Official C12 Clone - Build and Support Thread

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ripemedia

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Yes, you can try the other tube half first before switching back to the 12AX7.
Ok I tried the other side of the tube and got roughly the same results across R17: 109V / 74V. I then tried the original tube and the results seemed worse: 133V / 110V. I checked all connections again and verified that the resistors on the mic are correct and wired properly. Still not sure what’s going on. 😞
 

Matador

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12AX7 pulls a lot less current so that's not unexpected.

Ok, switching back to the 12AT7: with the grid hard grounded, you get the correct current, but with the bias set to 0V (which should be the same thing) the current is way less. What about if you hard ground (with a jumper wire) the junction of C10/R11/R15? Also ensure that the P4 node in the microphone measures 0V. You can leave it on the 'other side' of the tube (pins 6, 7, and 8) if it's easier. You can also leave out C10 and C13 if you had already removed them earlier.
 

ripemedia

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Thanks. I’m not sure what is meant by “the junction of C10/R11/R15”. Currently C10 goes to ground. Do you mean a jumper cable that connects all three? Like, where I have highlighted in the attached image?
 

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ripemedia

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Ok here's the latest. I jumpered the node to the ground pad and took 3 measurements, all with with at 0V (and I confirmed P4 in the mic is 0V): a) C10 and C13 soldered in, b) C10 removed, c) both C10 and C13 removed. All three readings were basically the same: 110V / 74V. I'm sorry this project is so problematic. I appreciate all of your assistance. I'm hopeful we can find a resolution.
 

Matador

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Ok here's the latest. I jumpered the node to the ground pad and took 3 measurements, all with with at 0V (and I confirmed P4 in the mic is 0V): a) C10 and C13 soldered in, b) C10 removed, c) both C10 and C13 removed. All three readings were basically the same: 110V / 74V. I'm sorry this project is so problematic. I appreciate all of your assistance. I'm hopeful we can find a resolution.
Can you take a picture of the bottom of the board, and how you connected to the tube PCB?
 

ripemedia

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Can you take a picture of the bottom of the board, and how you connected to the tube PCB?
Sure. Here are some photos. Let me know if this doesn’t show what’s needed. Thanks!
 

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Matador

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So if grounding the junction of R11/R15 doesn't show different voltages, what about hard grounding the 'top' of R15? That should cause the voltage to jump way down (just like jumpering pin2 and pin3 together).
 

ripemedia

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With the top of R15 grounded, here's what I got with bias @ 0V: R17: 67V / 11.8V. This seems to be more like what we're looking for.
 

Matador

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Yes, now it's drawing significant current.

This points to the 250M resistor as suspect, although I've never encountered an issue with one of these resistors: I would guess it's more likely the tube is drawing excessive grid current, which is causing the bias to shift negative (and you can't measure it directly at the grid without special tools). Bias shifts are almost always caused by leaking caps but since you removed all the suspect ones I doubt that is any issue as well.

Do you have a 100K resistor you can use to replace R17 (which you switched to do the 12AT7 conversion) and see if the stock 12AX7 biases up correctly? It it has the same problem, then you might try replacing R15.
 

ripemedia

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Thanks. I swapped R17 for the 100K resistor and used the 12AX7 tube. Here’s what I got across R17: 143V / 102V. This seems good. Not sure what it means.
 
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Matador

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So a 12AX7 at @0V bias with 100V on the plate should be flowing almost 2mA, but you are getting about 0.4mA.

Can you swap the 250M for a new one? If you don't have one, try the highest value you have (hopefully 1M or above).
 
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ripemedia

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I don’t have another 250M but I can order one. Could it be helpful to swap the two 250M resistors that are in the circuit?
 

ripemedia

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I swapped the 250M resistors and there was no change in the voltages. Do you think it would still be worthwhile to replace them?
 

SpaceEater

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I just built the PSU, but i have a problem. I get 11V for the heater, but it is not possible to trim it down. Any ideas what could be wrong? B+ and bias voltages are fine.
 

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