Official C12 Clone - Build and Support Thread

[Delta Sigma]

The R12/R13 voltage divider gives you 60V between two 0.5M resistors (I'm looking at original C12 schematic). R12 and R13 both have 60V drop across them (B+ = 120, R12/R13 junction is 60 then R13 is 0V at ground). The voltage drop only exists when current flows (120μA). R14 has no DC path to ground beyond the R12/R13 junction so no current flows and no voltage drops.

Once you put your DVM on the capsule side of R14 and reference to ground, you create a path for current to flow. If your DVM has an input impedance of 10M your new voltage divider is 60V across 260M (R14 + DVM) to ground. 10M/260M * 60V  = 2.3V.

Assuming your capsule and / or C11 aren't shorted, you can assume that the voltage appearing at the R12/R13 junction will appear at your capsule's backplate.

 

[Studiogearlover]

The R12/R13 voltage divider gives you 60V between two 0.5M resistors (I'm looking at original C12 schematic). R12 and R13 both have 60V drop across them (B+ = 120, R12/R13 junction is 60 then R13 is 0V at ground). The voltage drop only exists when current flows (120μA). R14 has no DC path to ground beyond the R12/R13 junction so no current flows and no voltage drops.

Once you put your DVM on the capsule side of R14 and reference to ground, you create a path for current to flow. If your DVM has an input impedance of 10M your new voltage divider is 60V across 260M (R14 + DVM) to ground. 10M/260M * 60V  = 2.3V.

Assuming your capsule and / or C11 aren't shorted, you can assume that the voltage appearing at the R12/R13 junction will appear at your capsule's backplate.

Thank you so much Delta Sigma.

My last question....if I am using this with only a cardioid capsule and no polarization switch at all, should I still feed polarization voltage through that line?

Or leave the P3 polarization voltage on 0 and connecting the back plate  to BP (where the R14 ends) and the FC to the grid/c11 ?

btw...i am measuring approx. -1V on the grid....again, no capsule and no transformer installed during this testing..

thanks a lot again

 

[Matador]

This was covered several times in the thread:  you can't measure high impedance (>1M approximately) with a regular DVM.  This includes the grid (which connects via a 250M or 30M resistor).  If you want to measure bias voltage, measure it in the PSU right where it is generated.

If you only need cardioid mode, then you don't need many components in the design.  Everything related to the rear diaphragm can be left out, and you don't even need to transmit it P3 to the microphone.

 

[Matador]

Going from the V1.3 schematic:  if you want cardioid only:

PSU: eliminate R11, C11, and the switch
In the microphone: eliminate C11 and R16.  You can leave the rear diaphragm floating (if your capsule has one)

 

[Lordward]

I'm having an issue with my build. I just finished the PSU and I get the following values:

B+ 120V
Heater 0.31-0.35
Bias - 1.03

I was able to adjust B+ and Bias but the heater doesn't change when I adjust the trim pot in either direction. I have a 180K load on as well. What might the problem be??

Edit: I forgot to mention that the heater value fluctuates between 0.31 and 0.35 on its own.

 

[Lordward]

I'm having an issue with my build. I just finished the PSU and I get the following values:

B+ 120V
Heater 0.31-0.35
Bias - 1.03

I was able to adjust B+ and Bias but the heater doesn't change when I adjust the trim pot in either direction. I have a 180K load on as well. What might the problem be??

Edit: I forgot to mention that the heater value fluctuates between 0.31 and 0.35 on its own.

I've solved it!  After reading through 117 pages of mayhem I figured out that there needs to be a jumper placed  in there.  Yay.  All values correct and trim-able.

 

[Lordward]

But all is still not well
Tube is glowing, that's a good sign right? 
I was able to adjust the heater, measuring on the mic, to 6.3VDC.  Got the bias to -1 at the PSU.  But the B+ I can only get down to 127 VDC.  Trim pot is turned all the way down.  Maybe I need to exchange a part? Or two?  or more?  Or just get a beer?

 

[Matador]

Are you sure you put the 100K pot in the B+ line?  How much adjustment range do you have?  Is this 127V with the 180K resistor?

All three trim pots look identical, but the one labeled '104' should be the one used for the B+ adjustment.

EDIT: and yes, you should get a beer.  In formal logic we call that a tautology.

 

[Lordward]

Are you sure you put the 100K pot in the B+ line?  How much adjustment range do you have?  Is this 127V with the 180K resistor?

All three trim pots look identical, but the one labeled '104' should be the one used for the B+ adjustment.

EDIT: and yes, you should get a beer.  In formal logic we call that a tautology.

Beer was great!  Forgot about the mic for awhile
Anyway, yes I am sure that I have the 100K trimmer (104) in this position.  I'll check how much range I have but I think I started with something like 170V.  I'm getting this 127V with the mic connected.  When I disconnect and use a 180K load resistor I can tweak it down to 120V.

 

[TLRT]

Beer was great!  Forgot about the mic for awhile
Anyway, yes I am sure that I have the 100K trimmer (104) in this position.  I'll check how much range I have but I think I started with something like 170V.  I'm getting this 127V with the mic connected.  When I disconnect and use a 180K load resistor I can tweak it down to 120V.

Sounds like a tube issue(new production 6072's are a bit out of specs sometimes), what tube do you use? You can raise R1-R2 value in the PSU.

 

[micaddict]

Sounds like a tube issue(new production 6072's are a bit out of specs sometimes),

Try the other half?
Or is that moot in this case?

 

[Lordward]

Try the other half?
Or is that moot in this case?

Funny, I had actually bought the parts for the early BOM which asked for higher values.  Even put them all in only to take them back out since the board has 68.1K listed on them.  Ok, I'll put back the 90K.

 

[micaddict]

I  meant the other "half" of the tube.

But again, this may not be the issue.
Others, wiser than I will chime in.

 

[rmburrow]

Sounds like a tube issue(new production 6072's are a bit out of specs sometimes), what tube do you use? You can raise R1-R2 value in the PSU.

There was a C-12 on evilbay for 20 grand.  One of these kits looks like a good idea, especially with a Thiersch capsule....  On the B plus regulation issue,  clamping B plus using a 0A2 regulator tube with a 8 k resistor in series from the rectifier to the 0A2 plate won't let the voltage soar to 170 volts..the VR tube will hold it at 150 volts and the adjustment should fall within range... (150 v zener will do the same thing but I never liked zeners across B plus....fwiw).  Clamping B plus with a 150B2 tube (similar to US 0A2 except for base connections)  was done in the older Neumann NKM supplies...

 

[bavabasra]

Heater Voltage no load is adjustable to 6.3 V. With load (tube in socket) I cannot get more than 4.2 V regardless of trimpot position.
All other Voltages are correct. Any ideas?

 

[Category 5]

Bad tube or heater shorting to ground on the mic side?

 

[bavabasra]

Bad tube or heater shorting to ground on the mic side?

Neither, nor

 

[bavabasra]

Problem solved:
I am using a toroidal with 2X 6.3V outs. I used only one and thus lost about 2V with load. I hooked the 2 low voltage outs in series and the rest is history.

 

[Matador]

6.3V after rectification diodes will give about 7.5V.  The LM317 needs at least 3V across it to guarantee regulation under load, which means you won't go above about 4.5V @350mA, which seems to match your case pretty well.

If you want to run only a single 6.3VAC secondary, and don't mind running the heaters a tad under-voltage, you can get a regulator with a P-channel pass device and cut the dropout down to around 1V at these currents.  For example, a TLV1117 can run closer to 1V dropout, which means you can trim to 6V and have about 0.5V margin.

 

[bavabasra]

Thank you Matador for the pro and accurate advie. However, although not imperative, I do like having the extra voltage at my disposal.