Official C12 Clone - Build and Support Thread

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ripemedia

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Do you have some 1M resistors to test with? Remove C13 and C10, and replace R11 and R15 with 1M.
I don’t have any 1M resistors. I’m considering buying replacement parts but I can’t find any 250M resistors available. Do you have a source for them?

I’m also considering rebuilding the circuit entirely. But before a do any more troubleshooting I would like to know if you think these issues I’m experiencing are likely isolated to the microphone circuit or do you think there could also be issues with the power supply circuit? Thanks.
 

ripemedia

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Thanks. I just cleaned the board again to be sure. I don’t see any residue, and when I tested again I got the same result.

When I was troubleshooting earlier I mentioned that P4 had been connected to ground at the xlr. I have since severed the connection, but could that have messed up the tube (or something else) while it was connected?

I also noticed just now that when the tube is inserted, P8 and P6 form a closed connection. In fact both heater pins on the tube go to ground when the tube is inserted. Is this expected?
 

Matador

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Thanks. I just cleaned the board again to be sure. I don’t see any residue, and when I tested again I got the same result.

When I was troubleshooting earlier I mentioned that P4 had been connected to ground at the xlr. I have since severed the connection, but could that have messed up the tube (or something else) while it was connected?

I also noticed just now that when the tube is inserted, P8 and P6 form a closed connection. In fact both heater pins on the tube go to ground when the tube is inserted. Is this expected?
Shorting P4 doesn't effect the tube other than making it impossible to bias it properly.

P8 is the heater supply, and it returns to ground via P6: when you measure across these nodes, you are measuring the resistance of the heater coil which is ~20 ohms (when hot), and may look like only 10 ohms when cold. So this is expected.

The bias supply P4 only travels through several components on its way to the tube grid: mainly R11 and R15. So something is disturbing the bias. To determine exactly which node is difficult, because the 30M and 250M nodes are too high resistance to measure with normal DVM's.

If you really want to eliminate the PSU, just insert a 47K resistor between the B+ node (P5) and ground (P6) right where it exits the PSU in the 7-pin connector (stick it right in the 7 pin connector, no cable needed). This should let you a) set B+ to 120V, and b) adjust the bias between 0V and about -5V.
 

ripemedia

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Shorting P4 doesn't effect the tube other than making it impossible to bias it properly.

P8 is the heater supply, and it returns to ground via P6: when you measure across these nodes, you are measuring the resistance of the heater coil which is ~20 ohms (when hot), and may look like only 10 ohms when cold. So this is expected.

The bias supply P4 only travels through several components on its way to the tube grid: mainly R11 and R15. So something is disturbing the bias. To determine exactly which node is difficult, because the 30M and 250M nodes are too high resistance to measure with normal DVM's.

If you really want to eliminate the PSU, just insert a 47K resistor between the B+ node (P5) and ground (P6) right where it exits the PSU in the 7-pin connector (stick it right in the 7 pin connector, no cable needed). This should let you a) set B+ to 120V, and b) adjust the bias between 0V and about -5V.
Ok thanks. I inserted the 47K resistor as suggested. I had to really increase the adjustment of B+ all the way but 118.6V is as high as the voltage would go. Does that seem appropriate?

Without the resistor in pace, B+ is about 250 with the adjust at max, and 205 when B+ adjust is all the way in the other direction.

Bias was still at 0 and I was able to easily adjust it to -1.
 
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Axelorox

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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?
 

Delta Sigma

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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?
Yes. Did you get an order confirmation? I believe my last order, I received a confirmation then didn't hear back for a while. Probably best to give him a couple weeks before you give him a nudge.
 

Matador

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Ok thanks. I inserted the 47K resistor as suggested. I had to really increase the adjustment of B+ all the way but 118.6V is as high as the voltage would go. Does that seem appropriate?

Without the resistor in pace, B+ is about 250 with the adjust at max, and 205 when B+ adjust is all the way in the other direction.

Bias was still at 0 and I was able to easily adjust it to -1.
That operation is exactly as expected, which is why I don't think the PSU is at issue. :)

You can measure the high impedance nodes, but it requires a FET-input op-amp and a couple of 9V batteries: is it something you are interested in trying out? If you don't want that, two, 1M resistors will allow you to determine if there is something wrong with the board itself (unlikely, but nothings impossible).
 

Axelorox

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I
Yes. Did you get an order confirmation? I believe my last order, I received a confirmation then didn't hear back for a while. Probably best to give him a couple weeks before you give him a nudge.
I did get a confirmation both on the site and by email. I'll give it another week
 

ripemedia

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That operation is exactly as expected, which is why I don't think the PSU is at issue. :)

You can measure the high impedance nodes, but it requires a FET-input op-amp and a couple of 9V batteries: is it something you are interested in trying out? If you don't want that, two, 1M resistors will allow you to determine if there is something wrong with the board itself (unlikely, but nothings impossible).
Ok great. I’m glad we can rule out the PSU. I have ordered replacement parts for the resistors and capacitors. I’m going to start by replacing the high impedance resistors and see if anything changes and then continue with the other parts. I’ll post back here when I have more info. Thanks!
 

ripemedia

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That operation is exactly as expected, which is why I don't think the PSU is at issue. :)

You can measure the high impedance nodes, but it requires a FET-input op-amp and a couple of 9V batteries: is it something you are interested in trying out? If you don't want that, two, 1M resistors will allow you to determine if there is something wrong with the board itself (unlikely, but nothings impossible).
I replaced the 250M resistors and re-cleaned the board. R17 values were the same: 109V / 75V. Could it be possible that the capsule is affecting the circuit? I am not using the stock capsule. It's a Chinese replacement "C12-style" capsule from wgtcenter.com.

I'm going to replace the capacitors next. Still waiting for those parts. Thanks.
 
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ripemedia

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That operation is exactly as expected, which is why I don't think the PSU is at issue. :)

You can measure the high impedance nodes, but it requires a FET-input op-amp and a couple of 9V batteries: is it something you are interested in trying out? If you don't want that, two, 1M resistors will allow you to determine if there is something wrong with the board itself (unlikely, but nothings impossible).
I replaced C10, C11, and C13 with new capacitors. The readings I got across R17 seem to be the same: 110V / 77V. I still don't know what's going on with this microphone and why it's not behaving as it should :(.

I checked and adjusted all voltages again to see where things are. Here's what I got:

Bias: -1V
Heater: 6.3V (measured at mic)
B+: 128V
R17a: 128V
R17b: 101V

B+ is with the adjustment all the way down and I still can't get it to 120V. This is the original issue we were trying to resolve.
 
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Matador

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Was there an edit? I thought you said you replaced the caps but the voltages shot up to 177V?

What is the highest value resistors you have for testing? Do you have 1M? Or even perhaps 500K?

Remove C13 and C10, and replace R11 and R15 with the highest value resistors that you have and we'll go from there.
 

ripemedia

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Was there an edit? I thought you said you replaced the caps but the voltages shot up to 177V?

What is the highest value resistors you have for testing? Do you have 1M? Or even perhaps 500K?

Remove C13 and C10, and replace R11 and R15 with the highest value resistors that you have and we'll go from there.
Yeah I edited the message. K2 was not connected properly, so that previous reading of 177 was inaccurate. I have two 499K resistors. Would those work? If so, after replacing R11 and R15 should I take another reading at R17 and with the tube inserted? Thanks!
 

shortyboyboy

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Where can one study to learn exactly what you guys are talking about? I’ve built
Mics, preamps, and eqs but only by following directions. I would love to formally understand what I’m doing like you guys do!! Thanks for any helpful ideas or links / videos you can provide :)
 

Matador

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Alright. Caps removed and resistors replaced. Here’s what I got with bias at -1: 129V and 103V at R17.
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?
 
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