Official C12 Clone - Build and Support Thread

[ripemedia]

Excellent. Now when it's up and operating, can you measure:

a) both sides of R11 (the 499K)
b) both sides of R15 (the 499K)
c) the voltage at the tube grid (either pin2 or pin7 of the tube)

Then yank the tube, and remeasure a) and b) above (again, while it's powered up).
Can you also confirm what type of voltmeter you are using?

Ok here's what I got. With tube installed:
R11: -1V / -0.8V
R15: -0.77V / -0.85V
Grid: -0.78V

With tube removed:
R11: -0.6V / -0.48V
R15: -0.4V / -0.47V
Grid: -0.40

And here is the voltmeter I'm using: Amazon.com. DT830D. Thanks!

 

[Matador]

You have some fraction of a uA of leakage current entering R11. I was suspecting tube grid current, however when you removed the tube, the current is still there. Even sub 1uA across 30M is a large drop. I don't know if it's inter-nodal leakage or a board defect...

With tube out, take the same readings again, however first a) disconnect the P5 wire on the bottom of the mike, then take them a second time after b) disconnecting the P3 wire (leaving P5 still disconnected). So fill out:

With tube removed, and P5 disconnected:
R11: ? / ?
R15: ? / ?
Grid: ?

With tube removed, and P3 and P5 disconnected:
R11: ? / ?
R15: ? / ?
Grid: ?

Do you have access to any op-amps? Like a TL072?

 

[ripemedia]

You have some fraction of a uA of leakage current entering R11. I was suspecting tube grid current, however when you removed the tube, the current is still there. Even sub 1uA across 30M is a large drop. I don't know if it's inter-nodal leakage or a board defect...

With tube out, take the same readings again, however first a) disconnect the P5 wire on the bottom of the mike, then take them a second time after b) disconnecting the P3 wire (leaving P5 still disconnected). So fill out:

Do you have access to any op-amps? Like a TL072?

Ok here's what I got: With tube removed, and P5 disconnected:
R11: -0.47 / -0.37
R15: -0.31 / -0.37
Grid: -0.31

With tube removed, and P3 and P5 disconnected:
R11: -0.47 / -0.37
R15: -0.31 / -0.37
Grid: -0.31

Hm. I see now that they are identical. Not sure if it's helpful. Hopefully we're getting somewhere! FYI I disconnected P3 and P5 at the PSU.

By the way, every time I have tested with the meter, the initial reading is very different and steadily drops. For example, when testing the first side of R11 the initial reading was over -1V but then stabilized at -0.47. The other side of R11 was initially over -0.5V and then stabilized at -0.37. Is this behavior common? I just want to make it clear that I am recording the values after they stabilize.

I do not have an op amp, but can get one. Would this one work? https://www.mouser.com/ProductDetail/Texas-Instruments/TL072IP?qs=5BZzbFV4k2v7IBrcArRPQw== Thanks.

 

[TillM]

Thanks Matador, that was it!

It was quite a hassle to fit everything in an original size C12 body. I can post pictures of the inside if anyone is interested.

I‘m also interested how you fit the pcb inside.

 

[Hank Dussen]

I did a sine sweep (into tube grid) to test the circuit because I find the mic really lacks highs.
Below is the result. Seems pretty flat to me.
Could it be the capsule (by Tim Campbell) is rather dark sounding? Bought it in april 2010...

 

[Tim Campbell]

I did a sine sweep (into tube grid) to test the circuit because I find the mic really lacks highs.
Below is the result. Seems pretty flat to me.
Could it be the capsule (by Tim Campbell) is rather dark sounding? Bought it in april 2010...

Well that is 11 years ago . You could send the capsule back to me and I can check it for you. Email me please.

 

[SpaceEater]

I would like to see how it fits in there.

I‘m also interested how you fit the pcb inside.

Sorry for the late reply. Here are some pictures of how ive done it. Transformer is still a bit messy installed, i still need to fix that.

I had to machine the body tube quite a bit, which wasnt that easy. The rails i made from some thinner aluminium strips. The socket is from switchcraft, Capsules is from Tim Campbell, X-former from Haufe, Tube is GE.

How ive done the rest should be visible from the pictures. If not, just ask

 

[mellsonic]

Hello
6 year ago i build two of this c12 clone.with tc 12 capsule
now one have a problem with the low frequencies,it Sounds like a low cut is on.where can i start to Check?
i change the tube but the Problem is still there
thank youin Advance

 

[Axelorox]

I ordered some in stock parts for this build from the Studio 939 website over a week ago but have yet to hear anything. Is this normal?

Three weeks since my Studio 939 order and still no updates or replies to my email... is there any other way to reach him?

 

[mrgrooves666]

I'm in the same boat as you. I have placed orders for around $516 bucks on his store so far, and I still needed a couple of more items. Currently I have 3 open orders, the last of them placed May 5. I've wrote him many times, no answer. I know others here can vouch for him, that he will come through, but I still find this an odd way to run a business. We are all busy. I have 5 kids and 2 jobs to manage. No communication with your customers is a recipe for disaster, however you look at it..

 

[mrgrooves666]

I'm in the same boat as you. I have placed orders for around $516 bucks on his store so far, and I still needed a couple of more items. Currently I have 3 open orders, the last of them placed May 5. I've wrote him many times, no answer. I know others here can vouch for him, that he will come through, but I still find this an odd way to run a business. We are all busy. I have 5 kids and 2 jobs to manage. No communication with your customers is a recipe for disaster, however you look at it.. Don't mean to disrespect or bash, just what it is.

 

[Matador]

Ripemedia sent me his microphone, and the root cause of the current issues is very interesting, so posting here for posterity.

He could not trim more than about 1mA of bias current out of the 12AT7, which I had diagnosed as either a leaking cap (like previous
builders had experienced), but the root cause: grid leak biasing. According to RDH4, grid leak (or contact potential) bias is most
pronounced when the cathode and grid are near the same potential: hence, if the cathode and grid approach within 1V of each other,
then grid leakage increases.

In this design, cathode is grounded, and grid sits at -1V. However with a 12AT7, being much higher mu than a 6072A, the grid leakage
current is much higher. Hence, when you try to trim the bias closer to ground than about -1V, grid leakage starts to rise, which
in effect cancels the bias from the supply trying to go more positive. This causes the bias to be 'clamped' at -1V, regardless of
how high the bias voltage is in the PSU.

I couldn't see it clearly until I wired up a TL071 in a unity gain follower configuration, powered by two, 9V batteries. The input
bias current of this FET opamp is typically on the order of a handful of pA, meaning that when measuring close to 0V the input impedance
can approach north of several tera-ohm. This is high enough to be able to probe the >250Mohm nodes with reasonable accuracy.
In fact, there are special purpose capacitive sensor op-amps than can be used which have even higher input impedances.

In any case, starting from -5V, and dialing down the bias in the PSU toward 0V, you can clearly see the bias at the gate of the tube
lower, then stop lowering at about -1.26V. This means that with as 12AT7 tube, you can set the bias voltage to 0V (dial the pot to minimum)
and the tube will happily idle at the correct -1V bias. . The left DVM is showing the PSU bias point, and the right DVM is reading the output of the op-amp which is measuring the grid of the tube. At -3.5V, we get -3.5V at the tube, however when set to 0V, you can see the tube is still seeing about -1.1V.

I had not seen this on the 6072A, as being medium-ish mu the effect is far less pronounced since the plate-to-cathode current is so much less.

 

[plugged1]

Ripemedia sent me his microphone, and the root cause of the current issues is very interesting, so posting here for posterity.

He could not trim more than about 1mA of bias current out of the 12AT7, which I had diagnosed as either a leaking cap (like previous
builders had experienced), but the root cause: grid leak biasing. According to RDH4, grid leak (or contact potential) bias is most
pronounced when the cathode and grid are near the same potential: hence, if the cathode and grid approach within 1V of each other,
then grid leakage increases.

In this design, cathode is grounded, and grid sits at -1V. However with a 12AT7, being much higher mu than a 6072A, the grid leakage
current is much higher. Hence, when you try to trim the bias closer to ground than about -1V, grid leakage starts to rise, which
in effect cancels the bias from the supply trying to go more positive. This causes the bias to be 'clamped' at -1V, regardless of
how high the bias voltage is in the PSU.

I couldn't see it clearly until I wired up a TL071 in a unity gain follower configuration, powered by two, 9V batteries. The input
bias current of this FET opamp is typically on the order of a handful of pA, meaning that when measuring close to 0V the input impedance
can approach north of several tera-ohm. This is high enough to be able to probe the >250Mohm nodes with reasonable accuracy.
In fact, there are special purpose capacitive sensor op-amps than can be used which have even higher input impedances.

In any case, starting from -5V, and dialing down the bias in the PSU toward 0V, you can clearly see the bias at the gate of the tube
lower, then stop lowering at about -1.26V. This means that with as 12AT7 tube, you can set the bias voltage to 0V (dial the pot to minimum)
and the tube will happily idle at the correct -1V bias. . The left DVM is showing the PSU bias point, and the right DVM is reading the output of the op-amp which is measuring the grid of the tube. At -3.5V, we get -3.5V at the tube, however when set to 0V, you can see the tube is still seeing about -1.1V.

I had not seen this on the 6072A, as being medium-ish mu the effect is far less pronounced since the plate-to-cathode current is so much less.

Is this inherent using any 12AT7? I use one in mine and didn’t have any of these problems..? Just wondering if maybe I’m missing something; it sounds great and all values are spec as to Chungers OP. Should I be using different measurements with a 12at7?

 

[Matador]

Yes, although every tube will be slightly different. Grid leak bias was common "back in the day" when directly heated cathodes were the norm. The big downside is tube-to-tube variation (bias is unpredictable and hard to measure). The tube will work at many operating points properly in a microphone so I don't see it as a "problem" in practice.

 

[duantro]

I’ve finished one of these c12 mics with a 6072 l/ beesneez ck12 and it sounds great, except for a ground hum. If the Iec is removed/ or ground lift plug is installed, it is dead quiet. It’s wired as recommended as far as I can tell. Shields are connected to shell tab on Gotham 7 pin cable, both xlr tabs are connected and jumpered to pin 7 in psu, ground of polarity vdc switch goes to pin 1 ground of xlr pad on psu pcb. I’m getting continuity from pin 7 to mic body. Shield tab is not connected to pin 7 in the mic body. Is this correct? I’ll start going through the 130 pages here and see if anyone has a smoking gun. I’ve built 14 plus mics and it’s also a rabbit hole with hum and different pcbs.
If anyone has any clues, please let me know.

 

[shabtek]

what if you lift an xlr-3 shield instead
(maybe a loop with the thing you are plugging into?)

 

[duantro]

what if you lift an xlr-3 shield instead
(maybe a loop with the thing you are plugging into?)

Thanks for the suggestion. I tried that and it’s still there. Shield and 0v ground (pin7) have continuity in the mic from mounting screw on pcb. Should I use platstic screws to break the shield here? Shouldn’t the shield only meet 0v at one point- at the psu pin7 to tab?

 

[shabtek]

Thanks for the suggestion. I tried that and it’s still there. Shield and 0v ground (pin7) have continuity in the mic from mounting screw on pcb. Should I use platstic screws to break the shield here? Shouldn’t the shield only meet 0v at one point- at the psu pin7 to tab?

that might do it; DC return and screen untied at mic

 

[duantro]

that might do it; DC return and screen untied at mic

I removed the screws and it still buzzes. I removed the shield in the cable at the mic a la emperor Tomato Ketchup's diagram for Max Kircher's mk7, Still buzzes like crazy. Triple checked my wiring, swapped the + and- of the T/14 transformer,...Buzzzzzz. Does the PSU ground to the case at just the xlr 7 pin tabs? I get no buzz until I plug the mic into the cable and it's buzzing. All of this goes away if I test with a 2 prong ground lift adapter. WTH is going on here?

 

[shabtek]

what is signal chain?
it is reassuring to know that it can be quiet, just gotta figure out the safe way