides of the tube.

Is there any particular reason why this one only uses half of the tube? Wouldn't I get more gain out of it if I were to parallel the grids, cathodes and plates and add a second zero ohm resistor? Either way I'm pretty sure it'll give me a bit more volume. I reckon I'll give it a crack and report back.

Kris

EDIT: Giving this some more thought, I'm pretty sure paralleling the triodes would reduce it's output impedance. Could this mean that I may need a lower ratio trafo? Or would my CM-2480 still be OK?

 

[Category 5]

The original C12 is a plate follower using only one triode.  Some variants of the circuit, particularly those that directly mod the stock HT-11A circuit, utilize the second triode as a cathode follower creating a constant current draw (CCD) amplifier.  You get hotter output and obviously higher drive capability (depending on your output transformer of course).  If authenticity is what you're after pick a side and only use one triode.  The C12's lower output level is to be expected.  It is not as hot as other tube mics, but still you shouldn't find noise too objectionable with any decent mic pre.

 

[Matador]

You can parallel both side, but there are several modifications needed in both the mike and the PSU in order to make this work.

Twice the current can mean twice the noise, all other things being equal.

 

[Millerjay]

Hey guys,

Hopefully someone can help me. I finished the build but I can't seem to get the B+ voltage below 175v. It actually measured 195v with no load and measured 195v when I connected the mic. When I tried to adjust the resistor to drop the voltage to 120v it slowed down when it reached 176v and seemed to stop. Am I missing something here? If so I would greatly appreciate any assistance.

Thanks

 

[Matador]

Read back through the thread and you'll find this issue debugged several times.

Start with eliminating the mike from consideration:  use a 180K resistor to simulate the mike (connect as a load from B+ to ground by sticking it directly in to the 7-pin output connect at the PSU) and make sure the PSU adjusts to the correct voltage.  Then you can debug further.

 

[Millerjay]

Thanks a lot. I actually figured it out that same night. I had the 1 and 6 wires crossed on my cable.

 

[Category 5]

If anyone hears back from Tim C can you have him contact me.  I've been trying to get In line for another pair for a few months.

 

[Melodeath00]

If anyone hears back from Tim C can you have him contact me.  I've been trying to get In line for another pair for a few months.

I was in the queue for another capsule, and I think it was supposed to be ready in mid-September.  I haven't heard from him, but I don't exactly NEED to be spending money right now anyway haha. Maybe I'll shoot him an email and see what's up

 

[gyraf]

Cat.5: Tim is starting to surface again. I asked him to contact you, once he's ready to take in new orders...

Jakob E.

 

[Category 5]

Cat.5: Tim is starting to surface again. I asked him to contact you, once he's ready to take in new orders...

Jakob E.

Thanks Jakob.    It's gotta be tough when you're the only guy in the world that can do what he does.

 

[TheJames]

In case anyone is interested...

I didn't like the cheapy connectors/inserts on the PSU and mic body.  The connectors are based on Switchcraft types and an order for a D7M (the 7pin XLR inserts are apparently unobtanium), D7F, and either a D3M (full connector) or QG3M (3 pin insert to reuse in either the Chinese connector shell or the Switchcraft shell from the D7M) will get you going.

I found these at reasonable prices (for 7 pin XLR's at least) from onlinecomponents.com in the US.

Life got in the way of me finishing mine up.  Now that I'm "free" again, I'm trying to wrap these up!

 

[Banzai]

Has anybody had the chance to directly compare the  CM-13114 to the AMI T-14?

Very interested in any feedback on the differences (and/or similarities).

 

[chunger]

Has anybody had the chance to directly compare the  CM-13114 to the AMI T-14?

Very interested in any feedback on the differences (and/or similarities).

Brian Fox at Fox Audio Research has posted a few comparison plots and audio samples isolating the transformers.  In general, CM13114 rolls off sooner in the treble and has a flatter bottom end.  The T14's bass response experiences a dip at around 80hz and the bottom drops off sooner than the CM13114 but the high end is more extended.

At the end of the day, the capsule has profoundly more effect on the overall microphone's response than the transformer in my limited experience (when dealing with good transformers. . and certainly both of these models qualify).  . . even the sonic differences between different copies of Campbell CT12 capsules and/or flipping front and back membranes had more of  an audible effect than transformer swap.

 

[skidmorebay]

I am finally asking for help, after days of fruitless troubleshooting.

I hooked up my C12 through a preamp and directly into my monitors. I only hear low-frequency hum. The hum doesn’t go away when the body tube is installed, so I don’t think it’s RF. I’ve replaced C12 and C13 just in case they were passing DC. When I ground the grid the hum goes away. The capsule seems to be working but the audio is overwhelmed by the hum.

If I inject a 50mv sine wave at the grid, I get a sine wave at the output cap (C12) of about 1V. As soon as I attach the signal generator lead to the capsule side of C13 the hum goes away and I can hear the sine wave through my speakers. So, I think the tube circuit is working correctly.
When I remove the signal generator lead, the hum comes back. The hum is loud and reads about 1.2V at the output of C12 on my oscilloscope.

I’ve tried three different tubes with the same result.

Other vital signs:

B+ reads 120V at the mic pcb (P5)
Bias reads -1.01V at the mic pcb (P4)
P3 (polarization) reads about 0 at omni, 57V at cardioid, 117V at fig-8 on my Fluke DMM at the mic pcb (P3)
Heater is at 6.3V at the mic pcb
I get 58.7V at the junction of R12 and R13 on the mic pcb
FB and RB are bridged at the pcb. I’m using the stock Alctron capsule.

I have gone through the mic with my multimeter and checked for continuity at every component - everything appears to be hooked up correctly according to the schematic. I have continuity from the mic body all the way to the ground lug on the PSU, and between P6 on the mic pcb and the mic body. The PSU grounds to the chassis at the 7-pin XLR, as per Chunger’s build guide.

Anything I am missing?
thanks!
JS

 

[JessJackson]

This mic sounds so much better with a fully passive power supply like original c12's

Most of you who built this mic with supplied psu board aren't hearing it's full potential.

 

[Category 5]

This mic sounds so much better with a fully passive power supply like original c12's

Most of you who built this mic with supplied psu board aren't hearing it's full potential.

What's the difference Jess?  The Mataching PSU is just like the original except for the regulated heater isn't it?

 

[Matador]

This mic sounds so much better with a fully passive power supply like original c12's

Most of you who built this mic with supplied psu board aren't hearing it's full potential.

What's the difference Jess?  The Mataching PSU is just like the original except for the regulated heater isn't it?

Yes, B+ is passive just like the original, with the exception of using a full wave bridge rectifier rather than a half-wave rectifier.

I await to hear how it's different, other than using an LM317T to regulate the heater supply rather than a Zener diode.