Official C12 Clone - Build and Support Thread

[Tim Campbell]

I've finished my C12 a while ago but it really lacks some highs compared to some other microphones I have build (EF47, U67, U47,...), while I understand that it should be a rather bright microphone.
I've build it with the best parts I could find (Tim Campbell capsule, 5 star 6072 tube, AKG transformer) so I'm guessing it might be the caps.

I used an 0,4uF PIO k42y-2 for C12, and what I believe are 4000 pF polystyrene caps for C10 and C11. These caps have a marking that says RUWEL 4000/10 500v.
And parallel with these 2 caps, as well as for C13 I used 1000 pF polystyrene caps. These are marked 1n0 F250 V9KS.

Maybe some of these caps are wrong?
Anyone any idea what could be the problem?

Hank when is the capsule from?

I hate to ask this but it actually happened once. You did remove the covers from over the membranes?

 

[ripemedia]

Where are you measuring the bias voltage?  I'm guessing you are measuring it right at pin 2 of the tube socket?  You can't measure it there, because of the 280M worth of resistance.  Measure it at the P4 node directly as it enters the PCB down at the bottom edge near the transformer/XLR connector.

Thanks. Totally got it RE: the board. Regarding bias voltage, I was measuring from inside the PSU. However, just checked again now measuring from P4 node at the mic and the value is the same. 0.03.

 

[Matador]

Thanks. Totally got it RE: the board. Regarding bias voltage, I was measuring from inside the PSU. However, just checked again now measuring from P4 node at the mic and the value is the same. 0.03.

So it goes from -1V to 0V after connecting the mike to the cable?  What about with the tube removed?

If it's still 0V with the tube removed, can you lift out C10 (just unsolder one side) and recheck?

 

[ripemedia]

I rechecked my wiring and couldn't find anything suspicious until I looked under the bell! Where I had bridged ground to pin 7, it was touching pin 4.  :-[. Fixing that fixed the issue! Sorry for all the troubles. It was my own dang fault.

Thank you for your help!

 

[ripemedia]

I have a question about B+ voltage. I am using a 12AT7 tube and have made the recommended adjustments: 33K resistors for R1 and R2; 47K resistor for R17 and when I check B+ voltage without the mic plugged in, it's 205V, and that's with the trim pot all the way down. With the mic connected, the voltage is 130V. I can't get it lower than that. Should try different resister values, or something else?

Thanks!

 

[Matador]

I have a question about B+ voltage. I am using a 12AT7 tube and have made the recommended adjustments: 33K resistors for R1 and R2; 47K resistor for R17 and when I check B+ voltage without the mic plugged in, it's 205V, and that's with the trim pot all the way down. With the mic connected, the voltage is 130V. I can't get it lower than that. Should try different resistor values, or something else?

Thanks!

You can raise up R1 and R2 to get more drop.  Your 12AT7 might just be pulling less current than typical.

Can you measure the voltage drop across R17?  One side should be 130V, and the other will be the plate voltage.  With that measurement we can calculate the "best" R1/R2 for you.

 

[ripemedia]

Can you measure the voltage drop across R17?  One side should be 130V, and the other will be the plate voltage.  With that measurement we can calculate the "best" R1/R2 for you.

That's great. One side of R17 is 132.5V and the other side is 106V. Thanks!

 

[Matador]

That's great. One side of R17 is 132.5V and the other side is 106V. Thanks!

Interesting:  you have 26.5V drop across a 47K plate resistor, which is 0.56mA.  Most of my 12AT7 tubes pull more like 1-2mA with a -1V grid bias at these voltages (my test microphone shows 61V plate voltage with 121V B+, which is ~1.3mA).

Are you sure it's a 12AT7?  Are you sure R17 is 47K?

 

[ripemedia]

Are you sure it's a 12AT7?  Are you sure R17 is 47K?

Yes the tube is the Mullard 12AT7 from the studio 939 site, and the resistor is labeled 4702F and when I measured it was 47K. Could there be something else that's affecting the voltage draw? I just measured again and got 132.9V and 105.4V for R17. FYI I am grounding at the PSU. Let me know if I should be grounding elsewhere. Thanks!

 

[Matador]

Yes the tube is the Mullard 12AT7 from the studio 939 site, and the resistor is labeled 4702F and when I measured it was 47K. Could there be something else that's affecting the voltage draw? I just measured again and got 132.9V and 105.4V for R17. FYI I am grounding at the PSU. Let me know if I should be grounding elsewhere. Thanks!

What do you mean "grounding at the PSU".  Do you mean measuring with the negative lead of your voltmeter in the PSU 0V node?  If so, that's fine.

Can you dial the bias to 0V (up from -1v) and tell me the drop across R17 again?

 

[ripemedia]

Can you dial the bias to 0V (up from -1v) and tell me the drop across R17 again?

Sorry. Yes that's what I meant -- negative lead at GND in the PSU.

Ok I set the bias to 0V and got 106.8 and 71.5 -- a drop of 35.3 across R17.

 

[Matador]

With 70V on the plate and 0V grid bias, a 12AT7 should be flowing almost 5mA, not 0.75mA.

Can you confirm that pin 3 reads 0V?
Can you measure the heater voltage right at the tube across pins 4 and 9?

 

[ripemedia]

Can you confirm that pin 3 reads 0V?
Can you measure the heater voltage right at the tube across pins 4 and 9?

Thanks. Here is the latest:

Bias: 0V
Pin 3 (in the PSU): 0V when the position is set to omni
Pin 3 (at the tube): 0V
Voltage across pins 4 and 9 at the tube: 6.3V

 

[Matador]

Hmm...pin 3 doesn't connect to the PSU...what were you measuring?

Last test: can you hard solder a jumper wire between pin 2 of the tube and pin 3?, and remeasure the voltages on R17?

 

[ripemedia]

Hmm...pin 3 doesn't connect to the PSU...what were you measuring?

Last test: can you hard solder a jumper wire between pin 2 of the tube and pin 3?, and remeasure the voltages on R17?

Sorry I mean I measured P3 in the PSU. I was confusing it with PIN3. Here are the new measurements with the jumper between pin 2 and pin 3:
Bias: 0V
R17a: 20.9
R17b: 73.6
Drop: 52.7

 

[Matador]

Now that is making more sense. With the grid forced to 0V, we have 20V plate and 1.12mA, which agrees with the datasheet pretty close.

Something is dragging the negative bias more negative and cutting off current, which means there is DC current in either R11 or R15. The two likely suspects are C10 and C13, with C13 more likely. What kind of caps did you use for these two? Polystyrene's?

Can you lift out C13 (you can just lift up and disconnect one leg if you don't want to desolder it completely), remove the jumper between pin2 and pin3, and remeasure? After this (to save time), you can also lift one side of C10 and remeasure again. 0V bias for the measurements is still fine.

 

[ripemedia]

What kind of caps did you use for these two? Polystyrene's?

Can you lift out C13 (you can just lift up and disconnect one leg if you don't want to desolder it completely), remove the jumper between pin2 and pin3, and remeasure? After this (to save time), you can also lift one side of C10 and remeasure again. 0V bias for the measurements is still fine.

Yes they are Polystyrene's. They're the ones from the kit at studio939.

Ok here are the measurements I got, all with bias at 0V and the jumper between pin 2 & 3 removed:

Lifted C13 only
R17: 108 and 72, drop of 36

Lifted C10 + C13
R17: 108 and 72, drop of 36

Lifted C10 only
R17: 107.7 and 72, drop of 35.7

These all seem to be the same as before.

 

[Matador]

Wow, that makes no sense. If you read the first 5-10 pages of the thread there was another builder than had the exact same issue, and it ended up being a leaking cap (which you removed both of).

Do you have another tube to test? The only thing I can think of is either a) the 30M or 250M resistor are bad, or b) the tube is leaking grid current.

 

[ripemedia]

Wow, that makes no sense. If you read the first 5-10 pages of the thread there was another builder than had the exact same issue, and it ended up being a leaking cap (which you removed both of).

Do you have another tube to test? The only thing I can think of is either a) the 30M or 250M resistor are bad, or b) the tube is leaking grid current.

I know. It doesn’t makes sense.

I have the tube that came with the T11a. Could that work for this test?

Also, what about trying the other side of the tube?

 

[Matador]

I know. It doesn’t makes sense.

I have the tube that came with the T11a. Could that work for this test?

Also, what about trying the other side of the tube?

Yes, you can try the other tube half first before switching back to the 12AX7.