Official C12 Clone - Build and Support Thread

[tonycamp]

Hi Matador,

I'm using the mic as a test load, plugged in, I'm getting 130vdc with the trimmer backed out all the way? The heater is perfect, and the bias is at .032 measured at xp1 pin on the mic, bias trimmer is also backed out all the way?

 

[Matador]

Hi Matador,

I'm using the mic as a test load, plugged in, I'm getting 130vdc with the trimmer backed out all the way? The heater is perfect, and the bias is at .032 measured at xp1 pin on the mic, bias trimmer is also backed out all the way?

XP1?  That is the primary of the output transformer...this should be sitting at DC ground (32mV is pretty close).

Bias is measured at the PSU "P4" node (or the middle node of the 5-pin header).  With nothing connected, you should get between 0V at one end of R3's travel, and about -1.75V at the other end.  This is because R6 establishes a bleeder current which should drop across R3.

 

[Winetree]

Finished build and ready to test.
Question; Does it matter which red wire from transformer goes to pin 5 or 6 on Xlr.
Same with blues wires to txp 1 or 2. They're  labeled + and -
If so how do you tell?

 

[tonycamp]

Finished build and ready to test.
Question; Does it matter which red wire from transformer goes to pin 5 or 6 on Xlr.
Same with blues wires to txp 1 or 2. They're  labeled + and -
If so how do you tell?

I believe you just have to designate one side or the other to + or -, then commit to it, the blue wire follows the polarity of the red wire right next to it, but then again, listening to my advise could be a fail ???, as i'm having some voltage adjustment issues? i intend on resolving these tonight...i hope

 

[tonycamp]

You really need a test load to set B+ and bias:  put a 180K resistor between B+ and ground right at the 7-pin cable jack on the PSU.

Thanx Matador,

Ok, i put a 182k(closest i had) resistor across  B+ to ground, i adjusted B+ to 120v, and the heater is good at 6.3v, but i get 0 action out of the bias trimmer?? Any ideas?

T

 

[Matador]

Finished build and ready to test.
Question; Does it matter which red wire from transformer goes to pin 5 or 6 on Xlr.
Same with blues wires to txp 1 or 2. They're  labeled + and -
If so how do you tell?

It's really doesn't matter:  just make sure you have primary vs. secondary sorted out and you'll be fine.

 

[Matador]

You really need a test load to set B+ and bias:  put a 180K resistor between B+ and ground right at the 7-pin cable jack on the PSU.

Thanx Matador,

Ok, i put a 182k(closest i had) resistor across  B+ to ground, i adjusted B+ to 120v, and the heater is good at 6.3v, but i get 0 action out of the bias trimmer?? Any ideas?

T

Well, that's not good.   The obvious place to look is at the bias trimmer pot:  it may be that you've dialed it all the way in so that it looks like a short.  Sometimes the pots can take 4 or 5 turns to "go back" the other direction.

I would start by reflowing the pot connections, then if that doesn't help I would take it out and make sure you can read 5K between the two outside pins, and that you can read a variable resistance between any outer pin and the middle pin.

 

[tonycamp]

I reflowed everything, i very carefully checked all product placement, it all looks A OK.
So here's what i've got now, i replaced the 5k bias trimmer for the heck of it, and as i was doing it i realized the stock wiring has pin 4(bias pin) jumped to ground at both mic and psu 7 pin xlr's(I believe that's for the stock cathode config??). I clipped those and now i have a properly adjusted -1.0v bias, 6.3v heater, but I still cannot get 120v out of the B+, its 130V B+ and the trimmer is backed off 100%? 

any ideas how i'm screwin up this time?

thanks
T

 

[chunger]

I reflowed everything, i very carefully checked all product placement, it all looks A OK.
So here's what i've got now, i replaced the 5k bias trimmer for the heck of it, and as i was doing it i realized the stock wiring has pin 4(bias pin) jumped to ground at both mic and psu 7 pin xlr's(I believe that's for the stock cathode config??). I clipped those and now i have a properly adjusted -1.0v bias, 6.3v heater, but I still cannot get 120v out of the B+, its 130V B+ and the trimmer is backed off 100%? 

any ideas how i'm screwin up this time?

thanks
T

You MUST disconnect the jumper from pin 4 to ground at both the microphone 7 pin jack and the power supply jack when constructing this kit.  We utilize pin 4 for the bias voltage while the original microphone did not.

 

[tonycamp]

Yes Chung, i over looked it, it kinda hides in there, it didn't help that i just left stock wiring alone. You have any idea about the 130v thing i mentioned?

thanx
T

 

[Matador]

I reflowed everything, i very carefully checked all product placement, it all looks A OK.
So here's what i've got now, i replaced the 5k bias trimmer for the heck of it, and as i was doing it i realized the stock wiring has pin 4(bias pin) jumped to ground at both mic and psu 7 pin xlr's(I believe that's for the stock cathode config??). I clipped those and now i have a properly adjusted -1.0v bias, 6.3v heater, but I still cannot get 120v out of the B+, its 130V B+ and the trimmer is backed off 100%? 

any ideas how i'm screwin up this time?

thanks
T

You can get 120V with the test load:  pop the tube off of the mike body, and with it warmed up, measure the grid, cathode, and plate voltages right at the tube socket.  You can probe the TK, TG, and TP nodes on whatever side you chose to "activate".

It looks like your tube isn't pulling much current for some reason...

 

[3nity]

Chunger & Matador
Hope this is not an issue ..
High impedance isolation teflon pins being soldered straight to pcb!!
well this is supposed to isolate high impedance from pcb isnt?

 

[tonycamp]

Ok, I know i'm a newb and all, but i tried everything i could think of and have been told to try, Matadors instruction to put a 180k resistor as a dummy load allowed me to get the proper B+ voltage. But then with the mic plugged in, and using 2 different tubes, i have not been able to achieve the proper b+ of 120v. I started to trouble shoot the mic next, and what i found seems impossible. R15 and R14 250m resistors read DEAD! Nothing, nada, zero, no continuity what so ever, open! I just got a new fluke 87v, and i thought i was doing something wrong, till i broke out the old MM and the same thing, no reading? Would r15 and r14 being dead, and a break in this circuit cause the voltage to stay up around 130v with the B+ trimmer turned all the way counter clockwise? Other 200m to 500m resistors test fine? I've been spending way to much time on this, so i ask, am i crazy or missing something?

thanx
T

 

[chunger]

Chunger & Matador
Hope this is not an issue ..
High impedance isolation teflon pins being soldered straight to pcb!!
well this is supposed to isolate high impedance from pcb isnt?

I inquired about this point with Matador in the prototype stages of the build. . . in this situation, we are pretty much using the part simply as another turret to facilitate easier soldering of the capsule leads.  I am not the technical lead on the project (I just take pretty pictures), but Matador explained to me that in this application, provided the pcb is throughly cleaned with isopropyl alcohol, the FR4 material is a good enough isolator for our purposes.  In use, the prototypes have not exhibited noise or other anomalies associated with leaks  attributed to the physical layout so I questioned it no further.  But, it's worth re-iterating especially in this section of the build that it is important to clean the board thoroughly when assembling the kit.

 

[3nity]

From what i see proper use of those isolation pins include different design.
Capsule leads and parts lead would solder to the pins instead of traces doing that.

Good to know anyways!

 

[Matador]

What is seen most often is that the connection of the bottom of the isolation pin is a floating via.  Then the other connections are made "on top".

I experimented with this in the first prototype:  isolating the bottoms of the standoffs means there are 4 extra wires connections that have to be made, two of which are critical ground junctions.

In order to combat noise, the soldermask extends right up to the plated turret hole.  This formulation of soldermask has a surface resistance of about 8Gohm per centimeter, which means that if the area around the turret is clean there will be no leakage paths to ground that are anywhere near the impedance of the actual circuit.  The layout in this area is controlled so that there is at least 1cm of solder mask between all of the high impedance nodes (notwithstanding the turrets).

So in short, we want to balance noise benefits with wiring complexity.

 

[Matador]

Ok, I know i'm a newb and all, but i tried everything i could think of and have been told to try, Matadors instruction to put a 180k resistor as a dummy load allowed me to get the proper B+ voltage. But then with the mic plugged in, and using 2 different tubes, i have not been able to achieve the proper b+ of 120v. I started to trouble shoot the mic next, and what i found seems impossible. R15 and R14 250m resistors read DEAD! Nothing, nada, zero, no continuity what so ever, open! I just got a new fluke 87v, and i thought i was doing something wrong, till i broke out the old MM and the same thing, no reading? Would r15 and r14 being dead, and a break in this circuit cause the voltage to stay up around 130v with the B+ trimmer turned all the way counter clockwise? Other 200m to 500m resistors test fine? I've been spending way to much time on this, so i ask, am i crazy or missing something?

thanx
T

I would be shocked if any common DVM/meter would be able to read this range of resistance.  The Fluke 87V tops out at 50M, which is pretty typical.

Again, please post the actual tube operating points (the grid, cathode, and plate voltages, or TG, TK, and TP respectively) and we can help you figure it out from there.  What you should see is:  TG at somewhere near the -1V bias level (the meter impedance will foul up reality in this case), TP sitting near half the B+ voltage (perhaps 75V or so), and the cathode TK sitting at ground.

It might also help if you can post some pictures of the interior and the tube wiring so we can confirm everything looks ok.

 

[tonycamp]

Thanx Matador,

Ok here are the readings

Tg1 grid bias =.032
Tk1 cathode=Grnded
Tp1 plate voltage=148v

 

[tonycamp]

2nd shot

 

[tonycamp]

3rd shot

thanx
T