Official C12 Clone - Build and Support Thread

[Spencerleehorton]

Hi Matador/Catagory5,

thanks for what you have said, at least now i know im getting the correct level of output, just would be better to have a 12:1 rather than a 10:1 transformer that its got in there.

regards

Spence.

 

[bsatriani]

Hi Matador,
I built a pair of C12 - it is a pleasure to build kit!

First note: I could get B+ set to 120v only increassing the R6 so I add 499k in series so the total R6 is now 969k(thougth it's less work than replace both 91ks).

Now,
I could say everything it's good, but there is one issue - a self noise wich is kind of annoying. I could hear it on the audio sample you posted but I put it on the background noise from your garage or your soundcard, then I could hear it on my builds - both of them - even it's less noissier than the one from your sample.
Now I've listen the audio samples from Tskguy - there is also present - it's a kind of a pink noise wich can be clearly hear on the HK12  samples but NOT on the C12 samples ( it's harder to hear it on speakers, but on headphones, it's easy - especially on the end of the vocals and ac guitar samples : on the C12 samples you can hear the console's hum but no pink noise, on the HK12 samples - console hum PLUS pink noise) - well, to be more exact, when A-Bing tracks, C12 samples do have a very little self noise wich is normal, but when switching to HK12 track - then seems like  adding a "spoon" of pink noise to the signal.
I first blame my tubes - a nos JAN GE6072 and a nos Mullard 4024(the second build it's a Elam251 mod and use 12at7), but hearing it on every sample I listen I think it's something else - maybe a PSU issue?
I saw that in the original C12 schematics there aren't any var-resitors on the B+ rail, nore any IC on the Heater rail. Could those components be the source of the noise? Or what else?...

It would be a fantastic mic if the noise would be removed!

Best regards,
Bogdan

PS Thank you Chung for your excellent build tutorial and for your support and help in sending me the replacement order when my initial order didn't arrive!

 

[Melodeath00]

For those interested in clips, this is my C12 (early 1960s NOS 6072, AMI T14, Tim Campbell CT12) versus my U47 (EF800, AMI BV8, BeesNeez K7) on my Martin acoustic

https://dl.dropboxusercontent.com/u/1579861/Acoustic%20C12%20vs%20U47.wav
https://dl.dropboxusercontent.com/u/1579861/Acoustic%20C12%20vs%20U47%20B.wav

 

[chunger]

Regarding noise, it is a well known fact in tube sourcing circles now that the supply o NOS GE 6072A tubes is very picked over.  It is starting to get very hard to find a truly quiet sample now. . .  And if.you do source a genuinely high grade tube from a reputable vendor, be prepared to pay a hefty premium.

As it concerns the specific audio samples posted, tskguys test mic for that session was using a NOS RCA tube at the time the origins and specs of which I do not know.  I am pretty sure the owner of that early original C12 has it fitted with one of the very best GE 6072A's. . . As supplies dry up, expect noisier tubes coming into service in mics.

A general note about the GE tubes. . . I think at this point it is safe to treat them as an "endangered species".  We should be vigilant and responsible to make sure the remaining samples are maximized.  I will strongly advise against the use of one of these rare tubes on a  budget Chinese capsule based build.  For those, use the Electro Harmonix 6072A or NOS 12AT7.  If you are using a campbell or tskguy or beesneez or other premium backplate-correct capsule and opt to source a NOS GE 6072A, please make every effort to make use of both triodes in the tube either by building pairs of mikes wired to opposite triodes and swapping.the tubes periodically or by re-wiring the tube socket of your single mic after some time.  I am also sourcing a batch of NOS 6072A's that test "mic grade" on only one triode.  This will be a good option for folks building single mics.

I have updated the build documentation to reflect the necessary component changes to use 12AT7 and 6021 tubes as well.  I will begin testing in earnest the Mullard NOS 12AT7 with the campbell as well as Chinese capsules in this kit.

A concerted effort to preserve supplies and maximize the sonic potential of these tubes is necessary at this time.  Lets make sure they do not go to waste.

 

[Category 5]

I've been collecting 6072 tubes lately and yes, they are more scarce now for sure but all of the 5 stars I've found except one have been dead quiet and sound great.  I'll say its worth sourcing a early/mid 50s specimen though.  The triple mica black plates with square getters sound significantly better than the 70s tubes I've tried or even the late 60s tubes I've tried.  I'm going to build another mic to explore the AT7s in this project but the really old 5 star 6072s are just killer in this mic.

On a side note I got my CT12s from Tim Campbell and have to say that dude is one of the coolest and nicest guys I've dealt with. 

 

[Woot]

Do you have any way of injecting a signal into the grid of the tube?

Unfortunately no, a scope etc is on my buy list...

Just wondering still if anyone can point me in the direction of how to troubleshoot my problem?

Thanks again,

Matt

 

[Matador]

Do you have any way of injecting a signal into the grid of the tube?

Unfortunately no, a scope etc is on my buy list...

Just wondering still if anyone can point me in the direction of how to troubleshoot my problem?

Thanks again,

Matt

Without any toos it will be a challenge.   If the tube rings when you tap the body, then it's probably amplifying.  I would look at the capsule connections. 

You can make a makeshift signal generator if you own a DAW and a sound card by cutting an unbalanced cable on one end and then using the two wires:  ground the shield side to the mike body, and use the signal side, along with a sine wave generator plugin, to inject a signal right into the grid coupling cap and make sure the tube is amplifying properly.  That would be a good place to start.

 

[pmcorbett]

Hey Matador/Chunger,
Can you tell what voltage R17 regulates...i.e where does the output voltage of R17 flow to? Just curious. I'm modding my mic for 12at7 conversion. Cheers.

 

[Category 5]

Just wondering still if anyone can point me in the direction of how to troubleshoot my problem?

Thanks again,

Matt

Probably a long shot but did you bridge the backplate pins?

 

[Matador]

Hey Matador/Chunger,
Can you tell what voltage R17 regulates...i.e where does the output voltage of R17 flow to? Just curious. I'm modding my mic for 12at7 conversion. Cheers.

It's the plate load resistor.  The output voltage develops across it, and leaves the mike via the coupling capacitor and out through the transformer.

 

[MicDaddy]

...but there is one issue - a self noise wich is kind of annoying. I could hear it on the audio sample you posted but I put it on the background noise from your garage or your soundcard, then I could hear it on my builds - both of them - even it's less noissier than the one from your sample.
Now I've listen the audio samples from Tskguy - there is also present - it's a kind of a pink noise wich can be clearly hear on the HK12  samples but NOT on the C12 samples ( it's harder to hear it on speakers, but on headphones, it's easy

Yes this is exactly the noise I've been chasing.

I first blame my tubes - a nos JAN GE6072 and a nos Mullard 4024(the second build it's a Elam251 mod and use 12at7), but hearing it on every sample I listen I think it's something else - maybe a PSU issue?

I have a 6072A GE from BOWIE.  I have since swapped out this tube with a lot of 5x 6072A EH (low noise microphonics) from Tube Depot.  It's not the tube.  The EH are not terrible by any means FWIW to those looking for tubes.

I've since done some hacking, removing the teflon pins and a handful of the turrets, floating certain components, leads and wiring several sections p2p.

Tube mics in general have a much higher self noise than that of transistor based true condensers? 

 

[micaddict]

Tube mics in general have a much higher self noise than that of transistor based true condensers? 

My Brauner Valvet X tube mic has a self noise below 9dbA.

 

[Melodeath00]

FWIW my C12 is not noisy. It has about the same level of noise as my point to point U47 build. Do you hear the "pink noise" in the acoustic guitar clip I posted? Any mic has noise, but actually, I found my Sennheiser e906 was much noisier than this tube mic once I turned up the preamp gain to get it as loud as the C12.

How important is it to get the bias voltage at exactly -1V? I found that the bias voltage and B+ seem to effect each other (I guess due to passive PSU?), and it's hard to balance them when adjusting. I think my Bias is at about -1.005V or so, with B+ at exactly 120V or possibly 120.1V. Is this "close enough" for long term use?

Thanks!

 

[Category 5]

Melodeath that sounds perfect.  I'll bet your meters margin of error is greater than that. Lol.  Also, both of those will fluctuate with the mains power so unless you are using a good regulator those will move slightly. 

 

[Melodeath00]

Thanks Category 5! Yes, I was wondering if the multimeter accuracy was even more far off than the 0.005V I measured, ha!

By the way, I had some intermittent noise (not pink noise, but a rustling sound) the other day, but I opened the mic and reseated the tube completely flush in the socket, and left it on for 24 hours, and the rustling appears to be fixed.

 

[Matador]

Some amount of noise is expected:  especially since the best tubes have largely been picked over.  It's SNR that's most important in any case.  I've found the noise on the prototypes I've built to be on par with other tube condenser designs.

As for the B+/bias dependency:  indeed, it is due to the passive nature of the PSU.  If you remember the tube curves I posed back a few pages ago (related to the 12AT7 conversion), moving B+ moves the load line around , which causes the idle current to change, which causes the bias point to change.  Moving the bias point changes the current and causes B+ to shift as well.

But the curves also show valid operation within a large cloud of B+ and bias points, so I wouldn't sweat it if it's in the ballpark.  I've designed the parts so that with a nominal tube, dialing the B+ adjust to 50K (or halfway) and setting the bias pot to 1.4K (about 40% up from zero ohms) gets a close starting point.  Generally adjusting bias voltage only causes a small change in idle current (perhaps 0.05mA or so) so setting B+ first is generally best.

 

[Woot]

Probably a long shot but did you bridge the backplate pins?

Hi Cat5,

Thanks for the reply, yeah I have. I built another identical mic at the same time which seems to be working fine, so not sure what I've done.

I will try Mata's suggestion of injecting a signal into the grid, see how we go.

Cheers,

Matt

 

[Spencerleehorton]

Hi Guys,

just checked my C12 pattern select and seems to be wired correctly but i only seem to be able to get Cardioid and Figure 8, no Omni? any ideas why this would be?

regards

Spence.

 

[bsatriani]

Yes this is exactly the noise I've been chasing.

I've since done some hacking, removing the teflon pins and a handful of the turrets, floating certain components, leads and wiring several sections p2p.

Any positive results?

 

[bsatriani]

FWIW my C12 is not noisy. It has about the same level of noise as my point to point U47 build. Do you hear the "pink noise" in the acoustic guitar clip I posted?

Can't tell from the clips you posted, they are edited(cutted) so short. Leave a few seconds before or after the recording source and then I might hear it,...or not....
Now, don't imagine that my builds have a hurricane inside them wich I call "kind of a pink noise", it is something subtle, but it is! 

Any mic has noise, but actually, I found my Sennheiser e906 was much noisier than this tube mic once I turned up the preamp gain to get it as loud as the C12.

Turning up the preamp gain increases preamp self-noise and so will increase noise in every mic(at least this happens with my preamps : SCA, JLM, DAV). A fair comparison would be done at the same preamp gain level/output level, and then lowering the loudest mic level into your daw to match levels.